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08-09-2020, 02:21 PM   #1
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Wide primes for astro

I've been doing quite a few backpack trips to do astrophotography lately and I see myself doing more. I have the Pentax 15-30 for when I'm not backpacking and it does great. But I want to hike lighter than that lens will allow so I currently carry a Rokinon 14mm f/2.8 for that kind of thing. My last few outings, I just can't seem to get the stars as sharp as I'd like. I've tried various methods with live view and just looking at the last photo in full zoom but it's always just pretty close. I'm sure part of this is user error but I'd like something to help cut down on that error. All the way until the focus ring stops seems to be past infinity on that lens too. I'm considering other options.
I was thinking the Iris Firefly sounded pretty interesting and that has a click at infinity as well as a locking focus ring which could come in handy. Reviews are good but not all gushing.
So I'd like to know how that lens is generally and specifically for big starscapes.
I'd also love to hear about any other lenses I should consider. I want the best quality I can get without it being especially big, heavy, or hard to operate in the dark when I'm tired. Needs to be full frame.
What do you think? Thanks.


Last edited by mattb123; 08-09-2020 at 02:32 PM.
08-09-2020, 04:02 PM   #2
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I have the Firefly and the features you described are handy. It did take a little bit of time to get make the adjustments for infinity focus. Specific to your question about how it works in large starscapes, Astro is the next area of photography that i want to learn so I can't add to that question. I've been spending a lot of time learning on the Lonely Speck Youtube channel. They sell a filter to help with finding focus on stars. I don't have it but the husband/wife team seem like good people are all about Astro. It is less expensive than a new lens.
08-09-2020, 06:57 PM - 1 Like   #3
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you may want to check youtube. There are instructions to get the lens adjusted so that the infinity is equal to the final stop. Maybe that helps.
this :
could be it.
08-09-2020, 08:03 PM   #4
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I had to adjust the infinity stop on my Rokinon 14/2.8 because it would hit the infinity stop before I got good focus. It took a couple of tries. Don’t worry about getting the setting exact, but make sure you can get focus without hitting the stop because the next step is to check the focus.

My first advice is NOT to use starts to check you infinity focus. I live near a coastal river with a draw bridge. The bridge has bright red and green channel lights mounted to the bridge, so I set up on a tripod 1/4 to 1/2 mile away at night and take a series of pictures at different focus distances. I make a note of what focus distance goes with each shot, then I upload the shots to my computer and check the focus of each shot. It is hard to see on the camera screen. You should see the dot of light get smaller as the focus improves, then as you go past infinity (if possible) the dot of light will start getting bigger. You can also use a street light, flashlight or the headlight of a car from far away. Other people have used a laser pointer mounted on a tripod shining on a rock or paper far away to focus on. You can use a long exposure at low ISO to get a clear shot and check exposure from close to far. Everything is stationary so you don’t need Astrotracer to check focus. Then just use the exact same focus distance for you astro shots later. (Focus distance may vary with temperature, but I have never noticed that much variation.)

It is possible to use a street light to check focus looking at a zoomed-in image on the back of your camera. Take a series of shots, zoom in and compare them. The one with the smallest dot of light is best focus. With a big light like a street light, you can see the difference on your camera back.

For my Rokinon 14, I stop down to f3.5, because I get some “blooming” with bright stars at 2.8 f-stop. For my Rokinon, my infinity focus is about 1/4” past the infinity mark. I put it at that mark and shoot. Make sure to check it occasionally... it is very easy to move the focus ring when adjusting the camera angle. Some people use tape to hold the focus ring.

The Samyang/Rokinon 14 can give great shots if you have a good copy and it is properly focused. If you use Astrotracer, you will get some distortion in the corners in the form or radial star trails. This is not a calibration issue or they would follow a semi-circular path. All ultra-wide lenses I have seen do this.

The Samyang/Rokinon 14 gives similar astro results To the DFA 15-30. I tested them head-to-head. I have never tried the Iris Firefly. I shoot with the K-1 full-frame.

---------- Post added 08-09-20 at 08:06 PM ----------

By the way, the Pentax FA 20/2.8 is not a good astro lens. Too much coma. Nice for daytime shooting.

08-10-2020, 10:13 AM   #5
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A friend of mine used to own the Irix 15mm he shot with a Nikon FF camera. Nice lens, definitely worth considering. I have the 14mm Rokinon, feel similar to you.

