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05-22-2017, 08:27 PM   #16
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Listen up...

I just got my first 24mm lens there, it is 40 years old, a rare find, the SMC Pentax 24mm f3.5 (the K series from 1975-1977 I think it was made). I am a K series fanatic!

I tested it in the back garden there in my urban area of a city with 250,000 population.

I imported the images into Lightroom and zooming it 1:1, with the lens fully open wide at f3.5 the stars are pinpoint with the least coma I have seen in a wide angle.

READ CAREFULLY:

I scaled up the 1:1 crop from my 15" monitor and the full APS-C frame measures 54" (4 1/2 feet wide) x 36" (3 feet high) from a Pentax K30 crop sensor body.

Prints 4 1/2 feet x 3 feet wide print will look brilliant. You can see the sensor noise at ISO 1600 but I haven't processed it but the resolution is astounding!

Quite frankly I am astounded by the performance of this lens, wide open, f3.5.

I never bought it as an astro lens but there you go, and if I told you what I paid for it you might not feel too good.

The f1.4 label of most of these modern lenses is a marketing ploy deliberately sucking in punters thinking they will be able to use them at f1.4, with the exception of a very few very expensive lenses.

40 years ago Pentax made lenses that were fully useable (good) wide open with realistic maximum apertures.

I 'll try and post later in the week with this lens if I get down to the dark sky area the next few days.

HAPPY


Last edited by nocturnal; 05-22-2017 at 08:34 PM. Reason: Aberrations in my grammatical language :P
05-22-2017, 11:30 PM   #17
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Interesting. I'm not discounting the capabilities of the lens but I have a hard time seeing an f3.5 as an astro lens. Sure, with some "help" like astrotracer or a tracking mount, any lens can be fantastic. Hell, I was doing 4 minute exposures at ISO100 the other night just to try things out. However, that just cause more work too. I'm no Photoshop pro so stitching photos together from multiple samples just complicates things a bit.

I will disagree that a f1.4 label is important in the simple respect that sometimes you just need to gather more light in a limited amount of time. I'm perfectly happy with the 14mm f2.8 but it just struggles at times with a 20s exposure to avoid star trails but still capture enough light. Maybe my hopes were too high that the 24mm could exceed those results but that's all part of the game isnt' it? If it was easy, everybody would have their cellphones out taking fantastic astrophotos.

Funny side story, when I was at Yosemite, waiting for the sun to rise, a very nice "kid" (easily in his 20's) came up to me and wanted me to take his photo with an ipad. In the pre-dawn light, with is back to the breaking dawn, it was about impossible to get any image at all. However, it struck me that it wasn't that long ago that cellphone cameras were mostly a joke for anything where nowadays, they could honestly do a pretty damn fine job of capturing images (unless you are printing large(ish).
05-23-2017, 11:12 AM   #18
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Sorry, now you've lost me. How does a 24mm at f3.5 gather more light that a 14mm at f2.8? f3.5 is 2/3rds of a stop slower that f2.8. On top of that, a 24mm lens only allows for roughly a 13s exposure before star trailing versus 20s-25s (23s really) on the 14mm.
05-23-2017, 10:29 PM   #19
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QuoteOriginally posted by colonel00 Quote
Sorry, now you've lost me. How does a 24mm at f3.5 gather more light that a 14mm at f2.8? f3.5 is 2/3rds of a stop slower that f2.8. On top of that, a 24mm lens only allows for roughly a 13s exposure before star trailing versus 20s-25s (23s really) on the 14mm.
No worries... this is the biggest and most important item to learn for astro, well one of them!

Now for the maths:

((24/3.5)/(14/2.8))^2= 1.88x the aperture area, and hence 1.88x the amount of light gathered through the larger aperture area.

(24mm aperture diameter/14mm aperture diameter) squared = a 1.88 times bigger hole to let the light through coming from a smaller and more concentrated field of sky view.

Right, so we are gathering light through a bigger hole for a smaller area of sky, this allows you in Yosemite to gather more light, image fainter stars with a larger magnitude (larger means fainter), thereby you can record more stars/galaxies/nebulae etc. etc.

Yes, on an untracked mount you have less exposure time with longer focal lengths but don't you have astrotracer?

