OK, here come my thoughts and musings…
First, RGB - any RGB value contains information about the COLOUR & ‘BRIGHTNESS’ of a pixel - supplement whatever word you like for ‘brightness’.
Next, CONTRAST - can be thought of as a ratio of brightest ‘brights’ to darkest ‘darks’ - just as monitors have a contrast ratio and print papers have a dmax and dmin contrast ratio.
The exposure compensation slider is - even though it’s supposed to be an ‘RGB’ control - very useful because it effects every pixel in an image to exactly the same degree.
If we take -1Ev out of an image we take every pixel down by -1Ev irrespective of it being a highlight, midtone or shadow.
HOWEVER - there is a caveat to this!
If we already have a shadow/black value of ‘0 RGB’ we cannot make it any darker by removing -1Ev, but we still drop all the lighter tones by -1Ev thus making them all 1 stop darker, so we DO effect the overall image contrast ratio - we reduce it.
The same can be said if we go the other way with +Ev comp - we will maintain our existing image contrast RATIO by adding +Ev comp right up to the point where our images brightest pixel hits 255(in 8 bit terms). If we continue to add +Ev comp we do indeed make ALL our darks and mids brighter/lighter, but those 255 pixels can’t get any brighter/lighter, so we still end up reducing the image contrast ratio.
But using the Ev comp slider WHILE avoiding CLIPPING of highlights/shadows does not - at least visually - produce any contrast ratio change, becuase it’s a blanket, non-selective, non-curve-based adjustment
But being an RGB style adjustment it WILL lead to a variation in all colours, which is really inconsequential in the grand scheme of things PROVIDED you have not white balanced said image. And even if you have, you can always amend that after the fact.
Curves on the other hand are designed to modify CONTRAST from the get-go, irrespective of whether they are composite RGB or individual channel curves, and they will all effect COLOUR too, with the exception of a Lab L* curve.
With Lab, the L*channel allows for CONTRAST adustment without effecting the colour of the a or b channels, and the lightness slider is a quick way of lightening the image.
Certain popular ‘info suppliers’ rave about curves as if they are something new. But when you’ve worked with Photoshop as long as I have (BarneyScan!) you’ll soon come to realise that everything is a bloody curve!
Sharpening is an edge-localised curve which is distributed on yet another curve.
When you blend layers you do so on a curve - just 'cos you can’t see it doesn’t mean it’s not there!
In Photoshop both curves and levels effect contrast AND produce colour shifts - unless you switch their blend modes from normal to luminosity. Which is one of the many reasons I use Raw Therapee etc to produce an image that needs final tweaks in Photoshop as opposed to a ‘finished’ image - thus leveraging the power of two different sets of tools to produce the final image - there are NO shortcuts in this game IF you want the ultimate in image quality; or if indeed your paying clients demand it. But that’s a whole other story…
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