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05-19-2017, 09:50 PM   #16
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Hi, I know this is a bit late but there is no mention of 'clear aperture' values between lenses.

For example, the Pentax 31mm Ltd @ f2.8 will allow 4.9x the amount of light through than say the Samyang 14mm @ f2.8 for the same exposure time.

Obviously, you would need to stitch 2 images of the 31mm together to roughly achieve the same field of view but... And a big but is... it will be a superior image!

I have the Samyang 14mm and it is quite good if one cannot be bothered with stitching and without tracking, but I have learnt that UWA lenses are not the way to go if one is getting seriously into astro due to the clear aperture issue.

You could even use the £25 Pentax-M 50mm f1.7 @ f2.8... and stitch 3-4 together. (12.75x the amount of light for same f number and same exposure time than a 14mm lens).

And with astrotracer for Pentaxians this is all the more possible to use longer focal lengths. I just bought the O-GPS1 for my K30 which has the fabulous Exmor sensor (most excellent for astro). It must have good dark current suppression technology built in, even if it is 4-5 years old.

Read Roger Clark's astro site if you have a few months to spare 😉

I would love to know more data (any?) on Pentax dark current tech for various models. 😎

09-18-2017, 04:39 AM   #17
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SMC Pentax-F 28mm F2.8

Actually, I mainly use my K-1 for landscape with two lens
Panagor 24mm f2.5 and my favorite lens SMC Pentax-F 28mm F2.8. I was very lucky to get a copy of this lens, It sharp right from f2.8. I shoot a lot of cityscape, city night, and long exposure stuff. Sample image below is at f11, focus at infinity and it is very sharp.

Last edited by anakhaled; 09-19-2017 at 03:14 AM.
03-13-2018, 06:51 PM   #18
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QuoteOriginally posted by nocturnal Quote
Hi, I know this is a bit late but there is no mention of 'clear aperture' values between lenses.

For example, the Pentax 31mm Ltd @ f2.8 will allow 4.9x the amount of light through than say the Samyang 14mm @ f2.8 for the same exposure time.

Obviously, you would need to stitch 2 images of the 31mm together to roughly achieve the same field of view but... And a big but is... it will be a superior image!

I have the Samyang 14mm and it is quite good if one cannot be bothered with stitching and without tracking, but I have learnt that UWA lenses are not the way to go if one is getting seriously into astro due to the clear aperture issue.

You could even use the £25 Pentax-M 50mm f1.7 @ f2.8... and stitch 3-4 together. (12.75x the amount of light for same f number and same exposure time than a 14mm lens).
I wish I looked into clear aperture before dropping the cash for a da*16-50 for astro. That math works out to a worse astro performance rating than your 14mm for significantly more.

Now I have been debating the 14,16,20 and 24 Sam/Rokinon lenses for a few days and just canít decide. If you broke your 14 today, which would you buy and why?
03-14-2018, 11:48 AM   #19
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QuoteOriginally posted by Greenneck Quote
I wish I looked into clear aperture before dropping the cash for a da*16-50 for astro. That math works out to a worse astro performance rating than your 14mm for significantly more.

Now I have been debating the 14,16,20 and 24 Sam/Rokinon lenses for a few days and just canít decide. If you broke your 14 today, which would you buy and why?
Your 16-50mm should/does have a higher clear aperture than a 14mm both at f2.8. The problem with the Da* 16-50 zoom is that wide open it probably suffers from heavy coma/astigmatism, whereas the Samyang 14mm f2.8 is excellent in this aspect, providing it is a good copy.

There is no way I'd have bought the DA* 16-50 for astro and I have a somewhat similar DA 20-40 f2.8-f4 Ltd for landscapes (superb lens but haven't tried it for astro as yet).

The way to go for astro is a K1 full frame with a Sigma 35mm f1.4, nearly the same FOV as a 24mm lens but with much greater clear aperture. This is the advantage of full frame or 645D or 645Z medium format as you can use longer focal lengths. All of this well beyond my budget!!!

03-14-2018, 03:34 PM   #20
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Proper focal length for your target is more important than clear aperture IMO. 35mm is tight for landscapes with a Milky Way background. Stitching the sky is okay, but foregrounds get tricky if stray light hits the scene and changes between frames.
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