Remove a Color Cast Part II (Threshold Layer)

In my January 27th post I described a way to remove a color cast by using Blur Average to establish an overall average color for a copy of the image, then switching to the original image and using a middle gray eyedropper in a Levels adjustment to sample the averaged image to reset middle gray in the original. This is a simple method that works in many but not all situations. There are times when a color cast is desirable. Sunsets/Sunrises, fireworks, night scenes, etc usually do not need or do not benefit from color balance correction.

Another method I often use actually does two things in one step. It balances the color to neutral and it also establishes a complete tonal range for the image, ensuring that there are black and white areas. The typical candidate for this image is one that has a color cast and is properly exposed, but has a histogram that is completely between and not touching the boundaries. By using a threshold adjustment layer it is easy to find pure black and pure white, then use the sampling eyedroppers in either Levels or Curve adjustment layer to set the corresponding black and white points. In the process of doing this any color cast will usually be removed. The resulting image will have a full tonal range, from pure black to pure white.

Here is a quick how to.

In this image of the Bow Bridge in Central Park, there is an overall warm cast, no black and no white in this image, making it appear dull and drab. In Photoshop, I start by creating an adjustment layer,

Select Threshold type,

giving me a layer that looks like this.

There is an adjustment slider that I will move left or right to find the threshold of black and white levels.

Next, I select the Color Sampler tool, and set the Tool Options to 11x11 average,

then I go back to the adjustment panel for the threshold layer and move the slider all the way to the left until the image turns completely white, then I move the slider slowly to the right until I start to see the first black areas,

sometimes it helps to zoom in so I can be sure to place my Color Sampler tool entirely in the black area,

it is important to reveal just the first area that shows up - the threshold of black - to ensure that I will get the smallest area of black. I use the Color Sampler eyedropper tool and click in the black area. This will leave a non-printing marker that will I will use later.

Now I repeat but this time moving the slider to the right to find the white point, dropping a marker on the white point. After selecting the two points I no longer need the threshold layer, so I can either turn it off or delete it.

Next I create another adjustment layer, either Curves or Levels. Either will provide eyedroppers that will be used to select the black and white points that I left reference markers on in the previous step. Using the black or topmost eyedropper to pick on the black point, and the bottom one to select the white point.

The image looks like this after setting the black and white points.

and for reference, this is the before view.

everything is more pleasing, there is a full range of tones, and the colors look better without the greenish/yellowish cast. Sometimes the effect is subtle, at other times it can be pretty dramatic. Hopefully you can use this technique to add a little more WOW! to your images.

Single Shot HDR - or How to Save Underexposed or Flat Images Using Tone Mapping

Went out this past Saturday and found myself at Jamaica Bay National Wildlife Refuge at the end of the day. The weather had been threatening rain all afternoon, but I took the chance to go there anyway. Aside from a nesting pair of Osprey, and a flock of Brants feeding at the shoreline, there was the sky. That kind of sky that you see before or after a storm. Bits of blue in the cloudless areas, the warm color of a soon-to-set sun reflecting off the numerous clouds, and a totally clear view of the whole spectacle - but my sights were set on the Osprey couple.

So I snapped off a few pictures without thinking. When I viewed the images on my computer, they looked pretty sad. The sky was correctly exposed, but everything else was drab and dreary. This was not at all how I remembered the scene, so I started thinking about how I might restore the original "feel" in the image.

There are a number of tools that can help you recover an underexposed image - Lucis Art, Topaz Adjust, the built-in tone mapping available in Photoshop - but I decided to use Photomatix Pro - mainly because I like the quality of the output and the relative ease with which I can get those results.

For a full description on how to use Photomatix Pro, look at my blogpost here. The process with a single image is similar to the one you would follow for a multiple image HDR after you merged the images into a single image. Basically you have two main options - Tonemapping and Exposure Fusion. The Tonemapping selection has two choices - Details Enhancer and Tone Compressor. I find the following workflow useful:

1. After loading the image, select a preset that gets you closest to the "look" you are trying to achieve.
2. Use "Strength" at close to 100% to  control how contrast will be affected by the subsequent adjustments.
3. Set white point, black point, saturation and gamma to please your eye.
4. Start making adjustments using smoothing, micro smoothing, contrast, microcontrast, luminosity etc - until you have gotten closer to your goal. 
5. If you end up with halos, use highlight smoothing to remove them.
6. Save and open the image in Photoshop - make whatever cropping, tone, contrast, color balance, sharpening and noise reduction adjustments you typically make. At this point you should be done. 

