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Since the photograph has already been taken by the time it is being edited, 'adjusting the exposure' of a RAW file must have a specific technical meaning (which I have struggled to find) Can someone explain that technical meaning?

Here are two possibilities:

'Adjusting the exposure' of a RAW file using a RAW editor is actually an intelligent editing function which approximates what the picture would have looked like if it had been taken at a different exposure. (If this is the case, how does it work? How does it differ to adjusting an image's brightness?)

OR

'Adjusting the exposure' of a RAW file using a RAW editor is a kind of filtering function that determines which information in the RAW file will be used to create the next version of the image? (If this is the case, how does it work, and what determines the initial settings before the 'exposure' is 'altered'?)

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The answers should take into account this part of the question: "How does it differ to adjusting an image's brightness?" I am sure there was a question once that asked for this specific difference, maybe just link to that. –  his Jul 22 at 20:20
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It depends on the editor in question. Not all RAW convertors do exactly the same thing. –  Michael Clark Jul 23 at 1:19
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I am not sure we can get a good single answer to this question, and certainly not a technically accurate one for all major RAW editors (given that some use proprietary technology.) I know that RawThearapy tends to be fairly linear in many of it's adjustments, where as Lightroom is most decidedly non-linear (i.e. adjusting exposure shifts the middle range of tones more than it does the highlights and shadows.) This question may be difficult to answer adequately. –  jrista Jul 23 at 3:33
    
Thanks for these comments his, Michael Clark, jrista, super helpful –  Tom Robinson Jul 23 at 17:22
    
Currently trying to work out if the two answers we have so far are contradictory/mutually exclusive and whether that is related to the above points (i.e. whether they apply to different converter systems) –  Tom Robinson Jul 23 at 17:24

2 Answers 2

It's my understanding that most raw converters apply a multiplier to linear values, either demosaiced or not. (The big exception is Adobe.) This mimics more/less exposure in the camera, and the end effect is that a file looks likes it had been exposed in camera at the net exposure (actual exposure + exposure adjustment in the converter).

But don't just take it from me:

From a post in Rawtherapee's forums by one of its main contributors(http://rawtherapee.com/forum/viewtopic.php?t=2589): "Exposure: Implements a straight amplification of the raw data, as if you had changed the ISO setting on the camera -- +1 EV on the exposure slider is the same as raising the ISO 1 stop."

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Hi :) Props for providing a sourced answer that does not require following a link elsewhere. Can you nonetheless include a link the the source you quote? Cheers! –  Cornelius Jul 23 at 15:48

The dynamic range that a RAW file takes in is generally much wider than the range used in producing a finished photograph. It contains the actual raw values of the amount of light gathered by each point on the sensor. When adjusting the exposure slider, it shifts the relative interpreted intensity of each pixel based on the amount of light that was accumulated by it.

The net effect is that it slides the dynamic range of the photo around the dynamic range of the RAW data. The absolute max black and white points of the RAW image don't change, but the black and white points of the image are moved around within that space.

This differs from brightness because brightness focuses on the mid-tones while trying to more or less preserve the black and white points, but makes the overall image's average luminosity higher, though there may be differences in what brightness adjustments do depending on the software you use.

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What are your sources for your answer? –  Andy Blankertz Jul 22 at 19:15
    
My source is simply knowing what is contained in a RAW file and understanding how that is interpreted. The multiplication of the values is the same thing as "shifts the relative interpreted intensity". I'm simply providing more detail about what the impact on the image is from doing that. Your processed image has a white point of white and a black point of black. If you leave your RAW value black and white point the same but multiply the values, it shifts the processed image within the DR captured in the RAW data. –  AJ Henderson Jul 22 at 19:42
    
I thought that the RAW white point was unconstrained wrt exposure adjustments. –  Andy Blankertz Jul 22 at 20:01
    
RAW is not an exposed or processed image. It has a min value and a max value. "Black" is 0, "White" is whatever the maximum integer value is for the raw format (depending on bit depth). But the actual processed black and white point are set for values within that range, generally not on the extremes. –  AJ Henderson Jul 22 at 20:06
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@AJHenderson Your answer describes the way Adobe RAW uses the term "exposure" and how moving the exposure slider affects the 8-bit image seen on the screen. Some RAW convertors do increase/decrease the black and white points when adjusting "exposure". Moving the "contrast" slider with those convertors also usually changes the distance between the black and white points. –  Michael Clark Jul 23 at 1:18

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