Digital Photography 101, Part IV

Setting the built-in exposure meter and exposure compensation.


In this article in the Digital Photography 101 series, I discuss setting the built-in exposure meter and exposure compensation. This is the fourth article in a series of five that deals with setting up the more important buttons and dials on a digital camera. As usual, the article goes a bit beyond camera setup to a discussion of the basics of using a digital camera. If you haven't read the first three installments of this article you might want to do so before beginning this article. Part I, Part II, Part III.

Exposure meters: Most digital cameras have two or three different ways of metering exposure, which means that you can pick the best metering mode for what you are doing at the time. Most digital cameras have provisions for spot metering, center weighted metering and evaluative metering.

Spot metering means the built in meter is reading light from just a very small spot in the image. When the camera is set to spot metering, a small circle will appear in the middle of the view finder and this is the spot that is being metered. I very rarely use this kind of metering but it can be useful if you need to make sure one tiny area has the correct exposure, such as person's face. It can also be helpful in making sure you have detail in both dark foregrounds or bright clouds. You can also use it to figure out the overall contrast of a picture--how bright the brightest spot is and how dark the darkest spot is. Since cameras only see five stops of contrast at most, it is possible to see if the picture has too much contrast to register detail in both the dark end and the bright end of the picture. In the old days of film, I used to be a believer in hand-held spot meters, nowadays I rarely bother with metering like this. There are other metering modes in modern digital cameras that will give much better results with far less work.

Another way to set the camera's exposure meter is to use the center weighted metering option. This will set the exposure for the picture by giving most emphasis to the brightness of the central area of the picture. This usually works alright, but again I rarely use it as there is a better option.

Most cameras have a metering mode called something like evaluative or multi-zone. In this method of exposure calculation the camera looks at many different zones in the image and figures out the best possible exposure for this particular scene. Some cameras use up to 60 or 70 separate zones when calculating the exposure. I leave my digital cameras set to this option all the time. In most circumstances it will work very, very well and you really don't even need to think about exposure; just let the camera do it all. For almost all exposure situations, modern cameras set to evaluative mode will do as good or better job than a professional photographer using a spot meter.

Exposure Compensation: There are a few times when the exposure meter will be fooled and it is necessary to override it to get the proper exposure. If most of the image is very, very bright like a beach scene or a snow scene, the exposure meter will try to make the whole picture an average mid tone and the snow or the white beach sand will turn out a muddy gray rather than the white snow or sand you saw with your eyes. In this situation the best thing to do is to override the automatic exposure meter by using the camera's exposure compensation button. What this button or dial does is increase or decrease the exposure in 1/3 stop increments. In a very bright snow scene I usually increase the exposure one to two stops or even three stops (in situations where there is nothing but snow in the picture) over what the meter thinks is correct. This will read as +1 or +2 or +3 on the exposure compensation dial and will make the snow a bright white and also make sure the mid tones and shadow values are not too dark.

For example, I once photographed a dark brown moose in a bright snowy meadow without exposure compensation and a whole series of shots was ruined. In the finished picture, the snow was a murky dark gray and the moose was pitch black with no detail at all, like a black cardboard cutout. A +2 exposure would have made the snow white and the moose the correct dark brown with plenty of detail. Unfortunately, since I was using a film camera with no LCD to check, by the time I saw the problem it was far too late to fix it.

The other situation when the built-in exposure meter may be fooled is where the overall picture is quite dark; the classical situation is the black cat in a coal bin, or in more practical terms, a landscape shot late in the day when it is almost dark. In these conditions, the exposure meter again tries to make the picture an average mid tone brightness and as a result radically overexposes the entire picture. To fix this problem, use the exposure compensation dial to adjust the exposure by a -1 or -2.

An exposure of -1,-2 or -3 is also the correct way to shoot night shots, say of a dark highway with red tail lights or a dark city scene with neon lights or house and buildings with tungston light. You want the scene to look like night with the lights glowing out of blackness. Check the LCD screen to see exactly how much exposure reduction you need.

As a matter of fact whenever you are in a difficult exposure situation and need to use the exposure compensation button, you should look at the LCD display on the camera and see if what you are doing makes sense in the real world or not; the LCD display will generally give you a pretty good idea of how good the exposure is.

Using exposure compensation when it is needed is very important. Two incidents come immediately to mind where I failed to use exposure compensation when I should have, and as a result ruined an entire day's shooting. One of the first times I did a lot of winter shooting I was in Jackson Hole in Wyoming where the Tetons are located. One morning I went out to shoot a scene where a beautiful creek wanders through a meadow with mountain cottonwoods and several wonderful old red barns. Before long it started snowing big, beautiful, soft flakes of snow that added a fresh layer to the six feet of snow that was already on the ground. Trees, overhanging limbs, fences and barns were all soon covered with the shimmering new snow and I started shooting like crazy, relying just on what the meter told me. I know I got a number of what should have been great shots, particularly several shots of a red barn framed by snow covered trees and rustic pole fences receding into a background of soft, misty Tetons. But I totally forgot to use exposure compensation, mostly because I was using film with no convenient LCD screen to look at.

Needless to say, when I got the film developed I had a lot of ugly, mid-tone greyish-blue snow instead of sparkling white snow and shadows that were so dark (blocked is the technical term) that all the detail was totally gone. If I had been shooting digitally using RAW mode I could have losslessly increased the exposure by one and maybe even two stops and maybe saved the pictures, but in those days of film and non-digital printing there wasn't a thing I could do and the whole day's shooting was a total loss. And of course I didn't discover all of this until two weeks later when I was home with no chance of re-doing the shoot. It is memories like these that make me really appreciate digital photography.