I have included several before and after examples below.

It's All About the Light . . .

I was asked to shoot an event this past weekend at a local restaurant - a sweet 16 party which included 110 guests. My thoughts directly went to what gear I would use and how I would do the lighting.

Available light shooting was out of the question - I was informed that the restaurant's dining room lighting level would be held low. I would have to use a fast zoom lens in any case, because a slower lens would have difficulty acquiring focus in such low light. I decided on an 18-50 F2.8 zoom for my D300, which gave me the flexibility of very wide to moderate telephoto, and would still be sharp at F2.8. But it would still not be fast enough to shoot available light, unless I used an ISO of 6400 or higher. The D300 image begins to look pretty crappy at ISOs higher than 800, so speedlight(s) would be the only logical choice.

Among the choices for lighting were on camera flash, which could be bounced for more even lighting, but I had hoped that I could do something a little different. I don't care for camera-positioned lighting because no matter what portable modifier you use, the quality of the light is flat and unappealing, with no contour-shaping shadows, except for that shadow that ends up under the chin and nose when you use those tall swiveling flash brackets that all the paparazzi use.  Another undesirable characteristic is flash shine - an area of perspiration-moistened or oily shiny skin that reflects more light, usually resulting in unflatteringly overexposed skin areas. With the expectation of shooting 100s of pictures there was no way I would spend days in Photoshop correcting shine.

I decided that the room was small enough to light the room with flash. It was time to mobilize the over half-dozen second-hand speedlights that I have collected over the years. All of them are made by Sunpak - 433D, 444D, 360D, and the venerable and highly sought after Auto 383. Each has a guide number of 120 making them as powerful as the best offered by Nikon or Canon these days. But more important - adjustable light output levels. I figured that with enough lights strategically placed I could illuminate the entire party room and keep the output levels low enough to be able to shoot the entire 4 hour event, take 400 images and not have to change the batteries.

I visited the restaurant the night before the event to check out possible locations for lighting. There were wall-mounted sconces that were large enough to conceal my lights, but they were too far from the ceilings. This distance is important since the light to subject is significantly greater when the light has the longer path to travel from the flash head to the ceiling then to the subjects. Also, there was a greater chance of getting the flash in the shot, which can work for dramatic effect if used judiciously, but definitely not ok for every other shot.This alternative was not going to work for me.

I decided to use super clamps to attach the speedlights and flash triggers to the chandeliers. With a little trial and error I was able to point the lights to cover the room with 7 speedlights, which left me one for the camera for low power fill. The ceiling-bounced flash was set to 1./4 power. This provided relatively short recycle times and low power consumption.

These were triggered with my favorite radio triggers, the Yongnuo RF602.

The quality of the indirect strobe lighting for all intents and purposes resembled available light -with some  wonderful benefits. Speedlights bounced off the ceilings spread light in all directions - softening up the shadows and providing lovely flattering light without any sign of harshness.

They freeze action by virtue of their short but intense burst of light. There would be no risk of motion blur. People were captured sharp and clear. No "tunnel effect" where the subjects are brightly lit and everything else is in dark shadow.

I could use a lower ISO (800) and still shoot at F5.6 - F8, the "sweet spot" for my lens as far as sharpness is concerned. And finally, I could take a long shot of the room and show all the people in it - none of that "tunnel" effect that is so common when camera mounted flash is your only source of light.

Below are two images. The first taken with a flash used with a bounce card mounted on a rotating bracket attached to the camera as the primary (key) light. The second utilizes the chandelier mounted flash bounced off the ceiling with a tiny amount of fill light from a camera-mounted flash using a bounce card. The power level on the camera's flash was either 1/8 or 1/16.

