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01-10-2019, 04:24 PM - 1 Like   #16
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QuoteOriginally posted by jawats Quote
So would it then be better just to experiment with my Tamron 28-75 2.8 and see what I need after tooling around with it for a time?
Yes, with that lens you have a good base to experiment with. You can also stitch, to get something wider. I shot this with the K1 and the 15-30/f2.8 @ f2.8 and 15mm. It's 3 images stitched together.You can also look at old Olympus Zuko, Contax Zeiss, Leica R and Nikon F mount lenses that you can swap mounts using remount kits from Leitax.com



01-10-2019, 04:34 PM - 1 Like   #17
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I,ve done some photos using astro tracer on K1 with FA 28-105, f 17-28 fisheye and while the fisheye worked well I prefer the non-fisheye look. At 28mm on the K1 I got some nice shots but I'm looking for something wider, right now I'm looking at a Sigma 20mm f1.8 EX DG or a tamron 17-35 dg f2.8-4 can't decide so I'd be interested in hearing from "peeperberry farm" about the Sigma 20mm. I want a good all arounnd landscape lens that will work well for astro as well.

I am finding that I have to cleanup the purple fringing in some of the older lenses.
01-10-2019, 04:44 PM - 1 Like   #18
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what little I have been able to get out and shoot with the Sigma 20:

Sigma 20mm f1.8 EX DG Aspherical RF | Flickr


I am not an astrophotographer....
01-10-2019, 05:18 PM - 2 Likes   #19
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Thanks for the link "pepperberry" they are very nice, they almost have a 3D look I'm now leaning toward that lens, I just looked at them on my phone I'll take another look later tonight on a larger screen. I think it would work great for astro, but might be a bit of larger lens than "jawats" wanted

01-11-2019, 07:53 AM - 1 Like   #20
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QuoteOriginally posted by Garry Conway Quote
I,ve done some photos using astro tracer on K1 with FA 28-105, f 17-28 fisheye and while the fisheye worked well I prefer the non-fisheye look. At 28mm on the K1 I got some nice shots but I'm looking for something wider, right now I'm looking at a Sigma 20mm f1.8 EX DG or a tamron 17-35 dg f2.8-4 can't decide so I'd be interested in hearing from "peeperberry farm" about the Sigma 20mm. I want a good all arounnd landscape lens that will work well for astro as well.

I am finding that I have to cleanup the purple fringing in some of the older lenses.
I have decided to purchase a Tamron or Sigma 17-50 f/2.8, based upon the following:

1. I have a Tamron 28-75, and that's not quite wide enough for astro.
2. I want something that doubles as a good indoor lens for sports events (TKD - close-up), and these fit the bill.
3. The rating of the Sigma and Tamron surpasses any of it's closest competitors.
4. I can get it for a decent price, depending upon which one I get and when.
01-11-2019, 10:57 AM - 1 Like   #21
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Plus one for a good copy of Samyang/Rokinon 14mm or 10mm. f2.8 is good enough when field is much wider than 18mm on a Sigma or Pentax zoom...
01-11-2019, 12:06 PM   #22
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QuoteOriginally posted by mtgmansf Quote
Plus one for a good copy of Samyang/Rokinon 14mm or 10mm. f2.8 is good enough when field is much wider than 18mm on a Sigma or Pentax zoom...
If I understand you correctly, you think that good copies of S/R 10mm or 14mm would be good for astro, as others have suggested. However, if I get a Sigma or Tamron 18-50 lens, f/2.8 would be fine for astrophotography? Let me know if I am correct here. Thank you!

01-11-2019, 12:21 PM   #23
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QuoteOriginally posted by jawats Quote
If I understand you correctly, you think that good copies of S/R 10mm or 14mm would be good for astro, as others have suggested. However, if I get a Sigma or Tamron 18-50 lens, f/2.8 would be fine for astrophotography? Let me know if I am correct here. Thank you!
I do not know any astro-photographers that shoot at 18mm and above, but others may prove me wrong...
01-11-2019, 12:47 PM   #24
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QuoteOriginally posted by mtgmansf Quote
I do not know any astro-photographers that shoot at 18mm and above, but others may prove me wrong...
Reading through some of the lists on sites like:

Lonely Speck’s Ultimate List of Best Astrophotography Lenses – Lonely Speck

The Best Lenses for Night Photography: A Case for Rokinon Primes - BorrowLenses Blog

Best Lenses For Milky Way Photography

How-To: Picking a Great Lens for Milky Way Photography

I see the Samyang / Rokinon / Bower primes mentioned quite often, esp. in lengths of 10mm, 14mm, 16mm, 20mm, 24mm, 35mm

In terms of zooms, I see:

