Vignetting Correction in RawTherapee

Started 5 days ago | Discussions thread
OP Alen K Regular Member • Posts: 316
Re: Test Your "Flats"

tradesmith45 wrote:

Should have mentioned a couple more things.

First is its a really good idea to test any flat you create by giving it a hard stretch so it shows any illumination "lumps" or artifacts in your methodology. For example fabric over a computer monitor can easily yield banding.

So far my most successful way of making a crude flat to find good adjustment values w/ WA lenses for LR & RT has been to cover the lens w/ 2 layers of fabric & place it a small distance from a completely white display on my computer monitor. But fabric texture will quickly start to show on stopping down. These interfere w/ measuring values from center to corner. Here's examples w/ stretching @ f1.4 & f2 for the Fuji XF16.

f1.4

f2

As you can easily see, this isn't working too well. Even so, these are still usable for getting much better values for use in LR & RT devignetting adjustments than can be achieved just eyeballing a night sky image.

Other limitations occur with the tiny Lenstip jpg downloaded files. Compression artifacts can quickly show up. These are more problematic than fabric/monitor unevenness.

Lenstip XF16 f1.4

I've bought a sheet of translucent white plastic to try as a better diffuser over the monitor but haven't tested it yet. Have various sky shots w/ fabric & those are same as using the monitor.

I think it's a great idea to use these problematical flats to model the vignetting correction in RawTherapee, which should almost always give a cleaner result. But it's also good to know how to make better real flats for situations where a modelled flat isn't accurate enough or other things need to be corrected as well (such as mirror box shadowing and dust motes). Thanks again for the tips.

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