You can see the difference - the girls in the lower image have softer features, you can see highlights in their hair, and the lighting is a bit more interesting.  The upper image has harsh lighting, the hair gets absorbed by the dark background, and there is that deep dark shadow under the chin and in the eye sockets.

This lighting approach cannot be used in all situations - sometimes the room is just too large, or the ceilings are too high. This demands some other form of bounce lighting, perhaps with more powerful monolights with radio or optical triggers, umbrellas or softboxes, etc. But for this application, the little guys were perfect, and everything worked out just fine.

Dreaded Lens Flare? Here's How I Deal with It.

Ask any photographer about lens flare - you will probably get an ugly expression accompanied by some choice expletives that I will not print here. It's a law of photography (that probably needs repeal) that you should always keep the sun at your back or side. The reasons are to provide better lighting on your subject, and to avoid - DREADED LENS FLARE - from ruining your shot.

I like to break rules, especially one like this. The many photographs of the setting or rising sun is one example that shows that I am not alone here. Sunlight filtering through the trees will also wreak havoc on images that are made with lenses that are prone to flaring. Lots of times these images are taken just before sunrise or just after sunset to avoid flare. If I like the light and composition I will typically shoot right into the sun if necessary to get what I want, then fix it later. There is a mood, an element of "drama" that results when you shoot into the light in these conditions that is hard to capture if you simply follow the rules. You can fix certain "features" in postprocessing, but you can forget about a bad composition that results from trying to avoid shooting into the light.

Needless to say, there are some obvious challenges here. First is the amount of light coming from the sun. It will fool your meter and make everything go to silhouette if you are not careful. HDR (covered in my earlier post) will take care of extremely wide contrast range and you can work in Photoshop using a combination of tools to tame down the flare.

Flare has two components. A strong color cast and a change in luminance.Where the flare occurs will determine how I go about fixing it.

Flare in an evenly toned sky or detail-less surface is the easiest. Just create a duplicate background layer and use the patch tool to select an adjoining area of sky and move the patch over the area that needs to be fixed. Done!

Flare that shows up in detail areas is more difficult. Here I employ a two-step process. First I remove the color cast. I start by creating a duplicate layer and selecting it, then I select a brush from the tool palette,

change the brush's blend mode to color;

then I select the eyedropper tool;

and and its sample size to 11x11 pixels;

I then select the brush tool again, and using the right click to enable the eyedropper, I sample a similarly colored area to use as my brush color.

I then paint over the flared area until all the green, yellow or magenta (or other color) is "neutralized." By using the color blend mode on the brush, the detail and texture is left intact, and I am only replacing colors.

Now you see it,

and now you don't!

At this point I usually flatten the layers once I am satisfied with the color removal. My second step involves adjusting the difference in luminance and/or reconstruction of the brighter area.


I decide what tool to use - content aware fill, clone, patch, or burn-in based on the type of repair I intend to make. If there is detail and the difference in tone is not that bad, I will burn in the area, which is the way I went with on this image.

Photoshop provides a dodge and burn tool, which lets you limit its effective range to shadows, mid-tones or highlights. I usually bypass this and create a new "dodge and burn" layer, fill it with 50% gray, set the blend mode to overlay or soft light, and use a black brush set at 15% opacity and 15% flow. I paint directly on the 50% gray layer. This darkens the area smoothly with minimal effect on color saturation or hue. The reason this works so nicely is that both overlay and soft light are contrast enhancing, but the closer to middle gray the tone is the less the effect. Anything that is middle gray is completely transparent to the layer below it. It is easy to build up density with black or white and be very precise about where you are dodging and burning. If you want to check your progress, turn the gray layer' visibility on and off. If you have to back off on an adjustment, use the opposite color brush - white to fix black mistakes and vice versa. If you don't like at all what you have done, just delete the layer and start over again.

Here is the final image:

What You Saw is NOT What You Got?

Consider the following scenarios. You are careful to set a proper white balance when you take the picture, then you come home, load up Photoshop (or the image editing program of your choice), and you get it looking perfect. Then you pirnt and something goes wrong - it looks like someone switched your files between the moment you press Print and you see the printhead moving back and forth creating an image. The results are just WRONG. Colors are different, the orange shirt is now salmon-colored, people are red-faced, a white dog is orange, the purple flower is now a bright shade of magenta, etc.

