When Sigma launch a new Art series lens the world tends to notice, so far sigma have wow’d the world with the 50, 35 and 24 art lenses. So when they announced the 20mm f1.4 DC Art lens every astro photographer got excited.
Astro photography is very hard on lenses, we demand fast wide angles with little or no optical imperfections. The ideal lens would be in the 16-24mm and have no coma or vignetting at a very fast (f1.4) aperture, so this looked to be an ideal match.
Thanks to CR Kennedy and Kens Camera for getting me a review copy, the lens is available at Kens for $1590 NZD.
The 20 art is a substantial lens, its similar in size to the 50 and 35 models but with the built in lens hood and cover its a couple of centimetres longer in your bag, this actually makes a bit of difference as it makes just longer than lenses ( 24tse, 16-35, 50 art etc) so a squeeze to get in and out unless you make additional room for it. Not only is it a little bigger its also quite a hefty weight. At 950grams its heavier than the 50 art, 50% heavier than the canon 24L (650g) and almost twice the astro shooters best friend the samyang 24/1.4 (550g). Its also a little front heavy but overall its a nice solid lens you are not going to fumble around with in the dark and with gloves on. I’d have liked to have seen a tripod mount or optional collar on the nodal point of the lens, I honestly dont understand why this isnt standard on any prime 100mm and under. it helps with balance and is great for panoramic shooting.
I dont have the means to test a lenses resolution or distortion etc, if you want these measurements there are plenty of places to find it. What we do know from those reviews is it does have some coma wide open and a fair amount of vignetting (3.5 stops in the corners at f1.4).
Luckily I got a nice clear night at Castle Hill to test out the astro credibility of the 20mm art. Focusing was easy, pick a bright star, live view at 10* and rack focus back and forward until sharpest, the manual focus with the lens was nicely weighted and easy to set, also seemed to remain well set for the whole time. Second up some test shots at ISO 1600 to see how the overall frame and corners looked. You’ll find a corner sample in the images showing the results.
My usual astro shooting is either a single image at 14 or 24mm , or a stitch of many image at 24mm or 50mm. For 24mm I use my canon tse mk2, its relatively slow at f3.5 but its such a good lens that I can use it wide open with little trouble. The 14mm Samyang is nice and sharp but has several issues that sees it relegated to test shots, timelapse and star trails mostly.
I’ve often found 14mm too wide for a single shot and 24mm too narrow, so 20mm of the sigma actually turned out to be a very nice focal length to use, its an unusual focal length but one that works really well for astro. Using the 500 rule we should be ok with a 25 second exposure, though I did notice a little bit of star trailing at 25 seconds it wont be noticeable in all but the largest prints, 20 seconds for ‘safety’ if you are worried (this does all depend on where in the sky you are shooting and at what latitude). Coma at f1.4 is certainly noticeable but its not that bad, but I like to avoid the worst of the coma in single framed shots so for this I would recommend stopping down to f2.2 or f2.5, this also gives you a reasonable depth of field, as usual focus stacking for near objects is a must. I also dont mind vignetting a little on single frame shots as it can add to the feel of the image.
For panoramics I prefer a longer focal length which is why I use the 50 mostly on the gigapan, or 24 when I am manually shooting with the novoflex. The advantage of a wider angle lens here is fewer shots to stitch and less time overall to shoot. A 360 panoramic with the 50mm can take 25-30min to shoot, this cuts into your ability to get different compositions and also can be affected by changes in light and weather. With the 20mm art I shot a two row (2 * 11) 360 panoramic in 11 minutes, far quicker for the whole scene, and stitching was a breeze in PT Gui.
When shooting panoramics coma is less of a problem as you are overlapping (35% in my case) and stitching these together tends to cover some of these issues up, so after reviewing the test images I decided to try a 360 at f1.6 and ISO 1600. Usually I am shooting at ISO 6400 and about f2.8, so this should result in lower noise for about the same light captured, but fewer shots and a lower resolution image overall compared to say an 84 image 360 on the 50mm. One issue that can cause problems with stitching here is heavy vignetting, if uncorrected this can cause banding on images, but between adobe camera raw and PT Gui for this lens at f1.6 it was easily fixable. Probably because there are fewer boundaries to blend together. Coma turned out not to be a big problem at this aperture for a blended panoramic and the resultant file (16000px wide) should print to a decent size without issues.
So how is it for more traditional daylight landscapes? Stopped down to f8 the lens is pin sharp to the corners and very impressive, seems to resist flare quite well too despite the bulbous front element. But that front element is an issue for filters. I prefer shooting with filters to control exposure as well as shutter speeds. Shooting on the Sony a7r helps as we have more dynamic range to recover shadow details so grad filters are less necessary.
A lot of people skip filters for exposure control and blend multiple frames in post, this works and I use this method when I feel it’s the best way but it still doesn’t allow you to control shutter speeds for long exposures. There is a solution to this, as with many of these kind of lenses. Nisi filters manufacture a filter holder made specifically for this lens. It costs $170USD for the adaptor and also requires 150mm filters which are themselves quite large and expensive. Nisi are great quality though and more affordable than some other brands. I’m assuming other makers will have systems for it too eventually.
Using my Novoflex VR pano head the 20mm makes some nice wide panoramics, these stitch together nicely giving massive and detailed files, it could replace my 24mm tilt shift for rotated panoramics I think.
I think it would be a great lens for use outside of landscape photography, it’s very sharp in the centre at f1.4 and the focus is fast and seems reasonably reliable. For wedding group shots and artistic wide angle shallow DOF images it’s going to be a killer tool to have along with you.
So it’s a decent all round landscape and astro lens but how does it stack up to the competition? Specifically the Samyang 24mm f1.4? The Samyang is a cheaper lens at around $900nzd, but it lacks autofocus and has a manual aperture control (this for timelapse can be an advantage). If you get a good copy of the Samyang it will have better controlled coma than the Sigma.
So for Astro photography this lens certainly has the ability to deliver what we want. The big question though is how does it measure up to the Samyang 24mm f1.4, which is much cheaper at $900nzd?
As usual there is no easy direct answer to this. The Sigma is much sharper lens overall and is quite amazing in the centre at f1.4, the Samyang needs f4 to match the sigma’s wide open performance here. The Samyang has slightly better coma wide open in the corners but this is quite marginal depending on copy. They both have about the same amount of vignetting wide open. Both really need a little stopping down to get the best results
Overall I think I prefer the Sigma over the Samyang, it’s a better all-round tool though it probably comes down to if you want autofocus or (easy) filters. It is a significant amount more than the Samyang though so if you are shooting purely astro and want to save $600 the Samyang is still an amazing lens, if you get a good one.. Samyang are not noted for consistently high quality control.
As I already have a good 24mm prime and shoot more than just astro I would be very tempted at adding the Sigma 20mm f1.4 to my gear bag, and might be my lens purchase for the year!
- Build quality
- f1.4 Centre sharpness
- Corner to corner sharpens for landscapes
- Fastest wide angle
- Coma at f1.4
- Bulbous front element