[xenomorph]

Hi pentaxians

I'm debating in my head as numerous Pentax users before me - which UWA lens to buy?
Main usage - landscapes, night shots of aurora and stars (+astrotracker), daytime long exposure with filters - is there one lens to rule them all?
Camera - K-1 Mk2

I have already decided to buy Samyang 24mm 1.4 lens, but what if I want to go wider?
First I had on my plate D-FA 15-30mm f/2.8 but it's kind of pricy (1500€) compared to other choices. Only recommended filter kit was NISI, which is expensive too. Then by comparing Samyang 14mm and Irix 15mm reviews I decided to go for Irix Firefly. Also Irix filters are affordable.
Got an offer 300€ for used Irix, however after viewing sample images made with these particular lens, I'm not so sure anymore. In the center it is really sharp, but in the corners not only soft but images look like stretched and distorted.
Moneywise it would be a good deal - to get Samyang 24mm and Irix 15mm for 600€ (compared to 15-30mm 1500€), but when I was going to make the purchase, I suddenly had such a disturbing hesitation about Irix that I decided to put it on hold.

I contacted one pentaxian aurora hunter for advice. What he told me was somewhat surprising - not at all what I had read previously in lens reviews.
Pentax 15-30mm -> good for pretty much everything besides starry sky shots. Problem - trailing stars in the corners
Irix 15mm -> again corners problem - trailing stars and soft corner sharpness

He recommended Laowa 12mm f/2.8 Zero-D. Never heard about that lens before Since I couldn't make up my mind about it by reading reviews (actually not so many out there compared to other lens) I decided to make this post and ask around some opinions. It is ofourse much more expensive than Irix Firefly and I couldn't find any used copies for sale. In fact, it's very hard to find used Pentax gear in EU countries, which means adding to price of the lens and shipping fee an extra ~27% just for taxes when ordering outside EU. Probably filters are not cheap either. This lens + Samyang 24mm together reach close to Pentax 15-30mm price already what to do...what to do?

Please tell me your opinions - especially - is it much better than Irix for astro/night shots? As I understand I can't really use K-1 built in astrotracker with such wide lens without having stars trailing in the corners? I belive general truth about astrotracker was that anything wider than 24mm will make stars trail in the corners?

Greetz!
Allan

[hcc]

The user reviews of the Irix 12 mm are very worthwhile IMO:

Venus Optics Laowa 12mm F2.8 Zero-D Lens Reviews - Venus Laowa Lenses - Pentax Lens Review Database

There are great shots shared by the reviewers, that will help interested buyers.

[pepperberry farm]

wouldn't the Irix 11mm be a closer/better comparison?

[UncleVanya]

These links may help:

https://photographylife.com/reviews/laowa-12mm-f2-8/2 not very flattering to the lens for Astro.

https://photographylife.com/landscapes/comparing-5-top-wide-angle-astrophotography-lenses The Irix is included in this comparison.

Note these were not tested on Pentax bodies. It is possible this doesn’t generalize to the Pentax versions.

Edit: obviously only the lenses available on Pentax are meant to be compared from this review. The fact the Laowa was dead last is the important finding.

[bdery]

There are in-depth reviews of those two lenses here on Pentaxforums.

Irix 15mm F2.4 Firefly & Blackstone Review - Introduction | PentaxForums.com Reviews

Venus Optics Laowa 12mm F2.8 Zero-D Review - Introduction | PentaxForums.com Reviews

Long story short, I prefered the LAowa by a fair margin, but the Irix can be WR so that might be important to some. And it's a fine lens in its own right.

[photoptimist]

Both the "trailing stars" and the "stretched and distorted" artifacts are unavoidable side effects of the imaging geometry of all rectilinear ultrawide wide angle lens (rectilinear means that all straight lines in the subject will be straight lines in the image).

The "stretched and distorted" issue happens because the angle between adjacent pixels in the center of a UWA image is much larger than the angle between adjacent pixels in the edges and corners. A round object (ball, clock face, or person's face) will be a nice circle in the center of the image but a weird stretched ellipse at the edges and corners. ("Does this lens make me look fat?" Yes it does!)

Star trailing with astrotracer happens because of this angular effect. The linear motion of the sensor inside the camera can only approximate the angular motion of the field of stars. Astrotracer cannot fix the fact that in a rectilinear image, the stars in the corners move faster than the stars in the center. The one exception is that pointing the camera at the North Star (or it's southern equivalent) can get an astrotracer image with no trailing.


The point is that all rectilinear wide angle lenses have these problems. In theory, a true fish eye lens (which has an equiangular imaging geometry) would not have these problems. However, fish eye lenses have the problem that most straight lines in the image (buildings, trees, the line of the Milky Way) become curved in the image.

[MossyRocks]

Star trailing with astrotracer happens because of this angular effect. The linear motion of the sensor inside the camera can only approximate the angular motion of the field of stars. Astrotracer cannot fix the fact that in a rectilinear image, the stars in the corners move faster than the stars in the center. The one exception is that pointing the camera at the North Star (or it's southern equivalent) can get an astrotracer image with no trailing.

