My images can be found at Dennis A. Mook | Photographer. Please take a look.
In the film world there were two different ways to optimize your exposures, depending upon whether you were using negative film or transparency film (slides). If you were using negative film, there was a much greater dynamic range and one tended to overexposure slightly to gain more shadow detail, reduce grain and the film would hold the highlight detail. The key was denser exposures in the shadow areas.
In transparency film, which had much less dynamic range, one had to decide, in higher contrast situations, whether or not to lose the highlight detail or shadow detail, as most often the film could not successfully record both. Typically, one chose to keep the highlight detail (your eyes go to the brightest parts of an image long before the darkest parts) and let the shadows go black.
Today, that rarely is an issue with modern digital cameras as the newer ones can record about 12 stops of dynamic range, more than twice that of slide film of yesteryear. That means that you don't have to bias your exposure to keep highlights or shadows. (If the scene does exceed your sensor's capabilities, we now bracket our exposures and combine them into HDR images).
The question for you is this: Is placing your exposure in the middle of the histogram's range the best place to put it to optimize your image quality? Not necessarily. It has to do with the linear capture of photos and the electronics of the digital world. Take a look at this graphic and see if you understand why it is best to place your exposure as far to the right of your histogram as you can WITHOUT your highlight areas being cut off on the histogram's right side.
Here are some things to consider:
1) Most good, non point-and-shoot digital cameras today can capture 12 stops of light dynamic range.
2) Those same cameras normally can make images in 14-bit, which is 65,536 bits of information.
3) Digital sensors record light in a linear fashion, i.e., each F/stop records either exactly twice or one-half of the aperture next to it.
4) Histograms record the light captured on a scale from 1-255. Below 1 is totally black and above 255 is totally white, with no detail in either area.
5) The brightest information will fully fill the "bucket" of light in your sensor. Anything more will spill over and go to the right of your histogram and be totally white with no detail.
6) The darkest information will put no light in the "bucket" and be totally black. In the other areas, the "buckets of blue light" (graph below) are either twice or half as much light.
Take a look at the graphs below and, hopefully I'm clear, and you can understand why it is important to expose to the right (ETTR).
If you can interpret the above graphics, it shows you several important things about how the image data is captured by your digital camera's sensor.
a) Fully 1/2 of the available light (data) is captured in the brightest tones of your image.
b) Another 1/4 of the available light (data) is captured one stop down from the brightest image tones.
c) So 3/4 of the available light (data) is captured in the top two F/stops of your image.
d) By the time you get below the middle tones, there is almost no data to make a high quality image.
e) By keeping your histogram as far to the right as possible, you capture the most image data.
f) Because there is almost no data in the shadows (left side) if you have to lighten those shadows in
in order to see detail, you lighten all the digital noise captured.
g) But if you make an image that is too light (keeping your histogram to the right) and then have
to darken it somewhat to make the tones look right, you then darken the too light shadows and
the shadow noise is lessened.
One caveat: You have to expose your image to the right, but not so far to the right as to cut off the
histogram which would result in losing highlight detail (above 255).
So, to achieve the absolute best image quality, it pays to carefully make your exposures so the histogram is as far to the right as possible without cutting off highlight detail. Then move your exposure slider in your editing software to the left to darken the entire image. The result is more data captured for you edit and less noise in the image.
Now, all that being said, does it really make a difference? If you listen to some people they will tell you in all practical purposes it will not. I say it will in a few cases. So, instead of practicing it sometime and not other times, when it may not be important, I practice it all the time so I practice being consistent.
Thanks for looking. Enjoy!
Dennis Mook
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