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 p.2 #7 · Spot metering question, please clarify | |
The first thing to understand about exposure is that it is always set for the light source. Only a change in light should change your exposure. The color or tone of the objects being photographed do not have any role in setting exposure. This is why an Incident Light Meter works. An incident meter measures the light directly and gives you a shutter/aperture/ISO combination that works.
That said, it is a fact of life that our cameras have Reflective Light Meters, and are forced to make an exposure determination based on the light reflected from the objects in the scene. Now...something that is white can reflect 90% of the light that falls on it, while something that is black may reflect only 5% of the light that falls on it. This is why setting exposure using the camera meter isn't straight-forward. However, once you understand what's going on you'll see that it's not difficult...it just takes practice.
Here's how the camera meter works...the meter first makes a presumption that the world is gray. And not just any gray, but 12.7% gray (meaning, a surface that reflects 12.7% of the light that falls upon it.) If you want to see what 12.7% looks like then open a painting program on your computer, set your painting color to RGB 100, 100, 100, and fill the canvas with that color. That's the gray that the camera presumes the world looks like.
Having made this presumption, the camera will now set exposure so that the scene (or more accurately, the metered area) comes out gray. This happens regardless of the actual color of the objects in the scene. If you take a picture of a white wall, it comes out gray. If you take a picture of a black wall, it comes out gray. While this behavior may not make the reflective meter the most accurate meter, it at least makes it predictable, and that's what really matters. If we can predict the error in the meter reading, then we can compensate for that error, and achieve correct exposure. It's the knowledge of compensation that you must learn and practice.
If you meter an object that is a perfect 12.7% gray, then your meter reading has no error. This is what metering the sky is all about. It just so happens that the sky, about 45 degree up, has the same luminance as 12.7% gray (luminance refers to reflected light.) Your typical healthy green grass also has a luminance of about 12.7% gray, and can also be used to set exposure without error.
Other references can be used to set exposure. An 18% gray card can be used. Why 18%? Because there are no 12.7% cards. Why aren't there any 12.7% cards? I don't know. Anyways...an 18% card will cause you to underexpose slightly, and the instructions that comes with Kodak 18% gray cards say to increase your exposure by 1/2 stop. So when using an 18% gray card, your compensation is +1/2 stop (or +2/3 stop.) This is applied to the camera using your Exposure Compensation function.
If you're photographing a light-skinned person, then you can spot meter the skin, and set EC to +1. That will give you correct exposure. If you're photographing evergreen trees then you can meter the trees and set EC to -1. For snow, you can set EC to +2.3. For light sand at a beach, +2.
If you practice this skill, what will happen is that you'll develop a table of compensations based on your preferred subject matter. For example, for his landscape photography, photographer Jim Doty Jr. uses +1.5 for birch bark, +1 for yellow Aspen leaves, and -1.5 for a buffalo mane. So if you like taking pictures of red fire trucks then you will eventually develop a compensation value that lets you meter a fire truck and get correct exposure.
Now the metering mode can be addressed. When you're ready to take control of exposure, then Spot metering is the mode to use. Spot metering allows you to meter a small area, excluding the influence of the surrounding tones. So you can meter just a face, or just an evergreen, and compensate your exposure reliably. Center Weighted metering is an old metering mode, used in the Nikon F1 which was introduced in 1968. Its metering area is too large to be used effectively. Matrix metering is the "full-auto" of metering modes. Matrix meters a scene and then evaluates it to determine its own exposure compensation. It works well most of the time, but when it's wrong it's very unpredictable. If you don't really want to be bothered with compensating exposure to get it right, then Matrix is the mode to use.
Finally, Manual, aperture, shutter, program...these are shooting modes, and have nothing to do with setting exposure. You simply use the mode that is appropriate for you needs. Exposure is determined the same way, regardless of what shooting mode you use.
I carry around a white-balance reference that I use to set a custom white balance and to set exposure. It's not 12.7 or 18 percent gray, but that doesn't matter. As has been done for evergreens and aspen leaves, I simply determined that a compensation of +1.3 gives me correct exposure. So I'm able to use the one reference for both white balance and exposure. So anytime I step into new light I will whip out my WB reference from my back pocket, start the WB process, hold the card in front of my lens, angled to catch the light, and press the shutter to set a custom WB. If I'm in constant light conditions I'll also press AE-L and lock exposure. It takes about 7 seconds to set both. The card goes back into my back pocket and I'm all set.
Now...with all that said, please note that digital photography has brought with it alternative methods of setting exposure. There's Expose To The Right (ETTR); there's chimping the histogram (a method I refer to as exposure via trial & error); there's also the ISO-Less school of thought...all methods enabled by digital. So while the information above applies equally well to film cameras, a person using a digital camera might want to explore other methods. I personally use the film ways of setting exposure.
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