during a backfocus tuning session for a new Sigma 50mm 1.4 Art lens, I found out that when it was used together with a cooled astro camera (I used a Touptek 2600 Color), it generated visible astigmatism in the corners (Picture 1). This happened always in the backfocus range of 44 to 45.5 mm. I tried removing the filter that was placed between the lens and the camera, but the astigmatism persisted.
Instead, when I mounted the lens on a modded Canon 77D body, the astigmatism in the corners disappeared (Picture 2).
Please note that my Touptek works correctly with 135mm focals.
What is happening? I suspect that, at such short focal lengths, the protective glass of the camera (which in my case is a UV/IR filter) is enough to cause this astigmatism. Or maybe I am doing something wrong.
Have you had a similar experience with such short focals? It seems to me that that only viable solution with a 50mm lens is to use a DSLR rather than a cooled camera.
Thank you in advance for your advice.
Mau
Picture 1: Image corner with Touptek TS2600 Color + Sigma 50 1.4 Art
Picture 2: Image corner with Canon 77D + Sigma 50 1.4 Art
