Sigma 18-35mm F1.8 DC
I just picked up this lens in like-new condition for a good price (Japanese seller on eBay). Since Sigma discontinued it in Pentax K-mount back in 2019 they are getting harder to come by. I bought it primarily for astrophotography with my K-3II because I didn't have a lens in that focal range with decent optical quality (critical for stars) or one that was fast (f/3.5 was the fastest!). The focal lengths it covers are good for landscape astrophotography (e.g., Milky Way), constellation portraits, meteors and aurora.
This thing is very sharp at all focal lengths, even wide open. There is some coma in the corners at f/1.8 that is visible on stars (always a torture test for lens aberrations) but I would use it at f/1.8 only for meteors and aurora, in which case star shapes in the corner are secondary to speed (aperture). By f/2.5, the coma is mostly gone and by f/2.8 star shapes are really good.
The lens being uncommonly big and heavy for a wide-angle zoom doesn't bother me. One thing that might bother me, however, is the autofocus behavior. It does miss focus a fair bit when using phase detect (whereas it always hits focus when using contrast detect in Live View). I haven't explored the lens's autofocus behavior yet in any detail. It is, of course, not a concern for astrophotography where I would always manually focus. It is a concern only for general photography. I don't know whether the autofocus behavior of this particular lens was ever tuned using Sigma's USB dock but if so, that would have been for a different Pentax camera. In any case, I might have to pick one of those up and fine tune the lens myself for my camera.
Does anyone on the forum use one of these lenses or has used one in the past? Opinions?
I haven't used that lens before but have heard about it. Interesting to hear your autofocus issues and I don't know enough about the lens to say if that's a general issue or just your lens. It does seem like a very versatile lens, even if a little bulky (but not really bulky for what it is, if you think about that.)
On a related note, I just bought a Rokinon 16mm f2 for my K-3 for milkyway stuff. I'm still learning the lens, and it's only had one outing (where the moon was in full swing, hah!) but it works really well.
This is at f2.8
Milkyway with moon by Snappiness, on Flickr
This is at f2. I think it performed rather well at f2, but I have less of an eye for astrophotography than you do. It's acceptable to me, at least.
Guadalupe Mountains at night by Snappiness, on Flickr
The real test for me will come with shooting the milkyway straight on without the moon, which I hope to do soon. Just takes a lot of planning and effort to get out far enough away from these bright city lights.
I was considering that 16mm f/2. It’s nice that the FOV is somewhat wider than the Sigma. But in the end I decided I probably wouldn’t use it much other than for aurora, which I expect to be shooting only rarely. Even at a higher cost, I can justify a lens that I will use more often. And, to be honest, I always wanted a Sigma Art lens. It was inevitable that I would get one at some point.
For really wide Milky Way shots (e.g., all or most of the arch), I will be using mosaics, tracked with Astrotracer, of course. To get such an image on APS-C in a single shot would require a very short focal length, at which point the aperture area of the lens is very small. I have already tried such a mosaic with the 18mm end of my Pentax 18-135mm zoom but will repeat next year when the arch is again in the correct position. And I have done Milky Way mosaics with a 28mm lens (had to be stopped down to f/4) and with a 50mm lens. They were not particularly difficult.
Tracked mosaics are one of the advantages of Astrotracer. It makes them relatively easy to do; no harder than an untracked mosaic. They otherwise require extra hardware bedsides a tracker and some extra effort when aiming each panel.
Some quick snaps of Orion taken in the course of testing the lens on my K-3II, very lightly processed. These are 30-second exposures using Astrotracer with the lens stopped down to f/2.2 at 18mm and f/2.8 at 35mm. Unfortunately, there were so many clouds that night that none of my exposures are free of them. Normally, I would have taken many exposures of Orion and stacked them for better SNR but that was not possible. I think, however, that the clouds have given the images a little extra charm.
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Quote from Alen K on September 19, 2022, 7:50 pmI just picked up this lens in like-new condition for a good price (Japanese seller on eBay). Since Sigma discontinued it in Pentax K-mount back in 2019 they are getting harder to come by. I bought it primarily for astrophotography with my K-3II because I didn't have a lens in that focal range with decent optical quality (critical for stars) or one that was fast (f/3.5 was the fastest!). The focal lengths it covers are good for landscape astrophotography (e.g., Milky Way), constellation portraits, meteors and aurora.
This thing is very sharp at all focal lengths, even wide open. There is some coma in the corners at f/1.8 that is visible on stars (always a torture test for lens aberrations) but I would use it at f/1.8 only for meteors and aurora, in which case star shapes in the corner are secondary to speed (aperture). By f/2.5, the coma is mostly gone and by f/2.8 star shapes are really good.
The lens being uncommonly big and heavy for a wide-angle zoom doesn't bother me. One thing that might bother me, however, is the autofocus behavior. It does miss focus a fair bit when using phase detect (whereas it always hits focus when using contrast detect in Live View). I haven't explored the lens's autofocus behavior yet in any detail. It is, of course, not a concern for astrophotography where I would always manually focus. It is a concern only for general photography. I don't know whether the autofocus behavior of this particular lens was ever tuned using Sigma's USB dock but if so, that would have been for a different Pentax camera. In any case, I might have to pick one of those up and fine tune the lens myself for my camera.
Does anyone on the forum use one of these lenses or has used one in the past? Opinions?
I have this lens I've heard it's a good astro lens, I've used it for cityscapes mainly and the coma has always been good. I have used it with the K-5 and K-70 I would recommend to calibrate the lens I found that helped the AF Fine adjustment helped.
Update: I have released a lens test report for astrophotography based on multiple exposures I took of the Orion constellation as described in my third post above in this thread. I was mainly concerned about the quality of star shapes, particularly in the corners given that stars are such a hard test for a lens and given that an astrophoto is naturally going to contain mostly stars. I tested at several different focal lengths throughout the (limited) range of the lens and at several different focal ratios (apertures) from wide open down to f/2.8. I did not test at smaller apertures since I felt that the star quality was quite adequate already at f/2.8 and because I didn't buy a fast lens to shoot it at a (relatively) slow aperture.
I also tested the effectiveness of the tracking of Pentax Astrotracer with such focal lengths. Due to the rectilinear distortion of a typical wide-angle lens, Astrotracer exhibits trailing artifacts in some corners of the image, the length of the trails dependent on exposure time. This is a known limitation, but I wanted to test it with this particular lens since as a zoom, its geometric distortion varies with focal length.
Rather than repeat the text of the report here, I will simply link to it as posted in the Astrophotography Forum at the Pentax Forums.