The immediate response is to go back to your image editor, or your printer dialogue and based on what you see in the print you start making adjustments - a little less magenta, a little more cyan, add some yellow - wasting tons of ink and paper in the process - and when its over you are wondering if your printer is broken.

Or, there is a different problem. Everything looks great on the screen, that cloudless blue sky is deeply saturated and perfectly smooth from edge to edge. You look at the print and it seems as if someone dropped coffee grounds during the print process. This is sensor dust. But there is a problem - you go back to the screen image and you can't see them. My guess is that you are using either a laptop or an inexpensive LCD/LED display with a limited bit depth. Without getting too technical, unless specifically noted in the specifications, all-purpose displays such as the sub $200 flat panels or those in laptops are not capable of displaying the more subtle gradations in tone and color. Sometimes his will come across as banding on a sky, where you have a very gentle shift in hue, saturation and luminosity from horizon to the top of the frame. Other times minute dust particles may have settled on your camera's sensor and blocking the light getting to the sensor directly beneath the particle, causing a "dust shadow" to appear. But in the bargain monitors, the bit depth is not enough to differentiate the subtle changes, making the dust spots invisible on the display. Most printers have a wider gamut, and are able to print this information,  The better the printer, the better the ablilty to display everything that is in the image - including the dust spots. Here is an example of dust spots:

If you don't see the dust spots, then there is a good chance you have one of "those" monitors. If you are serious about image quality, either sending it to others electronically or making a print - then its probablay a good idea to put a replacement display into the budget.

Well, as far as the first situation is concerned, there's an app for that. More accurately, there is a combination of hardware and software that you can purchase to address the difference between displayed vs printed colors and tones. There are a class of products called monitor/printer profiling applications that will fix the majority of the issues. These consist of a either a colorimeter or spectrophotometer that measures your monitor's native colors and gray tones on a test pattern, then creates a table of values that correct the differences between what the monitor shows and what the neutral standard is, and then builds a monitor profile that loads when you start your computer, making all the necessary adjustments. this way a green on screen will look like the green that will be printed - more or less. It's hard to do any image editing and color correction without at least the display being calibrated.


With the color accuracy of your display now under control, you have to address the print. If you use a printer manufacturer's inks and paper (and printer profiles if provided), or if you use a printing service. you are likely to get decent results - not perfect by any means, but reasonable. Most inexpensive printing services will use a hybrid technique, employing a digital projection onto silver halide emulsion paper, then processed in conventional wet process chemistry.  The more expensive houses will use high quality, color profiled and calibrated inkjet printers. Each paper type offered will have its own profile. the printers will use anywhere from 8 to 12 different pigmented inks - providing the widest color gamut and dynamic range possible. Very few affordable processes can even begin to approach the quality obtained from a properly processed inage printed on a 12 color image printed on rag paper in a color managed workflow.




Spyder Studio with monitor and printer profiling

Luckily the tools used to do this at the pro or commercial level are available in scaled down versions for the consumer. Datacolor and Xrite both offer affordable profiling solutions that work with most displays and printers as well as more expensive ones for professional printers and photographers. These create profiles for either printers or displays, or both.

The dust bunny situation involves being able to see the dust before you print. This requires using a display that is capable of showing dust.  At the present there are only a handful of displays that can do this - those which use e-IPS, S-IPS, P-IPS, H-IPS, AS-IPS, H2-IPS and UH-IPS. IPS stands for In Plane Switching, a display technolgy developed by LG Philips in 1996. LG makes nearly all the IPS panels currently available in the marketplace. For all intents and purposes the S, H and P-IPS panels are the ones to look for. The e-IPS is an adaptation of the technology to lower the cost, and more often than not can only display a color depth of 6 bits per color, or 2^6 x 2^6 x 2^6 = 64 x 64 x 64 = 262,144 colors simultaneously. This is not good for photo editing applications. The way 6 bit panels create 16.2 million colors is by rapidly switching between 2 colors at each pixel, creating the illusion of greater color depth - but while they look good for general applications, not being able to see all the information without switching is going to present problems seeing subtle things like dust and other artifacts. Sometimes you will see a panel specification stated in terms of a percentage of a color space. You might see something like 72% of sRGB. Most printers can now print close to 100% of sRGB, which means that the prints will show more color and tonal variation than your display, whicn in simple terms explains why you won't be able to see the dust bunnies on a $200 LCD/LED panel. With the exception of some Apple products and the Lenovo Thinkpad X220, nearly all displays are 6 bit.

