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02-20-2019, 08:29 PM   #1
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FA20mm vs A20mm for shooting night sky landscapes


I've recently had my first chance to try out shooting the milky way with my K-1 and FA31 (got some good shots at F2.8, 50 seconds, ISO 1600), and found it to be quite addictive. While shooting I was left thinking, I wish I could squeeze a bit more into the shot, I thought about stitching shots together for a panorama but at 31mm, might be a bit too much distortion, might try it at a later time with a longer lens. In the meantime I thought about maybe getting a wider lens, it doesn't seem that Pentax make wide angle affordable primes at this stage, so I had a look at what's available used.
There seems to be a few of these available on Ebay Japan that look in reasonable condition. Is the FA20 worth more just because of the autofocus? For night landscape I pretty much just set to infinity manual focus anyway.
I had a look at the Rokinon 20mm, but I think it's a bit large for my taste, the FA31 is about the max physical size I like to deal with.


02-20-2019, 09:29 PM   #2
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I have the FA20, and the build is the worst thing about it. Not that it’s bad, but there is a little barrel wobble on mine (although it doesn’t seem to affect the optics) and it just doesn’t feel as nice in the hand as the earlier lenses. However, as it’s only 10g heavier than the A, I imagine the plastic construction was adopted to keep the weight down.

The pancake lens hood (for both the A and FA) is enormous compared to the lens itself, and many people seem to not bother with them. If you shoot only night skies with it, I’d buy the A, personally. The AF is handy, otherwise, and seems to work well. In a comparison test here on PF against the DA21, there doesn’t seem to have been much between those two, although that was shot on an APS-C body at the time.
02-21-2019, 03:10 AM   #3
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there's also a Sigma 20mm f1.8 EX DG available used that's a very capable lens, especially for night/low-light shooting...
02-21-2019, 03:19 AM   #4
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The FA is bloody good. I haven't shot much at night time though. The build is an issue though. My copy jammed up and needed a service to restore the AF and aperture.

I have a Sensei 67mm plastic screw-in petal hood for it. It works well and takes up much less space than the whopping saucer (which I also have).

02-21-2019, 06:41 AM   #5
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I use the Rokinon 20mm on a K1. I find it quite good, and not so large. But I suppose next to a FA20mm or A 20mm it may seem large.
02-21-2019, 06:53 AM   #6
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For astro I would think either of them will work equally well. I like my A for general use (haven't shot any astro with it), not really missing the autofocus on such a wide lens, and I'd think it's better dampened for maunal focus than the FA is (although I have never used the latter.)

The big saucer hood is rather cumbersome. I rarely bother with it and can't say I have noticed much of an issue with flaring or reduced contrast.
02-21-2019, 09:33 AM   #7
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I have a 20mm A that lying about. Also, a Sigma 15mm fish eye that dates from the 1970's or 80's not sure which as this particular model seems to have had a very short run. It is not the "filtermatic" version which I think replaced this model. I also have Rokinon fish eyes, both the APS-C and FF versions if you're interested. Either one has a wider FOV than the Sigma. PM if you're interested in any of these BUT, shipping to Australia could be expensive and slow as a wind-driven banana boat.
02-21-2019, 10:22 AM   #8
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I own a A Pentax 20mm, it is very sharp. On a K-1, it is recommended to carefully check the focus, sometimes I have been surprised to see, after transfer to my computer, that the background was not sharp enough. In a nutshell: do not trust vintage hyperfocal law, DoF is leaner than you expect!

I used to own a Soligor (also branded as Spiratone) 20mm F/2.8, it is far less expensive than the Pentax and mine was sharp once stopped down to F/4.

02-21-2019, 03:16 PM   #9
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Thanks guys, I'm going to Japan in a couple months so maybe I'll hold off and see if I can find something in a shop there before pulling the trigger.

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