Sorry, I wasn’t very clear. I didn’t mean that I drag the histogram but rather the top right hand point on the linear ‘curve’ 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 doesn’t 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 “standard” 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 ‘whites’ 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 space…
Jacques
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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, that’s why you see differences between the two.
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scratch that – it shouldn’t matter actually. I will refrain from commenting further until I get back home and do some homework with pencil and paper 
I don’t 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 doesn’t change. Everything in between changes in proportion to it’s 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
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Exposure compensation is a multiplication operator; zero times EV equals zero.
For what it’s 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"…
Yep.
But the black point still changes when you change the exposure.
Then, something else is being done besides the exposure transform. You can’t 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. I’m 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.
I’m afraid not, sorry. or at least I don’t 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 doesn’t understand that basis, they’re just moving sliders around and hoping for the best. If that’s 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 “linear scaling”, 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 don’t either white-point to the lowest clipped channel or do a “reconstruction pet trick”, 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 we’re talking about to insure you’re understanding the specific effect of these transforms, on the appropriate input data.
Thanks everyone for the replies. I’m 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 I’m a bit confused still on that point . For what it’s worth, I found a similar discussion regarding the use of the two methods in Lr but I’m 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.
lol. If only it were that simple…
There are many levels between those extremes. I don’t fully understand the molecular structure of the gases in a light bulb… but I know when to turn the light on.
then please do elaborate. feel free to go as complex/detailed as you think is needed, I’ll do my best to follow…
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I did that in my first comment.
I see. thanks for clarifying
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