The image below is an exaggeration, but a good way to illustrate the difference between high and low bitrate displays. The dithering is the switching that takes place - this is a static image, but you should be able to still see the banding. The rightmost color is typical of what you will see on an 8 bit (x3 = 24 bit) per color display.

At the very minimum you should be looking for a panel that can display 8 bits, or 2^8 x 2^8 x 2^8 = 256 x 256 x 256 = 16,777,216 simultaneous colors. These typically can display a color space as large as sRGB, considered the minimum for photo editing, and the pricier versions can display up to Adobe RGB, a bigger color space. A rule of thumb is to get the largest color space you can afford., but not less than 98% sRGB. The specs that are meaningless to you are speed, brightness, contrast ratio, etc. These are all well-beyond what you need. In some cases flat panel displays can be too bright, making it difficult to profile.

There are a few very costly 10 bit panels, which if you do the math,  2^10 x 2^10 x 2^10 = 1024 x 1024 x 1024 = 1,073,741,824 simultaneous colors. These are absolutely breathtaking, but be prepared to spend more than $1100 for a 27"panel. The problem with these is that unless you can create a 10 bit workflow, such a display is overkill. Most cameras are 8 bit, as are printers. There are few photo editors that can work in 10 bits. It is easy to see that  having a 10 bit panel would be unecessary.

You can find a list of popular IPS displays with street prices here . The ASUS PA238Q seems to be the least expensive 8 bit panel that offers full sRGB display at $300. I suggest that you look for reviews or a list of specifications for any display you are interested it to ensure that it is suitable for your purposes.



Once you are able to see stuff like dust spots, you need to be able to remove them, and there are two methods to accomplish this. Each has its good and bad points. The more conservatie but costlier approach is to send you camera in for a sensor cleaning. Give to someone else to do and if something gets messed up in the process, they will (hopefully) take care of things. This can cost from $50-$100 and you can be without your camera for several weeks.



dry and wet sensor cleanng system

You can always purchase a blower, dust brush and wet-cleaning swabs and solutions for around the lower price of sending the camera out. A blower and a brush,should be standard equipment, since the majority of sensor dust is removeable with these gentler tools. Use the mirror lock up function to expose the sensor, and using a light to see what you are doing, use a blower that is intended for this to gently blow the specs away. Sometimes you need to use a little "gentle persuasion" to get the more stubborn specs off. Under no circumstances should you use the compressed air products, which use unfiltered air, and can blast microgrit across your sensor, permanently etching it in the process. Actually, you would not etch the sensor itself, but the low-pass filter that is in front of it. In any case, you would be looking at a costly repair, typically in excess of $200.


If you are daring enough, you might try the wet method. This includes a swab of lint free material attached to a paddle that you dampen with a cleaning solution and wipe once across the sensor. Any time you touch the sensor you run the risk of scratching the filter, so you need to be super extra careful and resign yourself to the $200 or higher repair should things go wrong. I have done it 3x on my D200 with no damage, but everyone's mileage is different. If you at all nervous about this, just send it in.


Exposure - How to Get it Right Most of the Time

With automatic cameras and their wonderful exposure setting systems it is not hard to get a good picture under normal circumstances. The ease with which even the simplest, least expensive point and shoot cameras can take a reasonable picture is astonishing. These little cameras are amazingly sophisticated, especially when you consider the low prices.

But it is when the not so normal circumstance presents itself that many newer photographers are at a loss. Strong side light, backlight, very bright scenes, low light action shots, water reflecting bright highlights, sunrises/sunsets, stage performances - these are just a few scenarios that can be challenging for a photographer that does not have a firm grasp of how to interpret the conditions and set the camera exposure accordingly. Camera manuals are of little help, since they are written for the non-technical user and for "average" lighting situations. Unless you take the initiative to investigate how exposure works on your own, you are likely to be in the "dark" as far as how it all comes together.