Tokina AT-X 11-20mm -
"" 14-20mm
"" 11-16mm
"" 15-30mm

Sigma 18-35 Art

Not many of these have Pentax mounts, or they are re-badged under different brands. The S/R/B primes may be the best way to go. Apparently, at least some astrophotogs use the range from 10 - 35 as workable in astro.
01-11-2019, 02:12 PM - 1 Like   #25
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Reading through these links, 16mm for aps-c is the narrowest recommended focal length for your aps-c Pentax (24mm equivalent for FF) unless you plan on doing a great deal of stitching and many shots. Personally, I rather crop a little rather than stitch a whole lot. You did say you wanted a prime and not a zoom? I forgot to mention the excellent 16mm Samyang at f2, but I still do not consider that wide enough and not too far off 18mm. Maybe you can rent some zooms and primes to help you find what you are looking for. Lastly, the 15-30mm Pentax is very big, heavy, and really made for FF cameras. Also, 24mm and 35mm lenses would only be on a list for full-frame shooting and not for your aps-c Pentax. As always, there are exceptions to the rule, so I am just pointing out general guidelines - as do your links...
01-11-2019, 04:37 PM - 2 Likes   #26
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My personal favorite is the Vivitar 28mm f2 'close focus'. The Komine made with a serial numbers beginning the the 28 i.e 28xxx. Vivitar (Komine 28xxxx) 28mm F2.0 MC Close Focus Wide Angle Lens Reviews - Vivitar Lenses - Pentax Lens Review Database It's sharp and fast, although if you're using some sort of tracking device fast isn't that important. I don't really care for going much shorter than that focal length because the distortion in the corners makes it hard to stitch frames. Here's a few of my night images taken with the Vivitar.























With a little patience (and luck) you can pick one up for about $100.
01-11-2019, 05:22 PM - 1 Like   #27
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QuoteOriginally posted by mtgmansf Quote
I do not know any astro-photographers that shoot at 18mm and above, but others may prove me wrong...
I use the Sigma 18-35/f1.8 with my K5IIs and the 15-30/f2.8 with the K1 - mainly because they provided the best balance between the physical aperture area (amount of light collected) and the field of view. Also, since they are zoom lenses, I can take advantage of the longer focal length which doubles the light collecting capability.

If you go too wide, you loose a lot of light. There are a lot of folks shooting with either the 24/f1.8 or 35/f1.4 in order to get the additional aperture area. The problem becomes how wide can you go with out loosing too much light?

I understand that the OP has a tracking mount where he can get several minutes of tracking, however the difference between a 10mm and an 18mm is about 7.8x. That's a lot of light to be missing.

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01-11-2019, 05:36 PM   #28
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QuoteOriginally posted by interested_observer Quote
I use the Sigma 18-35/f1.8 with my K5IIs and the 15-30/f2.8 with the K1 - mainly because they provided the best balance between the physical aperture area (amount of light collected) and the field of view. Also, since they are zoom lenses, I can take advantage of the longer focal length which doubles the light collecting capability.

If you go too wide, you loose a lot of light. There are a lot of folks shooting with either the 24/f1.8 or 35/f1.4 in order to get the additional aperture area. The problem becomes how wide can you go with out loosing too much light?

I understand that the OP has a tracking mount where he can get several minutes of tracking, however the difference between a 10mm and an 18mm is about 7.8x. That's a lot of light to be missing.

Good chart! Thank you. None of it seems to reference the Sigma or Tamron I was examining. Is there a formula out there?
01-11-2019, 07:44 PM - 1 Like   #29
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QuoteOriginally posted by jawats Quote
Good chart! Thank you. None of it seems to reference the Sigma or Tamron I was examining. Is there a formula out there?
We can come up with the numbers for the Tamron or Sigma 17-50 f/2.8. None of the numbers are brand specific. Only the focal length and aperture is used.
  • First we calculate the aperture diameter which is aperture diameter = (focal length)/aperture. Lets use 17 @ f2.8 the short end of the lens. In this case we get 17/2.8 = 6.07
  • Next we use the just calculated aperture diameter to determine the area of physical aperture.
QuoteQuote:
The area of a circle is the amount of space the circle covers. The formula for calculating the area of a circle is A = π*r2 where pi (π) equals 3.14 and the radius (r) is half the diameter.

Or converting this to use diameter we get the following formula for calculating the area of a circle is A = π*(d/2)2 or we can further refine this down to A = (π*4)*(d)2
  • So the area is calculated to be (3.14*4)*(6.07)2 = 28.94
  • The Light Collection is just the exposure time * the area. I included the light collection because I was comparing using various lenses across using the AstroTracer (with times of 60 to 90 seconds in order to limit corner distortion) vs a tracking mount (with time up to say 5 minutes). Personally, I decided to go with less equipment and thus the AstroTracer. It provides me with plenty of light collection and looks very natural. With a tracking mount you collect a lot more light, and it can become over done.

01-11-2019, 07:54 PM - 1 Like   #30
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Samyang 14mm 2.8 is awesome. Just make sure you get a copy that isn't decentered like mine was.
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