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11-25-2012, 01:11 AM   #1
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DA* 16-50: too much coma for astrophotography?

I want pinpoint stars throughout the frame. I don't care about barrel distortion but coma (non-round stars near edges) is a concern. Lenstip.com regularly tests for coma but they haven't tested the 16-50 yet. Their Polish site has a larger lens database and found plenty of coma for the 16-50. Test Pentax smc DA* 16-50 mm f/2.8 AL ED IF SDM - Koma i astygmatyzm - Test obiektywu - Optyczne.pl

Comments regarding the 16-50 and the night sky appreciated.

11-25-2012, 01:43 AM   #2
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I don't have the 16-50 but I'd that link results are correct I wouldn't use that lens. That being said if you printed it 8x10 or smaller you would have to look really closely to see coma distortion. Another thing to consider as I printed off a photo with large amounts of coma. Not one person on 12x16 ish print asked me what's with the slightly longer stars off centre of the page. Your average person looking doesn't pixel peep send or know what coma distortion is.
11-25-2012, 04:36 AM   #3
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Is there a wide angle zoom lens with no coma?
You most likely need to look for a prime i think.
11-26-2012, 08:28 AM   #4
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QuoteOriginally posted by Anvh Quote
Is there a wide angle zoom lens with no coma?
You most likely need to look for a prime i think.
Exactly my thoughts. Coma is an inherent feature of many zooms, especially wide angle ones. Prime lenses are much better suited for astrophotography - you still would need to stop down one or 1.5 f-stops.

Ben

11-26-2012, 09:39 AM   #5
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Primes don't guarantee freedom from coma. My FA 43 shows a severe amount and it starts just a little bit away from center. There aren't any WR primes, either.

I was considering the 16-50 because it's the widest and fastest WR lens. WR let's me run the interval timer without worrying about dew ruining my camera. For now I guess I will stick to the 18-55 WR. The 18-135 isn't enough of an upgrade for me. If the roadmapped DA* 16-80 or another WR option comes out I will reevaluate.
11-26-2012, 11:13 AM   #6
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QuoteOriginally posted by DeadJohn Quote
The 18-135 isn't enough of an upgrade for me. If the roadmapped DA* 16-80 or another WR option comes out I will reevaluate.
I doubt a longer zoom will have less coma, usually coma isn't on the top priorities of a photogrpahic lens designer.

the DA*55 might be a good bet, it's WR and this focal length is usually pretty easy to design with minimum aberrations.
11-26-2012, 11:26 AM   #7
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QuoteOriginally posted by bdery Quote
the DA*55 might be a good bet
Unfortunately, that doesn't seem to be the case:

Pentax smc DA* 55 mm f/1.4 SDM review - Coma and astigmatism - Lenstip.com
11-26-2012, 02:28 PM   #8
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QuoteOriginally posted by DeadJohn Quote
Primes don't guarantee freedom from coma. My FA 43 shows a severe amount and it starts just a little bit away from center. There aren't any WR primes, either.

I was considering the 16-50 because it's the widest and fastest WR lens. WR let's me run the interval timer without worrying about dew ruining my camera. For now I guess I will stick to the 18-55 WR. The 18-135 isn't enough of an upgrade for me. If the roadmapped DA* 16-80 or another WR option comes out I will reevaluate.
Your reasoning is flawed.

Dew will first form on the front glass, and you wont have usable pictures, no WR label on a lens will prevent this.
Dew will not ruin your lens or camera, long before the rest of your equipment is soaking wet, your front glas will be fogged, and if that happens, just don't store your wet equipment, let the dew first evaporate.
When using a fast non WR prime, you can expose much shorter than with the slow 18-55, so less time to get dew on the lens.
When you do not want dew, use lens heating, and preferable an old metal prime, which distributes this heat much better than a plastic AF lens.

11-26-2012, 02:49 PM   #9
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I have the DA 14 and tried it once last summer:


Even the stars are brighter :)
This is a 16:9 crop of the top part of a single image, so it`s basicly the whole frame with just the lower side cut off. The vignetting is in fact lightpollution but you can see some brighter stars on the right display some distorsion. Not sure if that is coma or something else.
11-26-2012, 10:03 PM   #10
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QuoteOriginally posted by glasbak Quote
Your reasoning is flawed.

Dew will first form on the front glass, and you wont have usable pictures, no WR label on a lens will prevent this.
Dew will not ruin your lens or camera, long before the rest of your equipment is soaking wet, your front glas will be fogged, and if that happens, just don't store your wet equipment, let the dew first evaporate.
When using a fast non WR prime, you can expose much shorter than with the slow 18-55, so less time to get dew on the lens.
When you do not want dew, use lens heating, and preferable an old metal prime, which distributes this heat much better than a plastic AF lens.
I agree that front glass fogging ruins photos before the entire camera is soaked, but I want the wide angle for time-lapse and star trails where the intervalometer is running unattended for a long time. I generally don't try on dewy nights but dew can't always be predicted. There have been nights when I come back to a dripping camera; the photos are unusable but I still don't want moisture getting past a non-sealed lens.
11-27-2012, 04:08 AM   #11
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QuoteOriginally posted by DeadJohn Quote
I generally don't try on dewy nights but dew can't always be predicted. There have been nights when I come back to a dripping camera; the photos are unusable but I still don't want moisture getting past a non-sealed lens.
Then simply put a plastic bag over the camera and leave the front lens clear, tape the bag to the lenshood or something like that.
11-27-2012, 08:54 AM   #12
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QuoteOriginally posted by DeadJohn Quote
Primes don't guarantee freedom from coma. My FA 43 shows a severe amount and it starts just a little bit away from center. There aren't any WR primes, either.
But primes are often better when it comes to aberration, it's as simple as that.
Also the faster the lens the more aberration most of the time.

Pentax smc DA 15 mm f/4 ED AL Limited review - Coma, astigmatism and bokeh - Lenstip.com

Pentax smc DA 21 mm f/3.2 AL Limited review - Coma, astigmatism and bokeh - Lenstip.com

Pentax smc DA 35 mm f/2.4 AL review - Coma and astigmatism - Lenstip.com


DA*16-50 stopped down might be decent though... since the examples are wide open
Test Pentax smc DA* 16-50 mm f/2.8 AL ED IF SDM - Koma i astygmatyzm - Test obiektywu - Optyczne.pl
11-27-2012, 08:57 AM   #13
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That DA 35 F2.4 has fantastically low coma and astigmatism btw.

But yeah, why not stop down 1 stop?
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