Lens Correction Question for Lightroom

[marshmellohi_]

I've been experimenting about my Samyang f/2 lens and I realized that the corners of my images aren't perfectly field flattened, and that brought me to the idea of lens corrections. In Lightroom, there is an option for lens corrections that correct both distortions and vignetting, so I was wondering if there were any disadvantages to applying those corrections to each and every one of my light frames before stacking? Correct me if I'm wrong, but I imagine this would correct the stars in the edges of the frame a noticeable amount, as well as make background calibration easier with vignette corrections. Thanks!

[vlaiv]

Geometric distortion is not the same as star distortion.

Geometric distortion represents different distance between two stars - depending on where you put those two stars in the frame. In ideal situation - it would not matter where you put stars inside of FOV - they should be separated by the same amount. This is however impossible to do, especially with wide field lenses because we are trying to map sphere to a flat surface (much like trying to picture the globe in an image). It can't be done in such way that you both preserve distances and angles as they are.

You need to sacrifice one or the other or make some combination. This leads to strange distortions in some lenses - and that can be corrected in software.

Star distortion is something else - it is the way how light is focused into a point - or rather not into a point. It has nothing to do with above geometric distortion. It is much much harder to correct in software, and in principle, in ideal conditions - meaning knowing perfectly characteristics of the lens and having no noise in the image - it can be done, but in reality - it can't.

If it was feasible - we would stop using field flatteners and coma correctors and there would be no need for that service mission to Hubble when they discovered that optics was flawed - it could be corrected in software. But reality is - it can't be done.

As far as vignetting, yes, it will work, but better practice is to use flat frames to correct for that. Flat frames will correct for vignetting - but also any imperfections in optical train - like dust particles or whatever creates slight light blockage. For this reason - it is the approach that is regularly used in astrophotography over "synthetic flats".

[marshmellohi_]

Wow, I completely forgot about the existence of flat frames! Thanks, I feel dumb for forgetting about them 😅

Also, thanks for the detailed response, your explanation really cleared up how distortions work.