Ha, if you can find one you could try the Zenit 16mm, but I don't think it's very hot wide open. Mine is a super start at f/8 or f/11.
08-10-2020, 11:59 AM   #6
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I don't own any of the mentioned lenses but on the ultra wide prime end have the Laowa 12mm f/2.8 Zero-D and it has been a good performer even wide open. It gets perfect at f/4 but even wide open there is only some very slight coma and if doing things like stacking it cleans up easy enough and even if not itsn't really noticable unless one pixel peeps.

As far a focusing ultrawides at infinity I find getting a really bright star centered is about the best one can do. I use live view with focus peaking turned off as that just bloat the star so that way I can tell when the star is minimized. Also if you have a dimmer star near in magnified live view I've found that when perfect focus is achieved those dim stars appear mostly on instead of as flickers. Beyond that I don't know of a good way to ensure proper focus especially with modern ultrawides being fairly long things will shift as the temp changes which can and does throw things off. Here is what the Laowa 12/2.8 can do wide open, I didn't really like how the colors turned on on this processing attempt (that is on me not the lens) but it does show its sharpness wide open.
08-10-2020, 12:15 PM   #7
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After a bit of research, I bet I'm just way over-shooting infinity. I'm going to do some more methodical testing and see if that may be the case. I didn't expect the markings to be as far off as everyone is saying they are.
08-10-2020, 02:14 PM   #8
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I'm probably going to say some stuff you already know, but some of this will hopefully be new.

There are many reports of Samyang/Rokinon/Bower lenses suffering from decentering quality issues. Fortunately for you, their 14mm seems to be more reliable than the 24mm. My first 14 was good but I had to return three 24's. Bloated, fuzzy, yet generally round stars are a focus or tripod shake issue. Triangular fan-shaped stars or curving halos are a lens problem.


Some lenses don't have a perfectly flat field, so I find a star around 1/3 of the way between the center and the corner as my focus guide. That gives a good compromise for sharpness across the frame. (If there's no good star 1/3 of the way out, reaim the camera to get a star in that spot, focus, then recompose.)


Focus with maximum liveview zoom. Bright stars are good for rough focus, but too-bright stars might look good even when slightly out of focus. That's why I prefer dimmer start to confirm focus. Ideally, find a star that only shows when in perfect focus, and fades away (because the light becomes too spread) when off.

08-10-2020, 09:51 PM   #9
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Actually I think my lens may not go all the way to infinity now that I tested. Just trying to focus on a daylight distant object using Live View I could never get it truly sharp. I'll try that calibration method when I get a chance.
Darn, would have been easier if I was going past infinity like I thought.
08-11-2020, 07:11 PM   #10
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Similar to MainNative I find the Rokinon 14mm a little 'soft' at 2.8 and usually stop down to 3.2. or 3.5 to get sharper results.

Surprisingly my preferred lens for astro-photography is the Sigma 10-20mm. Although its not a prime I can leave it open for longer, and I find I get better results than with the Rokinon.
08-11-2020, 08:45 PM - 1 Like   #11
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QuoteOriginally posted by mattb123 Quote
Actually I think my lens may not go all the way to infinity now that I tested. Just trying to focus on a daylight distant object using Live View I could never get it truly sharp. I'll try that calibration method when I get a chance.
Darn, would have been easier if I was going past infinity like I thought.
Don't calibrate for perfect infinity if you'll be using the lens in a variety of temperatures. Go a little past infinity.
08-12-2020, 09:18 AM   #12
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QuoteOriginally posted by DeadJohn Quote
Don't calibrate for perfect infinity if you'll be using the lens in a variety of temperatures. Go a little past infinity.
Yes always give your self some leeway to go a bit past to account for expansion and contraction.
08-12-2020, 09:28 AM   #13
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QuoteOriginally posted by DeadJohn Quote
Don't calibrate for perfect infinity if you'll be using the lens in a variety of temperatures. Go a little past infinity.
Thanks. Yeah I shoot in a wide range of temps, probably about -20 to 90F around here. I'll leave some room.
08-16-2020, 02:39 PM - 1 Like   #14
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QuoteOriginally posted by mattb123 Quote
Thanks. Yeah I shoot in a wide range of temps, probably about -20 to 90F around here. I'll leave some room.
I have an Irix 15, Blackstone and absolutely love it.

Last edited by TerryL; 08-16-2020 at 02:40 PM. Reason: Spelling
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