Untracked star trailing happens with all exposure times, it depends on how much you are zooming in (pixel-peeping), your sensor pixel count or how large your prints are. Ignore that 300/500 rule, that is only an acceptance for a small image or print, but like I said before, it depends on what you want, for example, a wall size print or a facebook photo. 😉

With say a 24mm or 50mm to image the full Milky Way, yes you will have to track and mosaic, but what you end up with and with practice/hard work will be superior imagery and a huge mega-pixel file to rival or beat digital medium format.

As to your lens, lenstip tested it and it wasn't good until f2.8/f4 and hence my earlier statements, you can read the analysis yourself...

Samyang 24 mm f/1.4 ED AS UMC review - Image resolution - LensTip.com

Maybe you should swap for a Samyang 14mm f2.8, that is what I've owned for a few years, and is excellent at f2.8 😁

Hopefully all this has helped you out even if you are still scratching your head, you certainly aren't the only one 😉

05-24-2017, 01:49 AM   #20
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Yes, I already have a 14mm f2.8. I was looking for something faster which is why I was trying this lens out. I've used astrotracer, did 4 minute exposures the other night in fact, but there is still something nice about the simplicity and authenticity of capturing an image without all of the stacking and blending and all that. There is also the time factor where in an ideal situation, you have the time to set up and take long exposures and multiple images but that isn't always the case.

I think understand what you are saying about light gathering. Maybe it's just coincidence on these two lenses but it mirrors pretty closely the ratio between at the "rule of 500" where it's a 13.8s and 23.8s exposure before the stars noticeably trail. Anyway, it's made me investigate and learn a little more today.

I've read the reviews about softness wide open. I just posted this thread to ask if what I was seeing was typical or not. As we know, quality control with the Samyang/Rokinon lenses can be a little dodgy so I was just hoping that others had use the lens for astro stuff and could confirm the same results or tell me that their copy did better. If my results are on par with findings from "accepted" lenses then so be it. But, as Clackers chimed in, his 24 was always sharp so it makes me question.

I also read this review which shows some 100% crops that don't exhibit the same weird bubble things (is there a name for this?) but then again, photos were taken on a Sony a7S.
http://www.lonelyspeck.com/rokinon-24mm-f1-4-ed-as-umc-review/

Last edited by colonel00; 05-24-2017 at 02:05 AM.
05-24-2017, 03:17 AM   #21
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Someone claiming a lens is sharp is subjective, sharp on internet sites at 800x600 maybe, not on a large print or pixel peeped.

The analysis shows it is soft unless stopped down and that is what you bought, f1.4 and other fast lenses came about for portraiture and for shallow depth of field for subject isolation, not for astro.

I have a 50mm f1.4 which besides having a lovely bright viewfinder is useless for astro. It is Ok for soft flower photography but as I have a superb Vivitar 28mm f2.0 for flower photography, it is sharp at f2.0 but I think the coma is bad outside the centre so useless for astro too but has bokeh to die for! 😃

The Sigma 35mm f1.4 Art lens is the best there is for sharp wide open with pretty low coma, if you want a light sucker that is kind of wide. Your Sam 14mm is good open wide but gathers less light due to the focal length, use your astrotracer with it. If you tried a proper tracking mount then these are far more hassle.

I wouldn't trust a word of that site you mention, they get paid for marketing products like f1.4 lenses for astro which as you found out don't fit the bill. On the other hand one can't argue with mathematics or physics. Creating superb astro images takes a lot of work and knowledge. Good luck with the reading and be aware there is a lot of convincing BS on the internet. Having a strong maths/physics background helps one decifer the BS out there! 😎

---------- Post added 05-24-17 at 11:43 AM ----------

Also...

Consider the Pentax plastic fantastic DA35 f2.4, it is sharp wide open at f2.4, the analysis shows it and some Pentaxians use it for astro, look it up on lenstip etc.

I would get one myself except I have the DA40 f2.8 Ltd which is a great lens for astro, sharp open with little coma.

I like small light lenses too, one reason being I hike up mountains to get the photography I want. No point carrying monster lenses about which cost megabucks and can't use them at f1.4/f2.0 etc.

Stick the astrotracer on which is tiny too and there you go, the glory of Pentax, no BS! 😃
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