This was touched on in the thread If I want my whitest whites to be 255,255,255 - do I adjust the RAW white points?
I wanted to see whether dragging the top right hand corner of the histogram to the left was equivalent to adjusting the exposure slider when setting the white point. First I tried the two methods on a grey-scale wedge and measured the lightness in the middle of the wedge when the white point was set to clipping. Both methods gave similar results.
However, when I tried it on a colour image (and adjusted so that all three channels are just clipping), the method using the exposure slider seemed to lighten the mid tones of the image more than the histogram method (judging by the histogram and visual observation).
Itâs not a very rigorous way of carrying out the comparison I guess and Iâm not sure why there should be a difference, if indeed there is one.
Any ideas anyone?
Does nothing, other than changing the rendering of the y-axis in log-linear view and the x-axis in log-log view.
What is your actual question?
I thought (maybe incorrectly) that you could set the whites to white by either using the exposure slider to move the histogram to the right or by dragging the top right hand corner of the histogram to the left and that in both cases the overall effect on the image would be the same. However, I have the impression that this isnât necessarily the case and would appreciate it if someone could help me to understand what is actually happening in each case.
As already covered in the original thread to which you posted a link, for a photographerâs needs you can set the white point either by adjusting the Exposure slider, or by adjusting a curve in the Exposure or Lab Adjustments tools.
Dragging the histogram in RawTherapee does not affect the image in any way. Are you not referring to darktable by any chance?
Sorry, I wasnât very clear. I didnât mean that I drag the histogram but rather the top right hand point on the linear âcurveâ as mentioned in the thread. Could the difference between the two methods stem from the fact that with the exposure slider, the whole histogram is shifted linearly to the right (including the black point), whereas if you drag the top right hand point on the curve to the left to set the white clipping point, then the black point doesnât move?
Good question, and one which should go into RawPedia. Maybe @heckflosse @agriggio or @jdc can help answer: what is the math (in layman terms, if possible) of the exposure slider and how does it affect the black point? How does it compare to the math of the âstandardâ tone curve?
What scale is the curve in, what do the curve grid lines represent?
Some tests:
Thank you @Morgan_Hardwood From the limited empirical tests I have done, it seems that the effect of dragging the top right hand point on the curve to the left is similar to the effect you get with the âwhitesâ slider in Lr i.e. the mid-tones are not affected in the same way as they are when you use the exposure slider to nudge the histogram up to the white point. It will be interesting to see whether this is indeed the case.
I am not at home, but in travel 
Just a general remark. All work in RGB is always «false» full of approximations which may seems pleasant but depend on working spaceâŠ
Jacques
exposure is just a multiplication by 2^{Ev}. the tone curve is gamma encoded so what looks like a straight line in the graph is actually a nonlinear curve, thatâs why you see differences between the two.
scratch that â it shouldnât matter actually. I will refrain from commenting further until I get back home and do some homework with pencil and paper 
I donât know the math but in simple terms:
When you change the exosure you change the white point, the black point and everything in between.
When you change the white point in the tone curve, the black point doesnât change. Everything in between changes in proportion to itâs position between the two.
well, of course it does matter. the formula should be something like this:
y = T(x^{\gamma})^{1/\gamma}
where T is the tone curve, and x \in [0,1] is the input value
Exposure compensation is a multiplication operator; zero times EV equals zero.
For what itâs worth, Guillermo Luijk has a dcraw tutorial that discusses this:
http://www.guillermoluijk.com/tutorial/dcraw/index_en.htm
Scroll down to " EXPOSURE CORRECTION USING CURVES"âŠ
Yep.
But the black point still changes when you change the exposure.
Then, something else is being done besides the exposure transform. You canât move a value that is RGB=0,0,0 off that with multiplication.
When it is determined that a particular camera has a non-zero value for its black point, that compensation occurs as a subtraction of that value from all measurements in the image. Iâm having fun with just that right now; my trusty D7000 had a blackpoint of zero, but my new Z6 has a blackpoint of 1008, so that has to be subtracted from the image early on to make it look right.
Ok, so black is black. And the rest of the image moves in relation to that. Is that a better way of saying the black point changes?
My comment was supposed to be a simple comparison of the two techniques in question, not a technical legalese definition of exposure.
Iâm afraid not, sorry. or at least I donât understand what you mean by that.
My comment was supposed to be a simple comparison of the two techniques in question
applying a linear curve is the same as multiplying
@Marctwo, the two techniques in question have a technical basis. If one doesnât understand that basis, theyâre just moving sliders around and hoping for the best. If thatâs your objective, have fun.
@Wayne_Sutton, I do the curve technique to adjust my white point all the time, in the manner described in the article I posted in a previous post. Both it and exposure compensation are âlinear scalingâ, or multiplicative operations, the adjusted variable has different meanings based on the supporting equation. I like the curve method because I can stare at the histogram, figure out a white point I like, and scooch the control point over to it. I find this to be especially important if there is clipped data in the raw image, as that gets shifted arbitrarily by white balance and if you donât either white-point to the lowest clipped channel or do a âreconstruction pet trickâ, you get the dreaded magenta cast.
Now, this is based on my software, not RT. RT is doing things for you that you need to tease from what weâre talking about to insure youâre understanding the specific effect of these transforms, on the appropriate input data.
Thanks everyone for the replies. Iâm still not sure I understand completely what is going on however If you push the exposure slider hard over to the right then the black point does indeed appear to stay put on the histogram while everything else above black is shifted to the right. Rawpedia maybe a bit misleading here @Morgan_Hardwood because it states that: Moving it to the right shifts the whole histogram to the right. This means this slider changes the black point (on the very left of the histogram) and the white point (on the very right). The formula indicated by @agriggio would presumably explain the difference between the two methods as far as the effect on the midtones is concerned although if I understand you correctly @ggbutcher the two methods are strictly linear operations so Iâm a bit confused still on that point . For what itâs worth, I found a similar discussion regarding the use of the two methods in Lr but Iâm not sure it adds anything to our discussion. The 4th post in the thread by MFRYE describes his observations using the two methods and these seem to stack up with what I have observed. two different ways to set black- and whitepoint
I agree with you @ggbutcher regarding the curve technique.