Histogram

On another post one person said that to do any good you had to look at all 3 colours separately. I have done this and noticed that for example all the blue was way to the right compared to red and green. How would you get the blue only to move to the left on a K5.

If you do that you will be artificially altering the relationship between the RGB cnannels and you may well not get what you expect. There are reasons for looking at three channels separately, but generally looking at the overall histogram is all we need to do.

Just look at the 3 separate channels to make sure that none of them are clipped/overexposed. They only line up in the left/right direction on a grey subject. If you have a lot of light blue it will be to the right.

The combined (white) histogram is misleading as far as exposure is concerned..... it's explained on the web somewhere, but I haven't bookmarked it.

EDIT: If you search Google loads of sites deal with it.
I can't see the one I looked at, but this one seems to
explain it and there seems to be a discussion below:

link

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I should have added to the above, that I find the Highlight/Shadow warnings much more useful than the histograms, particularly for whites that would be blown on swans etc.

I'm slighty confused, perhaps someone here can help me understand more clearly. I know this is about stills but with digital stills we are arguably dealing with something more akin to a video signal than an exposed "picture". We're all shooting video nowadays even when we take stills. Reading the flickr discussion linked above I'm a bit confused and slightly skeptical that some of the points made are nonsense. If we take digital stills as being like individual high res video frames (which is essentially what they are) then the detail and "resolution" comes from the luminance signal. Individual chroma signals will naturally be relatively low res compared to Luminance. Worrying overly about R, G or B clipping individually is potentially quite silly. Just because red is clipping DOES NOT automatically mean there'll be "no detail" in the reds/flesh tones because the green and blue contribute to the luminance signal and just because red clipped is not an automatic indication that "all detail is lost". The majority of the luminance signal comes from green, so if I was going to look at anything to judge detail and the danger of losing it, it'd be the green signal. Besides which, histograms are a cheap and nasty way of monitoring the "engineering quality" of your signal. OK for a rough guide but no good for proper evaluation. What we really need to see is a waveform. Then exposure and contrast ratio really could be judged and it'd be a genuine help. Histograms are as potentially misleading as they are useful IMHO.

To JohnBoy, you may be interested in the UniWB white balance then.

A very good read about the RGB histogram (and why is slightly misleading) is at:

http://www.libraw.org/articles/white-balance-in-digital-cameras.html

How to get the maximum signal (or minimum channels noise) from using UniWB is explained at:

http://www.libraw.org/articles/magenta-filters-on-digicam.html.

How to set UniWb on your Pentax camera, I forgot to be honest. It could be similar to this technique though: http://www.guillermoluijk.com/tutorial/uniwb/index_en.htm, if not Google is your best friend.

Lucian.

cracking link that Algi - nice one

The combined (white) histogram is misleading as far as exposure is concerned..... it's explained on the web somewhere, but I haven't bookmarked it.