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Subject: Re: Exposure to the right and tone placement



Bill,

In message #25 on this thread, you wrote

"However, if you inadvertently blow one stop of highlights you lose 2048 levels according to the prevailing wisdom for a 1.4x gain in shadow noise."

In fact, you don't. You get a full 4096 tones in and (if you convert to 16-bit) a full 4096 tones out. They may not be representing the luminances you want to capture, but you'll be using the full range of the camera. In an overrange scene, you may want to make the conscious decision to sacrifice a stop of highlight detail to retain shadows, but you aren't losing any levels, you're simply recording all the bright tones as one level, clipping all the highlight detail.

But if you underexpose by 1 stop, you'll get only 2048 tones in and out, period.

Hence placement of the highlight is the critical factor, not placement of the midtone.

Again, I DO NOT advocate overexposure, I advocate correct exposure, which while not particularly easy to achieve with the current tools is also not impossible. In the field I spot-meter the diffuse highlight and apply my predetermined exposure compensation for my camera. Shooting under controlled lighting is much easier.

Parenthetically, the raw conversion rarely if ever entails a simple gamma conversion from 1.0 to 2.2 (or 1.8 if you use a gamma 1.8 space as I do)—it's much more likely to be a complex tone curve that cannot be represented by a gamma formula. If you move black and white points, adjust midtone, add or subtract contrast, it's not a gamma conversion, it's a tone mapping from a linear-gamma space to a gamma-encoded one.

I've been pointing out the loss inherent in gamma conversions for over 12 years. Image editing always loses levels. Fortunately, our eyes aren't capable of distinguishing 4096 unique tones (the estimates on how many we can distinguish vary, but they're universally in the hundreds rather than the thousands). The trick is to throw away the levels we don't need and retain those we do. I have a distressingly large number of images that are one stop under that I've nevertheless been able to make work, but I have to work a lot harder than with slightly overexposed ones, and a whole lot harder than with correctly exposed ones.

Yes, you can take exposing to the right too far. That's called blowing the highlights. You can also take driving on the right too far—that's called mowing down pedestrians on the sidewalk. I don't advocate either, but it's generally safer to put one wheel on the curb than to plow into oncoming traffic, and it's generally safer to err on the side of slight (1/3 to 1/3 stop) overexposure than it is to underexpose. Some cameras offer more headroom than that, very few offer less.

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