Back in the day, before cameras had built in metering systems, a photographer would use a printed "exposure calculator" like one of ones shown in this link http://www.mathsinstruments.me.uk/page67.html or they would wing it, using a best guess estimate of how best to set the camera, using a printed guide that relied on "rules of thumb" to arrive at a close approximation of an exposure setting. Kodak used to include an exposure guide in the box with each roll of film that looked like this:

Believe it or not, following these guides resulted in pretty decent exposures. But for really accurate results in challenging light, pros and serious amateurs would turn to electronic light meters to measure light and translate the measurements into camera settings.

It was not until the early 60s (1960s, that is) that a Japanese camera manufacturer by the name of  Topcon introduced a single lens reflex camera with a through the lens metering system. Up until then some of the fancier cameras were equipped with external light meters, some of which were mechanically coupled to the shutter speed and aperture setting mechanisms. But the meters were not very sensitive to the extremes of black and white - and it was difficult to measure reflected light accurately. Cameras with interchangeable lenses presented another challenge, since the reflected light measured from a wide angle was not necessarily the same as the light from a narrow telephoto shot given the meter's fixed angle of view.

At the time many light meters were like the one pictured at the right, set up to measure the light  falling on a subject rather than the light reflected by it. This type of metering is called Incident Metering. The hemispherical piece on the top of the meter - the Lumisphere - would capture the light and present it to the meter sensor as having the same luminance as an 18% gray card. This was actually pretty clever, since the reflectances of the elements in the scene could not affect the reading. This is important as the meter and its scales were calibrated for 18% reflectance to render it as middle gray. So taking a reflected reading of an 18% gray card and an incident reading of the light falling on that card in the same setting would result in exactly the same exposure recommendations.

As the technology improved, reflected light meters became more accurate and sensitive. A German company named Gossen engineered a series of extremely sensitive reflective light meters, that had a little Lumisphere so that you could still take incident readings. They were somewhat modular, and had attachments that you could add to measure light in a narrower view, through a microscope, etc. Later models included a flash option.

Gossen Luna Pro Incident/Reflective Light Meter

Lumisphere in place for incident reading

Lumisphere moved aside, exposing sensor for reflected readings

Today, nearly all modern portable cameras use some form of reflected light metering system that measures the light coming through the lens and falling on the digital imaging sensor, or in the case of a film camera, the film plane. Professionals working in large format film photography using natural light often rely on a version of the above, or in the case of the Sekonic Digital Master L-758DR Light Meter pictured below, which can accurately pinpoint and measure a small specific element in a scene, using a very narrow angle of view, usually 1 degree, or it can function as an incident meter, and also has the ability to be triggered by a flash system, so it can perform incident readings of flash lighting. And it does this over a range of brightness that is far greater than what any digital camera can measure, with an accuracy of .1 fstop.

Using a light meter required a bit of thought in order ot get good results - and it didn't much matter whether you used incident or reflected in most situations. In the case of incident readings, you could take the reading from the meter 95% of the time without any exposure compensation and get a good image. You could also measure any part of the scene, and with your experience decide how birght you wanted the metered area to appear in your image, and compensate appropriately. The incident reading was more foolproof, while the reflected reading required more experience but gave you more control.

Consider the following example of a picture of a pair of cats, one white and one black.

If you were to measure the reflected light either using a camera or a light meter where you are able to isolate the entire cat, the black cat reading would tell you there is not a lot of light and suggest that you use a slower shutter speed or a wide lens opening or a high ISO (more sensitive to light) to allow more light to hit the film or camera sensor, and vice versa for the white cat. For argument's sake, a black cat might reflect 1 1/2 stops less light than middle gray, and the white cat 1 1/2 stops more. If you were to use the white cat's reading as a reference, you would have to add 1 1/2 stops more exposure - either by opening up the lens or lengthening the shutter speed. This would bring the tonal value of the cat from the middle gray the light meter assumes, to a brighter value - along with everything else in the scene. You could use the black cat as a reference and decrease the exposure - experience and sample measurements will help you to place the value of anything that you read with a reflectance meter in the right place.

