A couple of histogram stretch questions

[AlistairHowie]

Pretty basic questions, so bear with me please since I'm confusing myself a bit with histograms!

Firstly, when doing the very first stretch, should I be moving the white point a bit to the left and thereafter just moving the black point and mid (gamma?) points, or is moving the white point generally a no no?

Secondly, when looking at the initial histogram before doing any stretching, I find that the blue part of the histogram is regularly way over to the left, like in the picture below. Actually I did a single, gentle stretch before grabbing that screenshot, but the shape and positions of the curves are what they were like before doing anything. This was also the case for imaging M42 (I can't remember where it was for M13, and that's the only other image I've ever managed to take so far!)

The relative positions of the historgrams presumably reflects the underlying object being imaged? I have confused myself, though, as to the "meaning" of the relative positions of the histogram. I think I should be going into each R, G and B channel separately and moving the black point only to get the rising of the left hand peaks all aligned, before carrying on with combined stretches of the channels ... if I understood the advice of a well known SGLr in the South of France ... or did I misunderstand and would that alter the fidelity of the underlying data?

I think where I'm getting a bit confused is understanding what the relative position of the histograms means for my image, versus changing the width of the histograms.

As usual, all advice gratefully received.

[IanL]

The peaks (mostly) indicate the background sky in the image. See here for my post on reading a histogram:

http://stargazerslounge.com/topic/183214-which-exposure-length-is-more-favourable/#entry1904848

The relative positions of the colour peaks show that you have much more red and green in your image background than you have blue. If you mix red and green you get a sort of muddy khaki/brown colour, and if you look at the image that's exactly what you have got.

- You do not want to clip any of the pixels at the bright (right hand) side of the histogram, otherwise you are throwing away useful information. So you'd start by moving the white point slider to the right-hand end of the "tail". (You often can't see where the histogram tail ends as it will be plotted at a small scale, so you either need to zoom in a lot, or look at the % of pixels being clipped and make sure it stays zero, or some packages highlight clipped pixels in a different colour).

- To start with you do not want to clip any pixels at the dark (left hand) side of the histogram either; the sky is not black and you would end up with a rather odd looking image or black holes and blobs in your image background. So pull the black point up to the left hand side of the peak, again zooming in, using % or colour highlight to avoid clipping the background depending on what your tool offers by way of functions. (You may want to clip a small amount of the background later to smooth it, but this is normally less than 1% of the pixels and something to be tested after you have got a good stretch and done some noise reduction, etc.)

- Finally to get a rough colour balance you would try to align the histogram peaks by moving the mid-point sliders for each color channel. Bear in mind that this is only a rough balance and you'll probably want to modify it as you progress, but it gets you close enough that you can start working on things.

- Once you have done that, you may then want to do a second stretch on the balanced image with the typical inverted 'hockey stick' type curve to improve the contrast between highlights, midtones and background.

[AlistairHowie]

Thanks for the information Ian, very helpful, particularly your post in that previous thread.

Is there any way to zoom in on the histogram in Photoshop? I have CS4 but can't see any way to do that ... but then I am just learning .....

[IanL]

Not a PS user, but hopefully one will be along soon to help!