I once suggested a raw histogram (on the Adobe ACR forum, I think), and Bruce Fraser replied that it would not be useful because everything would be shifted so far to the left that it would be hard to read. He has a good point there, and a histogram with a gamma of 2.2 applied to the raw data might be more useful.
Either way would be an improvement. I look at RAW linear histograms, and don't find them hard to read, except that the upper stop is spread wide enough that the values are so spread out that they don't stack up. That could be easily addressed, though, by scaling the vertical more, to the right. Any histogram, IMO, should have a thick bar for the clipping value, so it is easy to see. The ones in my Canons are only thin dotted lines (not even solid).
A RAW RGB histogram need not be linear to be useful. It just needs to be RAW, without all the nonsense introduced by conversions that have nothing to do with exposure.
If consistency with the traditional histogram is a concern, I really don't think that this is useful for RAW shooting. Traditional histogram checking is mainly for checking the tones in your jpeg. I am only interesting in how much I'm clipping, or how far away I am from it. I am not looking for an ideal hump in a certain shape in a certain place.
Since most cameras allow some headroom, would you want clipping in the histogram to coincide with clipping in ACR without exposure adjustment or would you want it to show clipping only when the sensor starts to clip or the ADC starts to overflow?
Clipping is rarely a sensor event; it is usually a digitization event.
If the histogram shows you RAW r, g, and b separately, then you have the information you need for many purposes. You could determine where on the histogram your converter is reliable; you can avoid clipping all channels, or avoid clipping all but one if that's what you want.
I want to see the real RAW histogram (gamma is relatively irrelevant). If current converters can't use the data properly, then they need to be updated. Every RAW converter should have a true linear gain stage before any other processing, just like the EC dial on the camera. Unfortunately, some drag curves around along with the exposure (ACR nails extreme RAW highlights to 255 in the output, while the midtones go almost black with -4). There should be total linear freedom before any curves are applied; most converters act as if exposing to the right is not really a viable option, and RAW highlights are specular in nature. A converter also needs to know where clipping occurs in the RAW data. This does not always happen at 4095. Many Nikon and Canon cameras clip at different levels in different vertical lines of pixels (10D, 30D, and D200). The converter should detect clipping patterns, at least as an option, and clip uniformly. It should also look for patterns of different amplification in different lines. The are numerous artifacts in RAW data which can easily be circumvented.
Once you decide what you want, it is a relatively simple matter to upload a custom tone curve to the camera to get the desired results.
There's no such thing with Canons. All you can do is try to bring the JPEG parameters as close as possible, but they are still far off. The frequency response of the red, green, and blue filters in the cameras are not the receptive equivalent of the output of a calibrated monitor, even after WB. The correlation between RAW and an RGB output image is very weak with many saturated colors; saturation changes and hue shifts are performed on the review image, upon which the histogram is based. You could have red RAW values in the same image of 700 in both a grey and a red flower, and the red flower's 1000 could clip the review and red histogram, but be only 150 (out of 255) in the grey.
White balance is another consideration. If you are shooting with daylight color balance, the gain in the red and blue channels has to be increased by a factor of 1.83 and 1.36 respectively for a Nikon D200. Other Nikons and many Canons have similar multipliers. You may expose so the green channel is fully to the right, but the red and blue channels will be relatively underexposed. Do you want your histogram to show the true status of the color channels without white balance? If so you can load a special white balance into the camera such that the red and blue channel multipliers are 1.0 (UniWB).
I don't want any white-balancing in my RAW histogram; then it is no longer a RAW histogram.
Do you want the channels with daylight to be better balanced? If so, put a magenta filter over the lens to hold back some of the green light.
I've been doing that for a couple of years, but the histogram is still a JPEG histogram. There is much more difference between RAW and JPEG than just WB and gamma.
In sumary, a raw histogram is already available but it is not worth the trouble for most people.
I don't think so.
And what "trouble", if the camera actually had a RAW histogram option? You could see the review with the selected (or auto) WB, but see the histogram in all its RAWness. No need to discolor the JPEGs for a not-quite-RAW histogram.
For practical shooting it is often sufficient to know how the camera histogram with default tonal settings relates to the ACR or other raw converter rendering at default settings. This can be determined by a bit of testing. Similarly, most photographers do not bother with a magenta filter.
99.9% of digital photographers don't even know that daylight isn't the native WB of the camera. When I first posted the idea of using a magenta filter a few years ago, googling showed no one ever mentioning it before me. So, it is safe to assume that very few people are even aware of the possibility of improving the red and blue channels in daylight shooting.
Support is slow. I don't recall if the recent versions improve on it, but at one point ACR couldn't even WB an area where RAW R=G=B; it was outside its tint range. The CC30M filter I was using, however, works in daylight with ACR 3.3 (it's not a perfect daylight correction, though). I would use a magenta filter more often if I had a RAW RGB histogram. It's hard enough trying to guess the relationship of histogram to RAW in daylight WB; WBing for the magenta filter in the camera gives another relationship to know, so bracketing is probably the best policy.
I have UniWB uploaded to bank 4 of my D200, but don't use it very often.[a href=\"index.php?act=findpost&pid=68604\"][{POST_SNAPBACK}][/a]
Well, it's inconvenient to have your JPEG thumbnails with a color cast, and dark. And what you get is still probably very different from RAW.
I want a RAW histogram (linear or gamma-adjusted) with separate R, G, and B, and a review image that is sRGB, but flashes where the RAW is clipping. The flashing could be black alternating with the channel or channels that are clipped; for example, alternating between black and yellow where the red and green channels are clipped; white where they all are clipped, blue where only the blue is clipped.