Not long ago, I released a review of Sigma's newest Art-series lens, the 20mm f/1.4 Art. Unfortunately, Northern California skies have had bit of a tough time clearing up despite numerous requests from astrophotographers below, patiently waiting for news of this lens' nighttime, Milky Way performance. Last night, although far from perfect, areas of the sky did clear up enough to get a small consensus on how this lens fares when pointed toward the stars.
Although hard to tell with the naked eye, the sky still had plenty of haze in it. This meant my exposures brightened up with plenty of "noise" from light pollution in the area that was so lovingly held in the sky for me. Still, it was clear enough to see some stars, which meant it was clear enough for our first coma tests with the Sigma 20mm f/1.4 Art. That's all I'll say about the weather, so just remember that when looking at these. This is all just look into various types of distortion in stars at night at apertures f/1.4, f/2, f/2.8, and f/4. I won't say much except to fill in the basics. You can all determine the results and form your own judgments, here. Ready, set, go!
I don't have any side-by-side comparisons, but reading and examining results online, it seems that the Sigma 20mm f/1.4 Art holds up darn well. Darn well isn't gold, however, when you're comparing against a good copy of the seemingly coma-less Rokinon/Samyang/[insert one of a number of brand names here] 14mm f/2.8. But without having one with me to test, and with a few relatively quantitative measurements, I can say the Sigma should please all astrophotographers.
There is visible evidence of coma (comatic aberration) and astigmatism, but both are controlled quite well, especially when considering how wide this lens opens up. For some kind of quantitative measurement, we'll use The Lonely Speck's method of dividing the pixel length of these aberrations by the pixel height of the entire image to get the distance that the aberrations span as a percentage of the photograph taken (which should apply to any camera with the same sensor size).
In the image below (taken at f/1.4), both coma and astigmatism present at various lengths from 15 pixels to 25 pixels, depending on the area of the image, the brightness of the star, and the type of aberration being measured. Dividing these numbers each by 4016 (the vertical number of pixels of my D750 image), one discovers a relatively excellent range between 0.37 and 0.62 percent (The Lonely Speck considers anything below 0.4 percent as excellent, below one percent as acceptable, and above one percent as poor). Keep in mind this is at the far corners. The center of the image still displays excellent sharpness with no visible signs of any aberrations (as expected of most lenses' center-field performance). Read the captions for information about the following subsequent test images.
What you won't notice in these images is any jarring, overall feeling of "oh my god, it's everywhere" that you normally see. In this case, it's obvious we're measuring the brightest bluish star that's the only one with real signs of aberrations. In a real-world test where there is no haze or light pollution, this effect should be much more uniform. This was confirmed in quick tests I did with The Lonely Speck's downloadable test chart, which showed identical performance for every dot in every corner relative to one another.
If anything, the test chart images showed roughly a 10- to 20-percent improvement which I might chalk up to the lack of atmosphere to further distribute light. Maybe that's a stretch, but the point is, the results are uniform in the corners and repeatable in two completely different types of tests. Moreover, those tests allowed me to more easily discern that the maximum aberrations did in fact reach no more than about 0.62 percent when going just a few pixels in from the corners.
I do apologize for the imperfect sky, but I promise it's the best one we've had to date. In any case, the two tests still back each other up, and performance overall is still excellent. Below are crops from the centers of these images. You can all make your own judgments after one note: remember there was significant haze. Because of this, even the untouched image saved as a JPEG doesn't look great with the added noise reducing the presence of those nice, deep blacks we like to see. Therefore, the overall sharpness isn't going to look great, either. For reference, all crops are 700 pixels wide and 467 pixels tall.
For reference as to vignetting, below are the full images.
So, what do you all think? Worth it?
I didn't want to start the whole ISO talk, partly because it was so hazy that it's not really worth it. But, for those interested, here's a quick unedited and curves/saturation/sharpening/NR-adjusted image at ISO 800, f/1.4, four seconds.