by John Gerlach
Beginning in the mid-seventies, I wanted to become a professional nature photographer. One of the many things I wanted to master was exposing slide film which requires great precision to get it just right. During the mid-seventies, the typical advice I heard was to use a hand-held incident light meter or meter a gray card and shoot at whatever exposure you get from a reading off the gray card. Others believed in the "sunny 16" rule for determining exposure. All of these methods had some merit and worked some of the time. But, I wanted a way to determine exposure very accurately, fast, and it had to work all the time. During the late seventies and early eighties, I developed my own system of determining exposure that involved metering something, judging the reflectance value of what I was metering, making an adjustment for subject reflectance according to the tone of the thing that was metered, and shooting the exposure with great precision. It sounds like a lot to do, but once I got the hang of it, I could determine proper exposure for virtually any situation in mere seconds. The method worked extremely well for me for 25 years and actually got even easier to use when spot meters became widely available on top of the line camera bodies.
More than 50,000 nature photographers have seen my exposure program over the years and many who persevered became excellent at using the method. I was glad I could help them. I know hundreds attended my field programs primarily to learn to expose difficult slide film. Little did they know that I would have loved to use autoexposure, if only it was accurate enough for my purposes. When the Nikon F5 first came out, my wife Barbara was thrilled to get the new camera. I was shooting Canon by then. Some of the folks who first brought the Nikon F5 to my workshops said they didn’t need to learn the system I used because the color matrix meter generated perfect exposures every time. I am always up to a challenge so during the workshop we tested the Nikon F5 on automatic against figuring the exposure on manual with the in-camera spot meter. Of course, I got to pick the test subjects. I thought I could outfox this new color meter so I was careful to select subjects where I suspected auto exposure would have trouble. For example, one scene I selected was a beautiful autumn color forest that was nicely reflected in a still lake in golden dawn light. However, in the foreground, I had three trunks of white cedars framing the scene in the background. There was no light on the cedar trunks which filled about 25% of the frame. How would the meter know if we wanted the autumn color in the background properly exposed or the foreground properly exposed or something else altogether? Using manual exposure and the in-camera spot meter, I told my student to meter the bright orange leaves in the background and add 2/3 stops of light to the exposure which turned out to be a perfect exposure. The autumn colors in the background were perfectly exposed while the cedar trunks were in silhouette as we wanted. On automatic, the color matrix meter was thrown off by the 25% black in the foreground and overexposed the autumn color by about 1 stop. The Nikon F5 meter did recognize colors though and knew enough to add more light to a white snow scene. I tested it a few times and it added about 1 stop of light. Unfortunately, my method was far more precise because I knew snow on a sunny day needs about 1 1/3 more stops of light and a full 2 stops of extra light on a cloudy day. If a dark moose or bison is large in the frame and surrounded by snow on a cloudy day, then I even add 2 1/3 stops to get some detail in the dark fur without overexposing the snow too much. The F5 didn’t know what I wanted and didn’t give me the results on automatic that I wanted so I was never a fan of the accuracy of the Nikon F5's meter. It was still a great camera though, but Barbara continued to use the camera on manual and spot metered to get proper exposure.
I realize I got a lot of workshop and seminar business because of my ability to explain exposure in great detail. The system I developed has meant a lot to me, giving me control over exposure and earning me a lot of dedicated fans. Still, I only use the system because it works so well. The only time I ever used matrix meters with an automatic mode like aperture priority was to test it or when photographing animals that were moving toward me when shooting a variable aperture zoom lens. There was simply no way to meter the chest of an approaching lion for example, add 2/3 stops of light, and shoot because the lion would move closer and I would have to zoom the lens to a shorter focal length gaining some light. Instead, I just guessed what the reflectance of the overall scene was and set perhaps +1/2 stop of light compensation on the camera and shot using the matrix meter and aperture priority. This worked fairly well.
