My goal is to be able to analyse the falloff/vignetting of my lenses which is important for me to select the optimum aperture for astrophotography i.e. as wide open as possible while having falloff that is not too much. A friend initially discovered that vignetting information from all review sites we have scoured analyse the falloff after a tone curve is applied, or from OOC JPEGs, which also have some kind of “camera curve” applied. Imatest also gives the wrong suggestions for how to look at lens vignetting for this reason. So we have needed to perform our own analysis on linear data.
RawTherapee has the ability to “Save Reference Image” under the Color Management section of the Color tab. I note that the green channel’s numbers remain unchanged whether white balance is applied or not (I don’t apply it typically for this purpose anyway), and I am using the green channel’s numbers to calculate vignetting. There is no problem at all here. Everything works as expected.
For the sake of convenience, I would like to be able to not have to process and output TIFFs and just use the readouts within RawTherapee. Unfortunately, I cannot get the RGB readouts to match the “Save Reference Image” file.
- I initially assumed that by activating this function, which is “use working profile for the main histogram and navigator panel, otherwise gamma-corrected output profile is used” would allow me to see the RGB readouts which would match the “Reference Image”, i.e. linear data. Unfortunately, the values do not match. Why is this so?
I see that the values increase instead of decrease when “use working profile” is checked. Why does this happen? At the moment, I’m thinking it is because a TRC of gamma 2.4 and slope 12.96 is used while my output profile is currently set as ProPhoto RGB, which has a gamma of 1.0.
How then can I get the RGB readouts to match the “Reference Image”? I’ve also tried changing the TRC to Gamma 1.0 with slope of 0. This causes the RGB values to be slightly lower than the “Reference Image”, while the overall contrast is reduced, i.e. the ratio of the falloff in the corners to the center of the image is less, implying that the vignetting is less which isn’t the case. Since the “Reference Image” is definitely linear data, I’m now wondering if setting the TRC to gamma 1.0 isn’t really happening! How weird.
The G channel value readout keep changing as I mess about with various settings, so something I’m doing is certainly not right. It should remain constant. I’ve also tried selecting “no profile” for the Input Profile, but alas it also does not match the “Reference Image”.
I’ve probably overlooked something, but after messing about with this for a few hours and reading about how the Working Profile TRC:None works, everything is about as clear as mud. This TRC is independent of the tone curve which the input profile/camera profile has. When the Working Profile TRC is set to none, the preview is not of linear data; linear data looks much darker i.e. the “Save Reference Image”. However, the preview when Working Profile TRC is set to none is colorimetrically correct i.e. a photograph of a Colorchecker24 matches with the real thing/measurements of the real thing. I gather that this is a perceptually linear result, not linear data result, but I would like to be pointed to a clear explanation on this!
Here is a screenshot of the section of RT in question, I deeply appreciate any insight anyone can offer into my questions! Thank you!