At 12:11 AM 12/1/2005, David Mann wrote:
On Dec 1, 2005, at 12:40 PM, Rob Studdert wrote:
I just did a Q&D test using my *ist D and the camera histogram
responds
similarly to green and red. However what I didn't check is how the
histogram is
calculated, possibly if one channel is clipped and the others are
low the
aggregate RGB histogram will display as unclipped?
The driver for my scanner tends to round low values to zero when
displaying so I'm finding it very easy to blow out the highlights if
I'm not careful.
I think there is a good chance that the Pentax either rounds some of the
low values to zero or perhaps doesn't combine three channels before
providing the histogram results. I've noticed that sometimes the histogram
will appear to indicate that the brightest value captured is a mid-tone,
maybe 128 out of 256. However, if I adjust the white point lower in my
editing software to "correct" the overly dark scale, small highlight areas
will sometimes begin to clip, well before I have gotten anywhere near what
the histogram says is the brightest value recorded. Sometimes, the clipping
only occurs in one or two channels, which makes the highlight area look
discolored. There might still be detail in the highlight from the one or
two channels that didn't clip, but it still doesn't look good because of
the color shift.
In general, the histogram is a big help in setting exposure and contrast
settings on the camera, but there are still some cases where I think it
gives misleading information. Some sort of clipping indicator would be a
nice addition to the camera's preview features, or possibly provide a
numerical readout of the very brightest and darkest pixels captured in the
image, as measured on a 0-255 scale.
take care,
Glen
