I was very impressed by Ray Maxwell’s explanation of what a histogram on a digital camera shows in Maxwell’s House episode 60 [http://twit.tv/mh60]. The histogram is showing the amount of pixels that have a particular exposure and that exposure is power. So the further right on the histogram you go, the brighter the luminance.
However, the halfway point on the histogram isn’t necessarily the halfway point of the color resolution. What does this mean? Well, if the furthest right you can go on a histogram is brightest white, say, 256, one stop down (1/2 the exposure, thus, 1/2 the power) is 128. Two stops down is 64. Goodness…how many stops are there on a gray scale? 10? That means 32, 16, 8, 4, 2, 1….Does that mean that we run out of bits as we enter the dark end of the photo? Yup.
The previous episodes explain how this is managed using color spaces. This episode’s advice indicates a few bits of advice: 1) camera’s don’t themselves have ICC color profiles,
2) shoot in raw because that’s the full color resolution (common knowlege these days)
3) overexposing a bit will give you smoother tones, and a good way to get this effect is to set the “contrast” setting on the camera to “low contrast”.
4) the histogram you see on the camera is actually only possible after converting the raw image thru a color space (like sRGB).
5) what actually determines the information of the raw photo is a) ISO, b) shutter speed, and c) aperature. Other settings, like saturation, contrast, and white balance, are manipulations of the raw photo into JPG format (and that of course is done through color-space).
Neat stuff, good episode. I’m inspired to move towards using DNG format on my Pentax K10 now. The only bummer is that the Gimp doesn’t natively support DNG raw format (last I checked.)
Bitratchet 