In contrast, an incident meter would only read the amount of light hitting the subject, disregarding the brightness differences betweent the two subjects. So the setting for a picture of the black cat would be no different than for the white cat. The dark cat would reflect less light appear dark, the light cat would be light. Using the exact recommendation would result in a perfectly exposed image in most cases.

This is an important concept upon which all  exposures are based on. You CAN use a reflected light meter to accurately expose an image, but this is where experience and common sense come into play. It helps to think of the world in terms of shades of gray. To be more specific - 11 patches of shades of gray - from complete black (Step 0)  to complete white (Step 11), and nine more patches in between. These would be spaced "one f stop" apart, which simply means that moving from black to white, each step would reflect twice as much light as the previous. The table below shows typical picture elements and what their values might be:

The table works on the the premise that the average scene has a brightness range that generally does not exceed 11 f stops. This is a good thing, since most digital cameras have trouble recording an image when the brightness range goes over 10 stops. When encountering scenes with unusually wide brightness range the photographer must make a decision about what is more important - highlights or shadows - and adjusting exposure accordingly. Modern camera metering systems read entire viewfinders worth of tonal values, then do some very complex interpretations of what they read, taking an average of the entire scene, sometimes giving greater weight to the center area, or what the camera if focusing on, or taking into consideration the brightest areas and adjusting exposure to avoid overexposing these areas. But the one thing a meter cannot evaluate is what the subject matter is. A camera or handheld meter cannot tell that the light that it sees is coming from a black cat - and will suggest a camera setting that will result in a black cat being shown at value V on the chart, when in fact it is probably closer to III. It would do the same thing if it read a white wall. That decision is left to the photographer.

A very useful tool is an 18% Gray card - as long as you understand that it may give you an erronenous reading, but it will do this in a linear fashion. What I mean is that it could read 1/2 stop brighter, but it will affect all your settings by the same 1/2 f stop. The reason for this, according to Thom Hogan, is that meters are actually calibrated to 12% reflectance, not 18% thus making all readings go off by 1/2 stop. But, as always, your mileage can differ, so its always best to test your card under typical lighting situations and check the camera's histogram, (not the software's histogram) to see if the reading is dead center. If it is off to one side, you have to dial in enough exposure compensation to bring it back to center. You can visit http://www.bythom.com/graycards.htm to see a more detailed explanation.

I will use the next few lines to make some general statements about how shutter speed, lens opening and ISO interact, and how that affects your exposure setting, with the intent of following up with greater detail in future posts.

ISO + Shutter Speed + Fstop = Correct Exposure - they must always be in balance and this is ALWAYS true. If you use a lower ISO (less sensitive) you need to open the lens, or slow down the shutter speed. Remember, the Fstop number is the ratio of the opening of the lens to the focal length, so as you increase the F number the lens opening gets smaller. Just to totally confuse you, the shutter speed numbers on your camera represent the denominator of the fraction of a second that the shutter is open and admitting light to the film or sensor - so if the camera says 250, it assumes that you know that it means 1/250 of a sec, 4 would mean 1/4 second and so on.

Remember that fstop represents a doubling or halving of the light getting to the film or sensor. I'll start with ISO values, since these are a bit more intuitive. An ISO value of 200 is 2x as sensitive as 100. A value of 400 is 2x as sensitive as 200. To go from an ISO of 100 to 400 means that you are doubling twice - or 2 f stops.

Shutter speeds are similar. It's fairly straightforward to understand that if your shutter is set to 1000 (1/1000 sec) and you change it to 500 (1/500 sec) you will be letting in 2x as much light. If you slow it down to 1/250, you will be letting in 2x again more light - moving from 1/1000 to 1/250 you are adjusting the light by 2 f stops.

