In my photography I use Aperture Priority metering most of the time. I take a shot, look at the histogram, and use Exposure Compensation (EC) to add or subtract light as needed. Yes, this works great most of the time…but most of the time does not mean all of the time. There are shooting situations where it is best to not use an autoexposure mode (aperture priority mode, shutter priority mode, or program mode). This means selecting both the shutter speed and the lens aperture; that is, using manual exposure mode. In this mode, you have to physically set both the shutter speed and the aperture, and the camera remains at those settings — and exposes at those settings — no matter where you aim the camera. What you set is what you get, period. EC does nothing to change the camera’s settings. It affects the meter readout, but it does not in any way change the shutter speed or aperture on the camera, so turn EC to zero. In manual exposure mode, what you set is what you get. If you want to lighten or darken the image, you have to physically change the shutter speed or the aperture or the ISO. The camera itself does not change anything. What you set is what you get.
This blog post comes about because of a Joseph Van Os Photo Safari I recently led. We were standing on the top of a cliff overlooking the ocean, preparing to photograph birds flying past. If they birds were above us, the background was a pale milky blue sky. If below us, the background was a dark navy blue ocean. So what’s the right exposure? Any autoexposure mode would be biased by which background was in the frame. One solution would have been to work only those birds against the pale sky, or only those against the dark ocean. But the birds were not cooperating in that manner. They would swoop around, at one moment be above us and the next moment be below us. When I mentioned that this situation called for a manually set exposure, to compensate for the different background tonalities, the response I got reminded me once again how many photographers have never used a camera in the manual exposure mode.
If the light remains the same, once you correctly set a manual exposure, it is correct for all subjects in that light regardless of their tonality. OK, but how to set that exposure? Pick something, it doesn’t matter what, in that constant light, and adjust for it’s tonality. When you set the camera to manual exposure, an analog display will appear in the viewfinder with a “zero” point in the middle of a “plus” and “minus” line. “Zero” is the starting point. Change the shutter speed and/or aperture until this mark in highlighted, and you have set the camera to render whatever you aimed it at to be rendered as a medium tone. Go to a “plus” mark, and you have added light. Go to a “minus” mark and you have taken away light. So, aim the camera at a single tone, and meter it and only it. Physically change the shutter speed and/or aperture to render that area at whatever tone you want it to appear. Here’s a quick and dirty way to think about this. At the “zero” mark the camera will render that subject you metered as a medium tone. At the “plus one” mark, one stop open, the metered subject will be rendered as a “light” tonality. At the “plus two” mark, two stops open, it will be rendered as a “very light” tonality. At the “minus one” mark, one stop down, it would be “dark” subject; at “minus two” it would be an “very dark” subject.
Suppose you meter a blue area. Here’s what would happen as you change the shutter speed/aperture combination:
+3, whitish blue
+2, very light blue
+1, light blue
0, medium blue
-1, dark blue
-2, very dark blue
-3, blackish blue
Back to those birds…. The solution would be to pick an easy area to meter: either the pale sky, or the dark ocean water. Let’s use the pale sky; it was roughly “very light blue.” Set the camera to manual exposure mode, aim it at the area of sky where the birds would be, and adjust shutter speed/aperture until the “plus two” mark was highlighted. It doesn’t matter what combination shutter speed/aperture you use to get started. Pick the equivalent combination that gives the shutter speed needed or the aperture needed. Suppose you aimed your camera at that sky and the exposure combination that yielded “plus two” was 1/125 sec. at f/11. Well, you know that 1/125 sec. is way to slow a shutter speed to freeze a bird in flight. 1/125 sec. at f/11 is exactly the same as 1/250 sec. f/8, or 1/500 sec. at f/5.6, or 1/1000 sec. at f/4, etc. Need a faster shutter speed? Raise the ISO one stop, and you could shoot at 1/2000 sec. at f/4 (or any equivalent combination). Set the shutter speed/aperture combination you want to use, and fire away. So long as those birds remain in the same light, and the light itself does not change, the exposure will be correct no matter the background.
I can think of a number of situations where using manual exposure would be best. Consider working a black sand beach, with waves breaking white over it. The camera meter would read a black subject at one moment, a white subject at another. Or imagine you’re using a zoom lens to photograph a dark buffalo standing in the snow in Yellowstone. As you zoom the lens, the image changes from primarily snow to primarily buffalo. Or a situation I faced back in February while photographing red-crowned cranes. The white cranes were standing in a snow-covered field, but as they took flight the background became leafless winter trees, a dark toned area, and then to medium blue sky as the birds rose higher. In all three situations, so long as the light remained unchanging, the solution was to meter one area, set the exposure using manual mode, and shoot away.
If you’re not familiar with the manual exposure mode, I would strongly suggest some practice. But let me add one final statement: with all digital cameras, you cannot evaluate exposure or color via the LCD on the back of the camera. The histogram is your friend, whether shooting manual mode or one of the autoexposure modes. Learn how to use manual exposure mode, and learn how to read the histograms.