I made my big exposure mistake in the opposite direction when I first began shooting digital. I was in New England shooting autumn foliage around a beautiful pond in New Hampshire's White Mountains. By late evening I was all ready to call it a day and head for home when I noticed a back bay of the pond that had some really wonderful, bright red maples mixed with dark green spruce trees that were really stunning. By now it was getting very dark, but I knew that the Canon 1Ds Mark II camera I was using was capable of shooting in very dim light, so I went ahead and shot for another thirty minutes in this last dusk light. It was almost dark when I quit. I noticed in the LCD screen that the pictures were looking pretty bright and gaudy but I figured I could edit them later and they would be OK.

Wrong. The built in meter brightened up the scene to mid-day brightness and totally destroyed the soft, warm evening colors I was seeing with my eyes. The shots were so harsh and overexposed that even though I was shooting RAW I couldn't salvage them. I was able to go back the next day and reshoot the scene but somehow the light wasn't the same, not nearly as nice as it had been the evening before.

There was a double lesson in this experience. Not only did I get a a good lesson in the necessity of using exposure compensation but, once again, I learned that if you don't capture a really great scene at the instant you see it, the opportunity will never present itself again. Great shots last just a few moments in time and can never again be exactly duplicated; the light, the weather, the wind, the clouds and the scene itself always subtly change even over a few minutes, never mind a whole day, and are never again the same. Once a great shot is missed, it is gone forever.

I can't tell you how many times I have left a great scene without shooting it for some reason or another, expecting to shoot it later and then returning to find the shot totally gone, even though I was standing in exactly the same place, in what I thought was similar light. As a matter of fact, this happened to me just today. I was in the woods in Maine on a family outing with my three year old granddaughter and saw a wonderful shot of a small pond covered with swirls of bright red and yellow leaves. The scene was framed by russet and gold autumn foliage with bright white birch trunks and brilliant autumn colors reflecting in the still, black water and the whole scene gradually dissolving into the fog. And I had no camera with me.

I'll go back tomorrow and look for this scene again, but I know this shot is gone forever; it will never fall together exactly the same way again. It's one of those shots that I will file away in my find and keep looking for over and over, and maybe if I'm lucky I'll find something similar some day. After you learn the basics of photography, ninety-five five percent of great shots is being in the right place at the right time and, though I hate to admit it, this is mostly a matter of dumb luck. When you see a great shot, try to shoot it right now or it will be gone forever.

An important point when using exposure compensation is remembering to turn it off when you are done (i.e. set the dial back to zero). I can't tell you how many times I have forgotten to do this and ended up over or underexposing big batches of pictures. The way this usually happens is that I am shooting at the end of the day in the dim light of evening and I use exposure compensation to darken the picture by a stop or two. Finally I finish and I am tired and ready to quit for the day, so I forget that I set the exposure compensation dial at -2. And the next morning I am excited by the dawn I am shooting so I forget again and ruin the whole dawn shoot by underexposing it. Not good. Get in the habit of resetting exposure compensation to zero the instant you finish using it.

Also, be very careful about using minus exposure compensation. For almost all normal daytime scenes, digital exposures need to be as bright as possible without blowing out the highlights. This means you need to make the bright parts of the picture as bright as possible without washing out the detail completely. In practice, this means being sure that white clouds or the bright colors of a sunset still have detail and are not blank white. The reason for this is that every time you stop the camera down one stop the sensor is receiving only half as much light and since the dark shadows have very little light to begin with, if you stop down once or twice or three times from the exposure the camera recommends, there will be very little of anything left in the dark shadows. At this point random electric signals overwhelm the picture you are tying to take and the result will be lots noise in the dark areas and poor image quality in general.

Most digital cameras have this knowledge built into them and will automatically set exposure as bright as possibly without blowing out the highlights. It is easy to get into trouble here. The problem occurs when you are using a camera that has a digital viewfinder that shows you the digitally corrected scene. Since the camera is making the scene as bright as possible without blowing out the highlights, the scene may look too bright in the viewfinder and the temptation is to stop down, i.e. choose a darker exposure, because that one looks better in the viewfinder. Don't do this, the camera almost always knows what it is doing in a normal daylight situation. You can always make the scene darker when you edit it the computer later. However, if you underexpose the picture when you take it, you may not be able to lighten it to the proper exposure in Photoshop without also adding a lot of noise.

Prosumer cameras, the type of camera half way between the inexpensive comsumer cameras and professional cameras, often have a digitally active viewfinder like the one described above. I don't like this kind of viewfinder; they make it hard to see the details of the scene and tend to make exposure confusing. SLR type digital cameras, the ones with interchangeable lenses, have viewfinders that are optical, not digital; they give you a straight view of the scene like your eye sees, not a digitally altered one which is often not even accurate. I far prefer non-digital viewfinders, because I get a sharp, clear view of what I am shooting. If I want to check the final picture for exposure and composition, I use the LCD screen.

An additional advantage of more professional digital camera is that they have an overexposure warning built into the LCD screen: blown out (over exposed) areas of the picture (usually bright skies) will blink in the LCD display. Checking the overexposure warning is a the best way of knowing for sure that you need to use exposure compensation to stop down a little bit if you want to avoid blank white skies that cannot be fixed later. When you do this, keep in mind that you are probably also blocking up the shadows, i.e. eliminating all the detail in the shadows. If you want to know how to deal with this situation where the scene has too much contrast for the camera to handle and still get properly exposed highlights and properly exposed shadows for a perfect overall exposure, read the article on difficult exposure problems in my next newsletter which will be out very soon.

This is the fourth part of a five part series on how to set up a digital camera. If you have comments or questions about any of this, give me an email.

Go to part five of the Digital Photography 101 series