black/white points - when/how to adjust

[blinky]

I am after a little advise here folks:D I wonder if somebody can tell me how I know whether to adjust the black or the midpoint (or the white point?) sliders when trying to correct colour balance problems? Say I have an image where the red and green peaks are roughly the same but the blue is much higher, would I adjust the blue black point slider to bring the blue back inline or the the grey one? Both seem to accomplish the same thing but I assume one is better than the other - maybe saves some more data?

[Beyond_Vision]

Use the black point to align them. By bringing the black point just up to the left side of the histogram

[roundycat]

Use the mid point slider and set it to two, if we are talking Photoshop. That gives you a gentle stretch without any risk of clipping or saturating the bright values. Initially only the very brightest stars will be white. Do not under any circumstances adjust the white point as it gets adjusted to a surprisingly large extent by the mid point slider.

Do this two, three or even four times. Do not let the background get too bright but, in the beginning, do not move the 'black' point at all. Certainly do not follow normal advice and set it to the left hand side of the histogram. You should be making these adjustments by carefully examining the picture, not the histogram. We do not hang pictures of the histogram on the wall!

Adjust the black point, only if necessary, to give a background brightness in the order of 20-30%, absolutely no more than that and don't touch it until the background is obviously bright. All your data will still fit within 16 bits so you are losing nothing.

Never adjust the individual channels in the way you describe. When you have finished with Levels you should have at least the ghost of a picture which is now easily fine tuned using Curves. This gives you the choice of up to fourteen adjustment points for each iteration, a bit finer control than is possible in Levels. Target your Curves adjustments to the 'interesting' part of the picture and the bit where contrast increase is most needed. After one go of Curves you might then need to adjust the colour bias, use the individual channels in Curves to set the background to a neutral colour (if it should be neutral!) and make sure to anchor the curves in the mid range to upper levels so as not to change the brighter colours in the picture.

After this you can separately target the brighter colours by using the same technique. Ctrl-Click in the picture to place an adjustment point and move it very carefully, in each channel, to set the brighter colours where you want them. Then you can finish off the Curves adjustments if you wish but be aware that all adjustments are increasing contrast, even sharpening, and too much of Curves or High Pass or USM will ruin it.

See here Mono to RGB

Dennis

[Beyond_Vision]

Each to his own

[RobH]

Yep....each to his own!!

I would never ever touch the midpoint slider

To balance your blackpoint, go to LEVELS, and then double click the black eyedropper.

This will bring up a dialog box where you can set your blackpoint levels for each channel.

Set each of these to about 30.

Then go to a dark area of your image that doesn't contain any nebulousity or stars and left click.

This will set the blackpoints of each of the colour channels to the value you set earlier.

Don't touch the whitepoint.

Cheers

Rob