This is not an original concept, but one espoused by Michael H. Reichmann of The Luminous Landscape.  I hinted at it in yesterday’s post, and I think it deserves a better explanation.

Exposing for Digital

First off, the rule of thumb with digital photography is “expose for the highlights, develop for the shadows.” This is because light reactivity on a digital sensor is fairly linear – that is, each nanosecond X amount of light is received on the sensor. Now, the more light that is received, the “whiter” the detail gets.  Eventually it can’t get any lighter, and that pixel is flat-white.  At this point, we call it “blown out.”

So, with digital you shoot to avoid blown out pixels, which leaves shadows slightly underdeveloped.  Then you try to smooth things out after the fact – thus the need for noise reduction software, etc.

Exposing for Film

Now, the rule of thumb with film is “expose for the shadows, develop for the highlights.” This is referring to how emulsions react to light. One might think that emulsion reactivity, like a digital sensor, is linear – for every nanosecond, x amount of crystals react and you get some detail.  That isn’t true, however: it’s curved.  After a certain amount of exposure to light, the reactivity of the crystals slows down.  So, given a highly dynamic range, a film camera might be able to expose for the shadows and not blow out the highlights.

With film it’s all the more important: in digital, you’re virtually guaranteed detail in the shadows.  With film, it takes a small amount of time to start the chemical reaction.  If there’s not enough light, there’s no reaction – and instead of blown-out highlights, you end up with undeveloped shadows.  The same problem as with digital, but on the opposite end of the spectrum.

Shooting Digital with Film’s rules

Now, what Reichmann said was to shoot digital as if it’s film.  This won’t work with JPEG output, mind you: once JPEGs blow out a pixel, that’s it – game over.

But with RAW photos it’s different. See, RAW photos actually have a stop or two (in my experience, almost three with Nikon RAW) of flexibility past what you can see on a screen. So, the argument goes: expose for the shadows.  You’ll end up with a bright picture, but detail is still all around.

In post-processing you can subjectively reduce your exposure, until you get something you like.  Your shadows will be well-defined, noise will be reduced, and you can control parts of the exposure, just like in the old days in the darkroom.

Yesterday’s final pic is a great example of this in action.  The exposure was fine on the foreground, but the sky was blown out.  Subjective exposure tweaks brought it all back together.