Astro 20mm 1.8G Astigmatism? w/ Images

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nhr5005 Junior Member • Posts: 36
Astro 20mm 1.8G Astigmatism? w/ Images
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Hi All,

I recently acquired a used, but near mint condition, 20mm 1.8G lens. For wide angle shots I usually use an 18-35G but I wanted something faster for an upcoming trip where I might get to see the northern lights. I don't shoot a ton of astro stuff, which is why I looked for a deal on a used copy of this lens vs. buying a new one or a wider manual focus 3rd party lens. I also know the Z mount is supposed to be way better, but even if I switch to Z in the future, I don't think I shoot astro stuff often enough to justify purchasing that lens. I'm shooting on a D750 by they way.

Anyways--I got to test out its astro capabilities last night in a nearby state park. I have to say, I am initially pretty disappointed. The corners display what appears to be a tangential or sagittal astigmatism that is visible in the image on some stars even without zooming in to 100%! It's not just the corners either, I'd say this easily creeps into the image about 1/4 of the way on each edge which is hugely frustrating.

I can't say I nailed infinity focus here, but I tried several focus positions, UV filter and no filter, different shutter speeds, etc and the only thing that got these to go away was stopping down to about f4. That's all fine and good, but if I wanted to shoot at f4, I would use my 18-35 @ f3.5, which is wider and does not have this optical issue! By no means was this a dark sky location (it's about an hour from the city), so I can only imagine this issue would be WAY worse if the image contained even more stars or the milky way?

Did I get a dud copy here, or is this a known issue with this lens that I overlooked? (Kicking myself :-(...). I was just shocked to see this considering the pretty decent shots I get from my 18-35 vs. a fast prime lens.

Attached are two photos from last night, JPEG but exported straight from camera raw with no edits and no lens correction filters applied. One at f1.8 and the other at f4.

f1.8 shot w/ astigmatism. Ugh!

Seems to go away at f4.

 nhr5005's gear list:nhr5005's gear list
Nikon D90 Nikon D750 Nikon AF-S DX Nikkor 16-85mm f/3.5-5.6G ED VR Nikon AF-S Nikkor 50mm F1.8G Nikon AF-S Nikkor 70-200mm F4G ED VR +4 more
Nikon D750
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