Now things get a little hairy. Lens fstop numbers are not intuitive, since they represent a numerical ratio of the effective diameter of the lens opening to the focal length. In the simplest of examples, a 200 mm lens with a maximum opening of 100 mm in diameter would be listed as F2. If the same focal length were an F4 lens, then it would be 25mm in diameter. The difficulty is introduced when you realize what your high school geometry teacher was trying to get you to learn - the AREA of a 100mm diameter circle is 4 times the area of a 25mm circle and would let in 4x more light - or 2 f stops. With lenses, the standard would be 2 - 2.8
- 4 - with each interval representing one fstop. So to change from a lens opening of F4 to
F2.8 you would double the light coming in, or one Fstop, and again going from F2.8 to F2. Because of these relationships, if you double the shutter speed but close the lens down by one Fstop, the image will have the same brightness. You could also slow the shutter speed by one stop and increase the sensitivity (ISO) by doubling it, and end up in the same place exposure-wise.

But what constitutes "correct" exposure? Typically that is where you are able to capture all the information possible in your image. Which begs the question - "How much is enough? Too much? Not enough? A better working version of correct exposure is the setting that will correctly capture the information the photographer wants to show.

This image of a Bufflehead was a particularly challenging exposure situation - a mostly dark bird, with bright white markings, bright sun that was low in the sky causing deep, long shadows, and water. It was shot with a 600 F4 and a 1.4x extender, which meant that the largest lens opening possible was effectively F5.6, but to provide better image quality I needed F8. So that set things up for a shutter speed that was short enough to stop the wave action and any random small movements from the bird. The end result was to adjust to a  higher ISO - 1000 in this case - to ensure that all of the above conditions were satisfied.

Even with all of the above in place, there was still a looming challenge that I decided I would not try to solve in the field. The brightness range was greater than what my camera could record. The rule of thumb is if you want any detail in the white areas, take care not to overexpose them. But that meant that all of the dark areas would have been "lost in the mud." So I decided to compromise a bit of the highlight detail in order to get the subtle iridescence from the neck and sides of the head, and show the all important eye. The dark areas did in fact go to "mud" but I was able to selectively lighten, or "dodge"' the darker areas to reveal the texture and color of the plumage.

In the interest of keeping things a simple as possible - I will describe, in broad terms what happens when you tinker with the three elements of exposure.

ISO - the less sensitive (lower number) you use, the less noise/grain you will have in your final image. You will have greater detail and sharpness, and a broader "dynamic range" (more about this in a future post).

Shutter Speed - slower speeds let in more light but has less "motion-stopping" capability. This is not necessarily a bad thing - you want a longer exposure to show things like fireworks, headlights of cars in traffic at night, star trails - or a special technique where the photographer purposely uses a slow shutter speed and pans the camera with a moving subject, showing the subject relatively blur-free while totally blurring the background, thus giving the impression of extreme speed. On the other hand if you want to stop the beating wings of a hummingbird you'd better use as fast a shutter speed as possible.

Apeture/Lens Opening - big openings let in lots of light, however, all but the most specialized of lenses are sharpest at their widest opening. If you see a lens that is F2.8 or as big as f1.4 there is a good chance that the designer made that lens tack sharp at that opening. Many lenses have a sweet spot at F5.6-F11 where they are sharpest. Another phenomenon is depth of field - or moving away from the camera, and focusing at a specific point, at what distance do things begin to look sharp in front of the focal point, and at what distance do they become unacceptably out of focus. Smaller openings (larger number) give you the advantage of a deeper depth of field, while bigger openings (smaller numbers) will provide only a very shallow zone of sharpness. You have all seen pictures where the subject is nicely sharp and th backgrounds are all blurry and soft.  That is a dead giveaway that the lens was pretty wide open. Telephoto or long focal length lenses have shallower depth of field than wide angle lenses do at the same distance. But at the same magnification (image size on the sensor) the depth of field is exactly the same.All this means is that at the same lens opening a 200 mm lens at 20 ft is going to have the same depth of field as a 100 mm lens at 10 ft.

As you can see, there are a lot of things that must come together in order to get consistent results, but the most important thing is to "own" the fundamentals. These building blocks to exposure, once grasped with confidence, will allow everything else to fall into place. You will be able to intrinsically know what is possible without giving it a second thought, and what you need to do to with your settings to get the finished product looking the way you want.