For many years, I never really bracketed slide film either. However, when AEB (auto exposure bracketing) and shutter speeds and f-stops in 1/3 stop intervals became available on camera bodies, I began setting my Canon cameras that I used manually to automatically bracket the shot by 1/3 stop lighter and 1/3 stop darker. I did this because I might be off slightly when I guessed subject tonality, the film speed may actually be somewhat different than listed, or the processing could vary as well. Even shutter speeds and f-stops can vary a bit from what they are supposed to be. I call 1/3 stop exposure bracketing "tweaking the exposure". This system worked well for me for the past several years. Any slides that I thought were more than 1/3 off the exposure I was looking for were sent to the circular file except for some moody images of fog or sunrise/set images where I wanted an exposure different than how it looked in reality.
When I shot my digital Canon 10D for the first time, I set the camera on manual and exposed subjects just like I was shooting Velvia 50. I remember metering a yellow gray-headed coneflower and adding one stop of light. With the camera set on manual, I metered the bright yellow and the cameras meter balanced at 1/4 second at f/16 for example. I slowed the shutter speed down to ½ second to add one stop of light. Instead, I could have set the aperture to f/11 to get one more stop of light, but I wanted the depth of field that was available at f/16. I looked at my histogram and saw that I had a nice bell curve slightly to the right of the middle of the histogram. There was no clipping on either the left or right side of the histogram. And just to learn about histograms, I shot another frame where I metered the yellow and made no exposure compensation. Then I tried another where I metered the yellow and added two full stops of light. Checking the histogram in both cases showed I had moved the histogram to the left when I made no compensation and further to the right when I added two full stops of extra light. But, in both cases, I still wasn’t clipping either side. I was shooting raw digital files that offer the maximum control in Photoshop. Barbara showed me that by moving an exposure slider one way or the other, I could make the two images that I purposely improperly exposed look identical to the properly exposed image where I added one full stop of light. This demonstration was shocking to me! A Velvia slide that was over or underexposed by one stop was nearly always useless in my opinion. With digital, I could be a stop off and still make it a perfect exposure after the fact.
While I previously rarely used autoexposure because I required such exposure precision, perhaps it would be a possibility with digital. My last trip to Kenya in 2004 was a good time to test digital. Since I knew that virtually nothing in Kenya that I would be photographing was all black or all white, I though the exposure latitude of digital which is greater than slide film and the ability to adjust raw files to get the precise exposure I wanted in the computer might make a case for trying autoexposure. But, why try autoexposure anyway? I am very quick at metering on manual meaning I set the shutter speed and f-stop while monitoring the exposure meter in the camera, but it does take me a couple of seconds and is a distraction. If I didn’t have to turn the dials to set the exposure compensation I wanted, I would be slightly faster which isn’t critical when photographing a landscape, but most critical when wildlife action is the subject.
On the second day of my Kenya tour while in Samburu National Park, I set my Canon 10D digital camera to ISO 100, the meter to matrix or evaluative metering and exposure mode to aperture priority. I set the camera to f/5.6 depth of field to make certain my shutter speed was fast enough to be certain of getting sharp images. But, I didn’t overdo the shutter speed. When shooting lenses longer than 200mm, the minimum shutter speed I felt confident in shooting at is 1/125 second even when the camera is mounted on a tripod or cushioned with a bean bag. I actually prefer 1/200 second if I can get it. But, I don’t need faster shutter speeds such as 1/500 second and more so if I found the camera was setting such shutter speeds, I adjusted the aperture to f/8 to get more depth of field, knowing I still had plenty of shutter speed. Of course, I did want fast shutter speeds in the 1/500 to 1/1000 second range when photographing wildlife action like vultures flying to a lion killed wildebeest that was lying in front of me.
I found not metering to be a real strange way to photograph wildlife. Naturally, I was suspicious at first so I checked the histogram after each exposure to make sure no part of the scene was underexposed and especially not overexposed. The histogram graph makes this easy to determine. My camera even uses "blinkies" to show areas of the image that are overexposed meaning there is no detail in the pixels at that position. When I first used my digital camera, I was so used to shooting in low contrast light that I never saw any blinkies. When I learned my Canon 10D had this feature, I deliberately overexposed a scene by a few stops just to see the blinkies work. Blinkies are a great aid to making sure overexposure is avoided in the image.
