Quote:
Originally Posted by 2020BC
Thanks!
As you know, lenses have an iris to control the aperture. This lets you change the amount of light that is getting into the camera. Wide open, at maximum aperture (f1.8), this lens lets in a LOT of light and is therefore very "fast". However at f1.8 around the edges of the image you get "coma" where the stars are not round but instead look like little comets.
To get nice round stars right across the image field you need to "stop down" the lens by closing the iris a bit.
With this lens you get dramatic improvements with each stop down to around f4, and then after that you get slight incremental improvements. At f6.3, with this lens, I get stars that are pretty sharp right across the full width of the image. I've not tried it any slower.
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Thanks Bill.
No, I didn't know lenses had an iris. I always thought the iris was in the camera (like non-DSLR's have). Shows I know absolutely nothing about cameras