Mostly true, astrotracer does do field rotation but as you siad unless you are oriented so the camera has one of the celestial poles perfectly in the center of the frame (declination 90) the rotational portion tracking as you move away from the center will decrease. As one moves away from haing the center of the frame pointed at a celestial pole to pointing it more towards the celestial equator the movement changes from more rotational to more linear until one is pointed along the celestial equator where the movement is all linear. Here an ultrawide lens will show the most trailing. For long lenses this isn't a big issue, but for really wide ones it can be. That isn't to say there aren't benefits to using astro tracer with an ultrawide but don't expect the 20x increase over untracked shots in exposure time that you can get with other lenses. Personally I don't bother with astrotracer with my 12mm and instead will let software handle stacking shots to drive the noise down as a 15s untracked shot has a lot of potential and a couple hundred of them really has potential, especially if I took some dark frames at the end while packing up.

I've never noticed coma with my Laowa 12mm Zero D and I am a rather pixel peeping individual. As far as it's use for astro it is generally regarded as a very good lens for that and I would agree, even when shooting wide open at f/2.8. My second time every using the lens and my first real attempt at doing a milky way shot I managed this with it:

[UncleVanya]

I've never noticed coma with my Laowa 12mm Zero D and I am a rather pixel peeping individual. As far as it's use for astro it is generally regarded as a very good lens for that and I would agree, even when shooting wide open at f/2.8. My second time every using the lens and my first real attempt at doing a milky way shot I managed this with it:

Great shot! I’d be happy with that. On the topic of this lens and coma I find a lot of reviews that mention it but few reviewers used Pentax. Is it possible the K Mount version is better? That seems absurd on the face of it - but perhaps there’s something odd involved.

[MossyRocks]

Great shot! I’d be happy with that. On the topic of this lens and coma I find a lot of reviews that mention it but few reviewers used Pentax. Is it possible the K Mount version is better? That seems absurd on the face of it - but perhaps there’s something odd involved.

I agree that it sounds absurd, but I do wonder if some of what is shown as coma in shots may be something a bit off off center in the lens. Lenses that I have that show coma when doing astro (almost all of them) it is very uniform but given what I have heard on ultrawides where it seems to be a bigger issue and shows more readily I wonder if it is more sample variation than anything else. Even in the article it is mentioned that they thought the 2 copies they tested were significantly decentered further adding to my belief of that being the problem. Those point sources of light against a very dark background are really good at showing issues but in the sample shot provided for the article on the Laowa 12/2.8 that shows what appears to be coma it is stated as being the upper right corner but when I look at that image I would almost say it from the upper left given the pattern for the coma. Also at 25s for exposure for that lens one would expect to see some star trails which will soften things up a bit since they were using the rule of 300 (300/focal length) for shot length and depending on the location and where the lens was pointed in the sky (there will be noticeable trailing along the celestial equator).

In the end I may have just gotten an exceptional copy but I did bother WPRESTO a fair amount about it before buying his copy.

[disasterfilm]

If you're shooting on a FF sensor and shopping for a lens like this to shoot wide field Milky Way astrophotography, I think you'll find the sweet spot for focal lengths is 24mm early & late in the season when the Milky Way is flatter near the horizon up to a 45 degree angle, and in the middle of the Milky Way season where the galaxy is closer to vertical that 20mm is a great choice. 24mm and 20mm still gives you plenty of room in your image to frame in other compositional elements, be they ground-related or in other parts of the sky.

Two big reasons:
1) Compositional - the wider you shoot the Milky Way, the smaller you're making the really interesting parts of the galactic core. You're sacrificing good detail there in favor of the boring and less-detailed outer portions of the galaxy. And TONS of relatively empty black space on either side. it's just less compelling than similar, slightly tighter photo showing off a dense field of stars, with the Dark Horse nebula and the tight clusters of dark gas textures, and minimal lost space around the galaxy.

2) Effectiveness of the astrotracer. The wider your lens is, the less effective the astrotracer is at allowing you to extend your exposure times. Even at 20mm, you start getting a lot of corner trailing. It's not the fault of the astrotracer, it's the fault of optics in general. You're projecting a round surface onto a square sensor... there's going to be trailing. The longer your focal length, the flatter the spherical projection appears. 24mm is a good starting point, 35mm is great for extended astrotracer shots, and anything further is extra great as long as the lens is suited for it.

Disclaimer: I have the Samyang 24mm, 20mm, and 14mm on FF. And the Irix 15. And the Laowa 12.
I never shoot the 12mm, 14mm, or 15mm lenses on FF for astro. I keep them for other purposes.

[bilybianca]

I have the Irix 11mm and the Irix 15mm. Both are fine lenses, and they do deliver. But they are both huuuuge and heeeeaaavy. I seldom bring any of them in the bag "just in case", so for portability maybe the Laowa is the better choice.

Kjell

[northcoastgreg]

If you're shooting on a FF sensor and shopping for a lens like this to shoot wide field Milky Way astrophotography, I think you'll find the sweet spot for focal lengths is 24mm early & late in the season when the Milky Way is flatter near the horizon up to a 45 degree angle, and in the middle of the Milky Way season where the galaxy is closer to vertical that 20mm is a great choic

I generally agree with that. Years ago I purchased the Rokinon 10mm for astrophotography on the K-5. I wound up never using that lens for that purpose. The DA 21 actually became my favorite astrophotography lens on APS-C! With the astrotracer functionality, narrow focal lengths become possible, and for that reason I think 20-30 mm is more useful for astrophotography on FF than 19mm to 11mm.

I ended up choosing the DFA 15-30 for astrophotography, but I wouldn't necesarily say it is the absolute best option --- although it's very good all things considered. It is, however, rather heavy and expensive. Where it reigns supreme is in landscape photography --- and that's what I primarily use it for. If you're getting a lens primarily for astrophotography, the Rokinons are terrific for that.