Sigma 18-35/1,8 coma

Started Mar 7, 2014 | Discussions
chary zp Senior Member • Posts: 1,452
Sigma 18-35/1,8 coma

Is this coma? Does it make the lens unusable for star shots?

It goes away completely only at F3,2

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BillD7000 Contributing Member • Posts: 813
Re: Sigma 18-35/1,8 coma

chary zp wrote:

Is this coma? Does it make the lens unusable for star shots?

It looks like motion blur in the top shot, not coma...

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AksCT Regular Member • Posts: 210
Re: Sigma 18-35/1,8 coma

Calssic hallmakr of "coma" is when bright spots are off-axis. In both pictures, spreading is all over, most likely caused by motion blur.

Entropius Veteran Member • Posts: 4,222
Re: Sigma 18-35/1,8 coma

That's either motion blur or a hell of a bad lens. Probably motion blur.

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OP chary zp Senior Member • Posts: 1,452
Motion blur? Really?

AksCT wrote:

Calssic hallmakr of "coma" is when bright spots are off-axis. In both pictures, spreading is all over, most likely caused by motion blur.

Well, how come the "motion blur" is only present in the corners, namely going / in the top left corner and \ in the top right corner, and not present in the centre of the image? And how come that motion-blurred are only the stationary "sharp spot" lights and not the stationary buildings, for example? See the two church towers in the top left corner or the greenish flat windows in the buildings on the horizon on the top left corner.

If it isn't coma, what is this effect called? I seriously doubt it is motion blur, from stationary street lamps.

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OP chary zp Senior Member • Posts: 1,452
Re: Sigma 18-35/1,8 coma

Entropius wrote:

That's either motion blur or a hell of a bad lens. Probably motion blur.

Check my previous post (reply to AksCT) for full SOOC jpg. I'd say it looks like coma

http://intothenightphoto.blogspot.cz/2013/02/overcoming-coma-aberration-part-2.html

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dk76111 Junior Member • Posts: 45
Re: Sigma 18-35/1,8 coma

My take on this:

#1: Motion blur.  Everything moves in one direction and it looks like those "tails" are distributed evenly throughout the photo.  As in no difference from the edges to the center of the photo.

#2: OK, no tails.  No motion blur?

#3: Once again, no tails.  No motion blur.  However, for all three shots nothing is in focus.  Or I can't find it.

As for the "flare" perhaps with your exposure time you're "blowing out" the lights?

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kewlguy
kewlguy Senior Member • Posts: 1,932
Yes

That is coma aberration - and yes it will give you the same look with stars. You can stop down to f/2.8 though, not completely gone but still usable.

But - you don't have to spend $$$ on 14-24 - using any lens wideopen is never good for widefield AP. So, IMO, stick to the Sigma, stop down to f/3.2 AND buy a tracker. For this purpose, a Vixen Polarie should be great. it's also good up to 135mm (the longest I tried).

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