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Subject: Re: Exposure to the right and tone placement



Bill,

:-) Iím just having some fun with what I call a myth that a bit is an f/stop.

Ignoring the effects of gamma for a moment, my point is a little different.
Truths:
1) Light intensity varies by log base 2 as distance changes.
2) Exposure values are scaled by light flux at log base 2.
3) Digit encoding is based on bits scaled at log base 2.

Yes, it is true that the high order digital bit represents half of the range of values in a digitized number. But these three truths are independent.

Letís work some examples to illustrate the point that the high order bit does not represent an exposure stop.

Dalsa makes a high end sensor (FTF4052C) for digital medium format backs. The specs advertise this as having a 12 stop dynamic range. This produces 16-bit images.

Simply mapping these exposure stops to the bits you get 1.33 stops per bit. If we assume the scene only contains 5 stops of dynamic range (and we fill the histogram) each bit would represent 3.2 stops. If we assume the scene has 22 stops of dynamic range and we have a 12 bit sensor, each bit (including the high order bit) represents .54 exposure stops.

Trying to map a scene with 22 stops of light into digital, film, or human vision is unreasonable of course. Once you understand these relationships and your objective you can employ ETTR rationally.

Cheers, Rags :-)

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