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I have a basic understanding of curves. That is, I know the mathematical concept behind them, and I have an idea what happens when I change them. But this knowledge came from sources which assumed that I am working with an image editing programm like Photoshop/Gimp.

The sources also teach beginning enthusiasts like me to do as much as possible in RAW, to prevent information loss in the image. So when I saw that the raw tool I use has curves, I decided to try using them there instead of later in Gimp.

I am however mystified by the presence of two different curves in ufraw. They are described as follows in the user guide:

Base curve

Base curve imitates the functionality of Nikon's tone curves. For Nikon NEF files you can choose Custom curve if you want to use the curve that is embedded in the raw file. Choosing Camera curve will enable the embedded curve only if it was enabled in camera. All camera users can load curves to apply custom curves to their images. The famous white wedding curve (V3.5) from Fotogenetic was applied to the image on the right. Move the mouse over the image to see the original image with a linear curve. A +0.5EV was applied to the original image to equalize the luminosity of both images. The white wedding curve adds some details to the dress. UFRaw can directly apply The curve from Fotogenetic to your images.

The base curve is applied directly to every color channel. It is applied after the exposure and white balance setting so that it will effect each channel equally. It is applied before the gamma correction, meaning that it is applied on the linear data.

and the second one

The curve in the Corrections settings is applied to the luminosity channel in Lch(ab) space. The controls on the left of the curve editor are for controlling the black point. You will notice that they simply control the leftmost point of the curve. If your picture looks foggy, the auto black button Auto Adjust might fix it. The auto curve button Auto Adjust on the right tries to set a curve that flattens the histogram. It can add lots of contrast to your photo but sometimes the results look very artificial.

I don't know where to get tone curves for my camera (a D90). I don't know how to make my camera embed a curve in the RAW file either.

My question: When I want to change the way my photo looks, which curve should I use for what kind of change?

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up vote 5 down vote accepted

The Base curve is used to alter how the raw light data is interpreted. You can think of this almost as the input curves in other apps, they control how the raw data in the file gets pulled into the image editing space. This is often where fancy curves are used to simulate different types of film and such... that's very much what it's doing, adjusting the response of the film to the light; only digitally.

The Corrections curves are used more to control how the image is rendered out to the destination. This is very much more like varying the type of paper you're printing on. (especially in the B+W paper world.) Since this is just the raw conversion step on the way into gimp, that analogy breaks down a little... maybe it's more like how your film is developed into a negative.

In theory, both curves are capable of the same things. In reality, because of where they arrive in the pipeline, if you're doing ANY other changes with other tools in there, you probably want to do your adjustments with the Base curve, so that those other adjustments are applied after your curves.

disclaimer, I've not used ufraw in a long while, and the above is from memory.

THAT SAID... personally, I would only apply curves in the input process to do very basic adjustments, or to perform camera correction. For example, I have a Canon 20D, it's known to over blow red and under count green... so the camera correction for it very subtly corrects those two flaws. There are actually a number of curves for it from Canon... one for "natural", one for "technically accurate" etc... they've done testing in the lab and built profiles to do very specific things. I typically apply those at import just to normalize the image so that I know the image I'm working on is not tainted by what camera it was taken with. Once I have the image in my editing app (LightRoom for me, Gimp for you) THEN I use artistic curves to adjust the image to the way I want artistically. Why? typically the raw converteres aren't built for artistic play and undo/redo experimentation... they are built to convert raw to something editable, with a minimal adjustment to correct for the deficiencies of the camera.

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I've been curious about this question for a while, but haven't used ufraw. Nice to finally see an answer! Thanks, @cabbey – lindes Dec 19 '10 at 17:22

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