MarkBarbieri
04-06-2007, 10:11 AM
Want to know the trick to getting the best possible exposure when taking a picture? The histogram is the key.
Your camera probably has lots of different exposure modes - spot, partial, center weighted, evaluative, matrix, etc. For this lesson, don't worry about which mode you use because it doesn't matter. The reason is that we are going to use the histogram as our meter.
Set up your composition and take a picture. Now check the histogram. If you have a line on the far right-hand side of the histogram, your exposure is too high and you need to reduce your exposure. You can do that by shooting in manual and just lowering the shutter speed or aperture, lowering the ISO, or using exposure compensation.
If you look at the histogram and you have room between the rightmost part of the graph and the rightmost edge of the histogram, you want to increase your exposure. The trick is to get the exposure as far to the right as possible without hitting the edge.
Hitting the edge is bad because those pixels are "blown out." They are just plain white. Sometimes you can't avoid it. If the sun is in your picture, it's going to blow out. Specular reflects also tend to blow out. If you can't or won't change your composition or lighting to make them go away, decide what parts you can ignore and let them blow out. Most of the time, however, you should strive to keep everything off of the right edge.
Why try to get close? Your camera records better information on the more exposed (but not over-exposed) pixels than it does on the darker pixels. This is primarily because the signal (the stuff you want) to noise (the grainy, extraneous stuff you don't want) is higher for the more exposed pixels.
Won't that mean that my picture will be too bright? Maybe right out of the camera it will, but you can adjust it back down using your photo editor. When you do so, the picture will be better than it would have been if you'd shot it dimmer in the first place.
What about the left edge? Ideally, you don't want pixels piled up against the left edge either. These represent parts of the picture that are just plain black with no detail. However, these don't hurt the look of a picture as bad as blown out parts do, so if you can't get everything to fit your histogram, err on the side of leaving dark parts rather than bright parts.
When exposing for the right edge, make sure that you use the RGB histogram and not the average luminance histogram. The former shows you a histogram for each of the three sensor colors. The latter averages them together and shows just the average. If you take a picture of a bright red flower, you can easily blow out the red pixels while the average luminance still looks OK. You don't end up with white pixels there, but you do lose detail in the red colors, which is almost as bad.
Some older digital SLRs have only an average luminance histogram, so you don't have a choice. With those, you have to guess or bracket.
Your camera probably has lots of different exposure modes - spot, partial, center weighted, evaluative, matrix, etc. For this lesson, don't worry about which mode you use because it doesn't matter. The reason is that we are going to use the histogram as our meter.
Set up your composition and take a picture. Now check the histogram. If you have a line on the far right-hand side of the histogram, your exposure is too high and you need to reduce your exposure. You can do that by shooting in manual and just lowering the shutter speed or aperture, lowering the ISO, or using exposure compensation.
If you look at the histogram and you have room between the rightmost part of the graph and the rightmost edge of the histogram, you want to increase your exposure. The trick is to get the exposure as far to the right as possible without hitting the edge.
Hitting the edge is bad because those pixels are "blown out." They are just plain white. Sometimes you can't avoid it. If the sun is in your picture, it's going to blow out. Specular reflects also tend to blow out. If you can't or won't change your composition or lighting to make them go away, decide what parts you can ignore and let them blow out. Most of the time, however, you should strive to keep everything off of the right edge.
Why try to get close? Your camera records better information on the more exposed (but not over-exposed) pixels than it does on the darker pixels. This is primarily because the signal (the stuff you want) to noise (the grainy, extraneous stuff you don't want) is higher for the more exposed pixels.
Won't that mean that my picture will be too bright? Maybe right out of the camera it will, but you can adjust it back down using your photo editor. When you do so, the picture will be better than it would have been if you'd shot it dimmer in the first place.
What about the left edge? Ideally, you don't want pixels piled up against the left edge either. These represent parts of the picture that are just plain black with no detail. However, these don't hurt the look of a picture as bad as blown out parts do, so if you can't get everything to fit your histogram, err on the side of leaving dark parts rather than bright parts.
When exposing for the right edge, make sure that you use the RGB histogram and not the average luminance histogram. The former shows you a histogram for each of the three sensor colors. The latter averages them together and shows just the average. If you take a picture of a bright red flower, you can easily blow out the red pixels while the average luminance still looks OK. You don't end up with white pixels there, but you do lose detail in the red colors, which is almost as bad.
Some older digital SLRs have only an average luminance histogram, so you don't have a choice. With those, you have to guess or bracket.