7 May 2009

Expose to the right

Yashica-Mat, exposed to the right

I found a link to this useful article on the Luminous Landscape web site which has alerted me to shortcomings in some exposure readings: expose to the right ! Some things are not intuitive if you have been raised on film.

The key passage for me was this:

A 12 bit image is capable of recording 4,096 (2^12) discrete tonal values. One would think that therefore each F/Stop of the 5 stop range would be able to record some 850 (4096/5) of these steps. But, alas, this is not the case. The way that it really works is that the first (brightest) stop’s worth of data contains 2048 of these steps — fully half of those available. Why? Because CCD and CMOS chips are linear devices. And, of course, each F/Stop records half of the light of the previous one, and therefore half the remaining data space available.

This little table tells the tale.

  • Within the first F/Stop, which contains the Brightest Tones 2048 levels available
  • Within the second F/Stop, which contains Bright Tones 1024 levels available
  • Within the third F/Stop, which contains the Mid-Tones 512 levels available
  • Within the fourth F/Stop, which contains Dark Tones 256 levels available
  • Within the fifth F/Stop, which contains the Darkest Tones 128 levels available

This realization carries with it a number of important lessons, the most important of them being that if you do not use the right-hand fifth of the histogram for recording some of your image you are in fact wasting fully half of the available encoding levels of your camera.

The histogram of this picture reflects everything from deep black to bright white, and I just have to remember this when I take pictures outdoors as well.

And no, I’m not putting that old 620 film in the Yashica-Mat!


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