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Solar Gravity 2018

Author Topic: Lens Issues  (Read 557 times)

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Offline philrosenberg

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Lens Issues
« on: 15:23:49, 20 January, 2017 »
Hi
I have just started thinking about astrophotography and have purchased a second-hand Rokinon 8mm fisheye for my Canon 40D with the intention of using it for some wide angle Milky Way and star trails shots.

I wondered if anyone has any experience with this lens. I seem to be getting very bad coma over the whole frame. I can't find a huge number of examples using this lens, but what I have found indicates that there is an aspherical element and coma is well constrained.

I've included a test image. I've only had the lens a week and the only clear night I've had was with a full moon and I didn't have time to get too far out of the city (It's about an hour's drive for me to get good dark skies). I also haven't cleaned up hot pixels. But the quality is good enough to see the obvious coma. I'm rather disappointed and considering trying to send it back if possible.

One possible issue with this lens that I found out since taking the shot is that infinity is very badly calibrated and often people need to shoot with the focus at a few metres to get the best infinity focus. Could this be the issue? I would have thought this would have resulted in general blur, rather than coma? Plus the depth of field on this lens is huge so actually it shouldn't bee too bad an effect.

I just wondered if anyone had any thoughts. Is this the quality I should have expected? Is there a way to improve it in post processing?

Thanks

Phil
IMG_9777 by philip_rosenberg, on Flickr

Offline Einari

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Re: Lens Issues
« Reply #1 on: 16:03:24, 20 January, 2017 »
Stars are very critical test for any lens. I'm sure your lens would be fine for daytime use.
Also Rokinon/Samyang lenses seem to have large variance of quality.
Might help if you would stop down your lens few stops.
Tapio

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Offline chris.bailey

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Re: Lens Issues
« Reply #2 on: 17:06:21, 20 January, 2017 »
You have to spend a lot of money to get a well corrected fish-eye but that does look particularly bad. Something around 15mm would be more forgiving.

Chris
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Offline Nomis Elfactem

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Re: Lens Issues
« Reply #3 on: 20:38:17, 21 January, 2017 »
That looks to me like one of the elements is misaligned/tilted slightly in the north/south axis... does it look as though it's taken a knock ?

S.
Simon

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