RT - best way to correct the noise floor/black level

Processing
1

I am not using an S-shaped curve in RT, so the blacks are not really black because of the noise floor. I can shoot 10 darkframes at the same ISO as the target picture, stack them and now I know what the real black level is. But in RT, “Black” setting depends on exposure, so every time you adjust the exposure, you also have to adjust that setting, which is time consuming. What is the best way to do what I’m doing, preferably that works with 16 bit output (underexposure indicator is based on 8 bit preview)?

Edit: tiff darkframes do not work, so even if I’m gonna use the darkframe menu I’ll have to somehow stack everything to a single DNG, and since I won’t be able to fine tune the level of correction manually, there will be some added noise

Edit 2: the darkframe menu is not the solution. Did a quick test with a single frame and the black level does not change

2

This is not RawTherapee specific. If you change the exposure the lightness over the entire range is changed. The implemented change becomes stronger the more you go to the highlight side of things, though.

Every time you use a slider instead of a curve, S shaped or otherwise, you will raise or lower the effect over the entire range. The underlying algorithm might favour certain parts, but it is done globally nonetheless. As a side effect you might end up having to play the back-and-forth game between certain sliders.

3

In Darktable if you change the black level in the exposure tab it will stay there no matter what the exposure slider settings is

4

I had someone on a discord chat asking about amp glow; does RawTherapee’s dark frame subtraction correct for nonuniform black point error, or does it only use that to find hot pixels?

5

This is only true (darktable and RawTherapee alike) if the RGB value of the blacks are pushed to 0,0,0. Any other value will also increase the blacks when increasing the exposure slider.

Setting black to 0,0,0 means that you clipped your blacks, though.

Use the colour pickers in both programs to check my statement :wink:

6

I’m sorry, tested it and you are right, but the required exposure correction to bring back the shadows should be extreme, like +7 EV
Edit: it automatically moves the exposure slider

7

Not an RT dev but you’ll probably want to use Flat Field correction for that

8

I am curious why you don’t.

It seems to me that you would not run into this issue if you did use curves. Be it the film-like curve in RawTherapee or filmic-rgb (tone curve if using display referred) in darktable. Bot have adjustable toes and shoulders to massage shadows and highlights into place.

But I’m sure I do not have all the context surrounding what you are trying to accomplish.

9

Sounds like some astrophotography processing… :slight_smile: