This page contains tips about photography.
How the camera calculates exposure
Sunday, May 20, 2007
Automatic Exposure
This is something that you can try out for yourself. It doesn't matter what kind of camera you have - film or digital, point & shoot or SLR... Although these kind of experiments are easier to do with digital (faster and less expensive) than with film.
Anyway, key is that you leave the exposure to the camera. There are several ways to achieve this, setting the camera to "P" or "Auto" mode is probably the easiest one. Actually, any setting manual will work (in Aperture priority mode or Shutter speed priority mode, the camera still makes the exposure).
So, find some black material, and some white material, as shown in the picture here. In this case, the black material is the back page of a binder (with, I might say, pretty black paper at the back. It is one of those fast-clip binders with a transparent front where you can shove about 30 pages in, that are held by a single clip in the middle of the gutter). The white material is the front of a two-ring binder. Nice white shiny material, can't get it any brighter than that, unless it starts emitting light by itself of course.
Anyway, get your camera close. Really close. Don't worry if you're too close to focus, because focussing is not the issue here; it's the exposure. Now, take an image of both the black and the white surface. Again, from real close, so there's nothing else visible on the picture.
Can you tell what is what? These pictures are straight from the camera. Contrast and brightness have not been changed, although white balance has been adjusted to correct for any false color casts.
The camera has no knowledge of what "the outside world" looks like. Therefore, whenever a picture is made, it has to make some assumptions. The basic assumption that (virtually) every camera makes is that on average, a picture is 18% gray (0% would be totally black, 100% would be totally white), or, in Ansel Adams speak, "Zone V".
Therefore, with nothing else in the picture, the black and the white surface will come out exactly the same. For the same reason, pictures of snow landscapes will come out grayish as well. Don't think that it happens only with digital cameras, because those film pictures that you shot during your skiing vacation came out great? That's because the developing & printing lab is correcting for those kind of exposures. The solution, of course, is to "overexpose" pictures that are predominantly white (or light), and "underexpose" pictures that are predominantly black (or dark). The terms underexpose and overexpose are quoted because we're not really under- or overexposing, only compared to what the camera thinks is the right exposure.
Another solution is to lock the exposure (using the AEL button, if your camera has one), by aiming the center point of the viewfinder to an "18% grey" object. When in doubt, use some light colored skin, bare concrete or grass as "18% reference". And don't forget to chimp (but use the histogram!) - don't feel bad about chimping, pro's do it too!
Anyway, key is that you leave the exposure to the camera. There are several ways to achieve this, setting the camera to "P" or "Auto" mode is probably the easiest one. Actually, any setting manual will work (in Aperture priority mode or Shutter speed priority mode, the camera still makes the exposure).
So, find some black material, and some white material, as shown in the picture here. In this case, the black material is the back page of a binder (with, I might say, pretty black paper at the back. It is one of those fast-clip binders with a transparent front where you can shove about 30 pages in, that are held by a single clip in the middle of the gutter). The white material is the front of a two-ring binder. Nice white shiny material, can't get it any brighter than that, unless it starts emitting light by itself of course.
Anyway, get your camera close. Really close. Don't worry if you're too close to focus, because focussing is not the issue here; it's the exposure. Now, take an image of both the black and the white surface. Again, from real close, so there's nothing else visible on the picture.
Can you tell what is what? These pictures are straight from the camera. Contrast and brightness have not been changed, although white balance has been adjusted to correct for any false color casts.
The camera has no knowledge of what "the outside world" looks like. Therefore, whenever a picture is made, it has to make some assumptions. The basic assumption that (virtually) every camera makes is that on average, a picture is 18% gray (0% would be totally black, 100% would be totally white), or, in Ansel Adams speak, "Zone V".
Therefore, with nothing else in the picture, the black and the white surface will come out exactly the same. For the same reason, pictures of snow landscapes will come out grayish as well. Don't think that it happens only with digital cameras, because those film pictures that you shot during your skiing vacation came out great? That's because the developing & printing lab is correcting for those kind of exposures. The solution, of course, is to "overexpose" pictures that are predominantly white (or light), and "underexpose" pictures that are predominantly black (or dark). The terms underexpose and overexpose are quoted because we're not really under- or overexposing, only compared to what the camera thinks is the right exposure.
Another solution is to lock the exposure (using the AEL button, if your camera has one), by aiming the center point of the viewfinder to an "18% grey" object. When in doubt, use some light colored skin, bare concrete or grass as "18% reference". And don't forget to chimp (but use the histogram!) - don't feel bad about chimping, pro's do it too!
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