I notice when using the multiply blend mode with exposure that even with all of the sliders set into a neutral position the image appearance is changed. Is there a reason?
Yes, since a pixel with a brightness of 50% will be turned into 50%*50%=25%.
This will tend to increase the contrast in the midtones and highlights, and decrease the contrast in the shadows. You can see this because the amount of contrast is represented by the slope of the output curve.
If you remember your high school algebra, if we have multiplicative blending with y=x^2 then the slope \frac{dy}{dx} = 2x, so the larger the x (ie. the brigher the pixel), the larger the slope and hence the larger the amount of contrast. For small x (ie. in the shadows) then the contrast 2x will also be small.
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Try reverse multiply as well to see how that works…you will need to mask part of the image to see how this differs
Thanks everybody for letting me know what I already knew.
The question that I was asking was why are there changes in the image when everything is set in the neutral position that is before I have made any changes in the sliders whatsoever I do understand how it works but why is there a change in my image when I do absolutely nothing.
To understand why the image changes when you do multiply blending, you need to understand what multiply blending does.
Say you have a pixel of brightness L_{in}. Then, the effect of exposure module is to multiply this by some value, based on your slider, so the output before blending is L'_{out} = \alpha L_{in}.
But, this is not the end of the story, because you have enabled multiply blending. This means that the output after blending equals the output before blending times the input. That is, the output after blending L_{out} = L_{in} * L'_{out} = L_{in}*\alpha L_{in } = \alpha L_{in}^2. But, if you leave the sliders in default position, then \alpha = 1, so the final output of the module after blending is L_{out} = L_{in}^2. Obviously this is going to change the image, as the previous responses have indicated, and which you claim to already know. So, I’m not really sure what your question is…
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Its the blend mode…blend in addition and see what happens or screen if you use the display blend modes…this is why they are so powerful…
For example use an instance of the tone curve module…I like the effect over the same done with rgb curve …add it …its a no op…should not change the image …its a strait line…now blend it at around 10% opacity in subtract blend mode…nice darkening contrast adding effect… multiply also could be used… this is the same thing that happens with you blend exposure as a no op with no slider moved…doing it with the curve you can also adjust the effect…
Do this with divide at 100% you will get white and subtract you will get black…
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Perhaps there is a slight misunderstanding here.
If I have understood you correctly, @davidvj, you think that the blend modes should become active only when you change something on the sliders.
If that was the question, the answer is no, the moment you select a blend mode, it is activated immediately, regardless of the change in the sliders.
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To me this does not sound like the right way to address people trying to help you.
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It can be a little bit frustrating when the answers received indicate that the question is not being fully read … or correctly read. If I offended others with my retort, this I do regret.
Enough said I think.
You asked about the blend mode, and we answered thinking that you don’t know how the blend mode works. Both Matt and I (and I think Todd, too) thought that was the case; only after your clarification did Boris realise what the misunderstanding was. So, I think maybe it was not the question not being fully / correctly read; rather, it was the question not being correctly asked.
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You are, of course, totally correct.
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