Let’s look at histograms closer. You don’t have to be a math whiz to understand what a histogram tells you. It is a very simple bar graph that even I can understand. A simple histogram shows you a graphic representation of the gray scale values from 0 (black) to 255 (white). Black is on the far left, pure white on the far right, and shades of gray that gradually lighten to the right are in-between. There is no perfect shape to a histogram since it merely shows you the range and amount of tones (shades of gray) in the image. You could have a nice bell curve if the majority of your image was around middle tonality, but some whites and blacks also were present. On the other hand, you might get a spike that goes off the top of the histogram. A subject that would do this is a set of cottontail rabbit tracks in the snow. Since virtually everything in the image is close to white, a properly exposed image should produce a histogram where the vast majority of the values are near the right side of the histogram graph where the near white values are found. You could even get a histogram where you get a spike at both the dark and light portions of the histogram. Photographing a dark moose standing in a field of snow on a cloudy day will produce this result since nearly all of the tones in the scene are either very dark or very light with little else between these values. There is absolutely no problem with a spike of tones that go off the top of the histogram chart. It just means a lot of tones exist at those values.
Histograms do show you when you have problems though. Any histogram that produces a spike on the far left (dark side) or far right (white side) indicates potential exposure problems. While you may have some pure blacks in the photo that you want to be black, it’s usually not a good idea to have a large spike on the left side of the histogram because you may get digital noise which looks like grain in a print. However, digital cameras are quickly reducing the noise problem with each new generation of cameras. Above all, it’s important to avoid clipping the highlight or right side of the histogram because overexposed pixels have no detail. An excellent histogram typically comes close to the right side of the histogram without clipping any of the highlights. But, you must be careful because most histograms only show you an average of the red, green, and blue color channels. A histogram might not show a spike on the right hand side when it averages the three color channels, but a single color could actually be overexposed without this being shown. Some cameras such as the Fuji S2 and the top of the line Canon EOS bodies do show all three color channels at once. Many cameras are designed to make overexposed areas blink when you examine the image by looking at the LCD on the back of the camera. These overexposed areas that blink are typically called the "blinkies" by digital photographers. Looking for the blinkies is a fast way to see if part of the image is overexposed. If you do get blinkies, then you must reduce the exposure so they are no longer present. If you are photographing a landscape, it might also be possible to use graduated neutral density filters to reduce the contrast in the scene so the digital camera can handle the entire dynamic range of light.
However, it is possible to make an excellent exposure and still have a small spike on the right side of the histogram. If your image has specular highlights that might come from light shining off water or metal, then it could be fine since no detail would be in the specular highlights anyway.
It’s clear digital photography is rapidly changing the way most photographers do business. Due to the wider exposure latitude digital has over slide film and the control you have over exposure in the digital darkroom, getting an excellent exposure isn’t as difficult as it was with Fuji Velvia 50, the last film I shot. I welcome this change even if it does cost me some clients who no longer need my expertise in learning to expose slide film. There are many exacting details I have learned about exposure that I know I won’t need anymore. If I can forget a few things which is easy to do for me, perhaps there will be more room in my brain for the new information I must know such as monitor calibration, printing, and even how to use a digital projector.
Even though I may find that I no longer need all the details that I had learned over the years about exposing slide film, I am glad I have the background anyway. I think it is still worth knowing why we have to open up or add more light to the exposure when metering very light or white subjects like snow. Conversely, it is worth knowing why I had to subtract light when metering a black or very dark subject. When I have the time, I still plan to meter manually and compensate for reflectance. In fact, just yesterday I was photographing Barbara riding her horse among snow covered aspens. Since I had plenty of time and the light wasn’t changing, I metered the snow with my digital camera on manual, added two stops of light, and fired a shot. I checked the histogram to determine that the white values in the scene were close to the right side of the histogram, without clipping anything and no blinkies appeared on my LCD display. It was important to be close to the right side of the histogram without clipping anything because I wanted to get as much detail as possible in the dark red fur of the horse and the dark clothes Barbara was wearing. I used that setting to fill my CF card with 156 shots and all the exposures were excellent.
