Exposure Controls Density, Development Controls Contrast
How a film-era axiom still applies to help you create perfect digital pictures
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When I was in high school, my photography teacher was quite a character. Mr. Colgan challenged students to one-armed push-up contests, and promised that if you were to bring him a box of Cuban cigars and a bottle of Napoleon Brandy, he'd give you an A for the semester. As far as I know, nobody ever took him up on either count.
Aside from being lots of fun, Mr. Colgan was also a great photography instructor. One of his favorite sayings, repeated over and over on a daily basis, was this: "Exposure controls density, development controls contrast." As we were still fully ensconced in the film era, this saying was all about the fundamentals of creating a good negative. Expose correctly to add the right amount of density to a negative (ensuring it wasn't "thin" from underexposure or too dense from overexposure) and then change where and how those tones play out on the negative based on development. Increase contrast with higher temperatures and greater agitation, or mellow things out with less development at lower temps and without extra agitation. The point of all this is that even now, 20 years later, this darn saying is still stuck in my head. But I haven't processed a roll of film in nearly a decade, so how can I turn this useful photographic knowledge from the film world into something that works with digital capture? Simple—it already does.
Exposure Controls Density
While this knowledge is most helpful if you're shooting RAW files, it also works with JPEG image capture. When capturing digital files, overexposure will cause you to blow out your highlights. Do it too much and you'll never get them back. But using the "expose to the right" axiom and you'll create the pixel density—more photons captured in more pixels on the sensor—that can be useful for increased detail and lower noise. Wrangle those pixels precisely in place during processing (or what we now call post-processing) and you'll have the right amount of "density" in your pixels, based entirely on the exposure you made in the camera. I find it helpful to think about density simply as "image-forming information," and to think of a pixel as a bucket filled with light instead of water. If you overfill your sensor's pixels, they will blow out and eliminate all detail—and you'll never get it back. But if you fill them just shy of overflowing, you'll have a lot more image-forming information in each pixel than just the "few drops" of light that are collected in a darker toned pixel/bucket. So to maximize image-forming information, avoid underexposure at all costs and go with the brightest exposure you can without blowing out highlights. Your histogram offers the ideal way to check your progress, as does the clipping preview for shadows and highlights. In programs like Lightroom, any areas where you have obliterated image-forming detail—be they in shadows or highlights—can be instantly viewed in bright colors. This is a great visualization of how exposure controls density.
Development Controls Contrast
In the digital workflow film development has been replaced by post-processing in the computer. How you move pixels around determines not only the ultimate tonal values, but the contrast range evident in the finished image. Thinking about this one backwards may help it make perfect sense: How would you adjust contrast in the exposure while you're taking a picture? Sure, you can (and should) use lights and fill cards and reflectors to control contrast and get it where you want it to be readable in the exposure, but beyond that, once we're talking about the contrast of the finished print (or in this case, digital image file) you've still got a ton of flexibility. With digital images, unlike film, that contrast can be modified at almost any step of the way after capture. Another way to think about it: where is the 'contrast' button on your camera? There's nothing that allows you to make a RAW file look more contrasty or a little flatter at the time you release the shutter, so to control contrast you need to adjust your development. It just so happens that development now occurs in the computer.
So the next time you find yourself overwhelmed by all the different factors that go into creating a perfect digital image file, boil it down to these two basic facts: exposure controls density, development controls contrast. Tackle them one at a time and you're well on your way to making better pictures.