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03-03-2019, 09:00 PM   #1
Jay Joslin
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Recommended Lens for K3-II Astrophotography Produced Anything But Stars

Rokinon 8mm F3.5 Ultra Wide Aspherical Fisheye Lens for Pentax

This was the lens that, for better or worse, was recommended as the best pairing for under $1K. Hoo boy. Results were very out of focus, despite all of my efforts when shooting in a great dark sky location in Puerto Rico. Like fishing, I took my time and enjoyed the occasion so I wasn't too put off at the time. But after getting home and trying everything, every single shot looks like it was shot through gauze on an astonishingly poor quality cell phone. In fact, I was able to get some shots with my iPhone of the bright starfield, but not this great camera (which is brilliant in all else with the kit lens).

Having enjoyed successful sessions with other cameras (Sony Alpha, Samsung NX), I know this camera can do it. Astrotracer was correctly calibrated, and all settings as were recommended in other threads were meticulously set. Then, when success was clearly elusive, I gingerly modified the focus to just below infinity, F-stop, bulb to various other increments. All produced abysmal results.

What could've gone wrong?

Thanks!

03-03-2019, 09:54 PM   #2
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Did you purposely use a focus of less than inifinity? Stars are all at infinity, you shouldn't use less than unless the markings on your lens are off(which they can be).
But for stars you should focus by using live view to zoom in on a bright star. Adjust the focus until you the star is as small and sharp as possible...that's what you should consider infinity.
Also some of the reviews I saw of this indicated that it was soft until f5.6, so if you were shooting wide open, that also might be part of the issue.

I'd choose the 14mm or 16mm Samyang's over the fisheye for astro.

Last edited by SteveinSLC; 03-03-2019 at 09:56 PM. Reason: Additional comment
03-03-2019, 11:03 PM   #3
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Your best bet would be to use live view magnification to verify correct focus.

Also, with a lens as wide as the 8mm fisheye, the astrotracer will likely produce blurry corners and only allow for relatively short exposures (perhaps in the 15-30 second range). A bit of trial-and-error would definitely be the way to go in ensuring that the focus is correct, and the exposure time reasonable.

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03-04-2019, 11:00 AM   #4
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In daylight with Live View and LV magnification focus on something as far away as you can see.

Tape the focus ring at that point using "gaffer's tape" on some other type that doesn't leave a lot of residue.
Use just enough tape that you won't jostle it of position in the dark.

I do this with any lens that I want to use for stars, etc.

03-04-2019, 12:29 PM   #5
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Samyang 16mm f2 would be the best astro lens you can buy for pentax.
03-04-2019, 01:53 PM   #6
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QuoteOriginally posted by Trickortreat Quote
Samyang 16mm f2 would be the best astro lens you can buy for pentax.
Or the 24/1.4.

Though the 15-30 and perhaps the 11-18 may also be equally good, though more expensive and in the case of the 15-30, much larger.
03-05-2019, 12:28 AM   #7
Jay Joslin
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Thanks everyone! Duh! Why didn't I think of gaffer's tape? Probably because I was in Puerto Rico and actually trying to vacation vs fixation!

Couldn't find a Samyang 16mm in a K-Mount, though they do have a 14/2.8. Rokinon has a 16, but aren't they about the same? And good to know that I can be disabused of the whole fisheye notion.

I think I was trying to futz with focus as trial and error as infinity was producing the gauzy effect just as much as a step back - just figuring that something could've been off.

Thanks everyone! I was a little intimidated that as a relative noob to K-Mounts and getting astro right, I'd fall all over myself. You are all incredibly kind.
03-05-2019, 12:31 AM   #8
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Yup, Rokinon and Samyang are the same. Which is marketed where varies, but the product doesn't.


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03-05-2019, 12:38 AM   #9
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QuoteOriginally posted by SteveinSLC Quote
Or the 24/1.4.

Though the 15-30 and perhaps the 11-18 may also be equally good, though more expensive and in the case of the 15-30, much larger.
Hmmm.. i find 24mm to be to wide for landscapes.
Id rather go with sigma 18-35mm f1.8 and use it at widest end
03-05-2019, 06:58 AM   #10
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My 8mm FE only considerably sharpens up at f/5.6. And I had to significantly adjust the focus scale (screw under the focus ring rubber clamps it).
Maybe yours is similar.

My 12mm FE however is sharp across the frame (K-1) even fully open at f/2.8.
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