It’s clear that getting a great digital exposure is much easier for my students than it was with slide film. There is nothing like getting instant feedback to see that you got it right. I still think metering with the spotmeter in the manual mode and compensating is a viable option. But, the method I have taught since 1978 will be simplified greatly. And due to the latitude of digital capture, I won’t be speaking of 1/3 stops anymore since I don’t feel digital requires that kind of accuracy. Here’s a summary of how I would compensate for subject reflectance when metering with my digital cameras thru-the-lens spot meter in the manual mode.
Black or very dark subjects-subtract 1 ½ to 2 stops of light
Dark subjects-subtract 1 stop of light
Middle tone-no compensation
Pink, yellow, light blue, etc.-add 1 stop of light
White-add 2 stops of light
After applying these compensations and taking the image, always check the histogram to make sure nothing is clipped on the right side of the graph to avoid overexposure problems. But, don’t get carried away too. There really is no need to keep rechecking your histogram if you are shooting a series of similar shots where you are metering the same and the light isn’t changing. When photographing Barbara riding her horse in the snow, I metered the snow and compensated for the snow by adding two stops of light. Checking the histogram proved that everything was fine. I rechecked the histogram again after about every 40 shots just to be certain nothing had changed. It didn’t.
To sum everything up, the light sensors in digital cameras can record a larger dynamic range of light than slide film handles. With adjustments that are available in software, exposures that are fairly close to being right on can be made perfect, especially if no data is lost from being clipped at the dark end or especially the light end of the graph. This makes the possibility of effectively using automatic exposure for subjects that aren’t all dark or all light in reflectance quite possible. I found using aperture priority auto exposure with the matrix meter to be quite effective on my last Kenya safari where nearly everything was some shade of brown, tan, yellow, or gray. Virtually nothing was all dark or all light. On the other hand, photographing bison or elk in Yellowstone during the winter was another matter altogether. I found the best way to get a perfect exposure was to meter the white snow on manual, add 2 stops of light, and shoot a 3 frame bracket using AEB at +2, +1 1/2, and +2 ½ stops of light. My goal was to make the snow as white as possible without clipping the highlights (no blinkies) to keep detail in the snow and to get the most detail possible in the dark fur of the bison or elk. You want the snow tonal values at the far right side of the histogram to avoid losing detail on the dark side and getting a problem with noise in the shadows too.
When photographing closeups or landscapes or anything else that doesn’t move, I still prefer to meter manually with my thru-the-lens spotmeter, compensate for subject reflectance, and shoot a three frame exposure bracket using AEB which stand for autoexposure bracketing. By the way, AEB on my Canon cameras can still be used when shooting manually. All it does is automatically bracket the exposure I think is right by giving me a second exposure ½ stop darker and a third at ½ stop brighter. I have my cameras set for ½ stop intervals, although they could be set for 1 full stop or 1/3 stop intervals. I find 1 stop intervals to be too large for my needs and 1/3 stop intervals don’t make enough difference to bother with when shooting digital. When I shot slide film, I always used 1/3 stop intervals though.
The nice thing about digital cameras is you know you are getting an image and you can check the histogram to make sure the exposure is what you are seeking. If it isn’t, then in most cases you can adjust the exposure. In the manual metering mode, just change the shutter speed or f-number a bit to change the exposure. In an automatic mode, use the exposure compensation button to adjust the exposure. Properly exposing images with digital capture is easier than slide film and we all welcome this change. But, great images demand much more than excellent exposure. The digital revolution has made exposure easier so now we’ll all have more time to devote to the other key factors that make up a great photograph. These factors include developing excellent shooting habits to make sharp images, finding the most photogenic subjects, learning to be able to quickly see a unique situation, composition, camera viewpoint, and the effective use of light in terms of color, contrast, and direction. That’s a welcome change that digital has made possible. We’ll all have more time to spend on the creative side of image making.
