My “go to” lens for astro-landscape photography used to be a Rokinon 24mm f1.4 manual lens. It seems to be standard advice given out within various night photography groups and publications that the Rokinon 24mm f1.4, along with the Rokinon 14mm f2.8 are the standard for budget minded photographers who want to photograph the Milky Way. After using these lenses for a couple of years now, I have come to a different view, and have been updating my lens collection accordingly. The lenses I have been migrating toward are the Sigma series of Art lenses, specifically the Sigma 20mm f1.4, Sigma 24-35mm f2.0, and the Sigma 50mm f1.4. Since my switch, I have been asked repeatedly about my reasons for the switch, and my impression of the Sigma vs. Rokinon performance. Rather than responding with the same answers multiple times, I am writing this post so that I can simply send a link to the post, and all who ask will get the same uniform response.
First, a disclaimer. I am, by no stretch of the imagination, an expert on lens performance, nor do I possess the necessary experience to opine generally on this topic. But I do feel I can contribute some constructive observations on the lenses in question based on my own particular usage and expectations thereof. So in this spirit, here are my thoughts. As always, YMMV.
The first consideration is cost. Looking at Amazon or B&H, one quickly finds that the Rokinon series of lenses costs roughly half of what the Sigma lenses retail for, some slightly more, some slightly less. If cost were the only consideration (or perhaps the major consideration), the Rokinons would win hands down. On the other hand, if longevity is a consideration, the Sigma lenses come out ahead. The quality control of the Sigma lenses has proven to be excellent in real world use, while the Rokinon lenses are build to a less rigorous physical standard. Additionally, there are many Rokinon users that report having to return Rokinon lenses because of an issue of decentered lens groups, leading to half of the image being out of focus. I have had to return two copies of my Rokinon 24mm before receiving one that was not decentered.
The Rokinon lenses are fully manual lenses, while the Sigma lenses are automatic. The standard story line is that we shoot in manual mode when doing night photography, so it does not matter that the Rokinon lenses are manual. Phooey!! Maybe for you it might not matter, but for me it has become a deal breaker, and here are my reasons why.
1) On a fully manual lens, the EXIF data recorded with the RAW image file (or JPEGS, for that matter) does not contain information on the lens that was used in the shot, nor the f-stop that was used to produce the image. Try going through images you took a month or year ago, and see if you can identify the lens used or the aperture. The same people who advise that a manual lens presents no problem for night photographers seem to contradict themselves, when in other contexts they recommend comparing coma testing results to see what settings are acceptable in actual use. How can you compare the results of two or more exposures, when the EXIF reads f(???) and f(???). Also, has anyone ever asked you what exposure settings were used to create an image, and you had to guess because there is nothing regarding aperture in the EXIF data? How about lens focal length? Are your observational skill sufficiently developed so at a glance you can determine whether an image was taken with a 14mm, 20mm, 24mm or 35mm lens just by examining an image? Mine certainly are not, and I do not like to guess about these settings when examining and critiquing past images.
2) On a fully manual lens the aperture must be set with a ring on the lens barrel. I shoot at night here in the Ozarks as a regular routine, and also in other locations where the temperature and dew point converge. To eliminate fogging, I must wrap the lens barrel with a dew heater or with hand warmers to prevent fogging. This means that I must unwrap the lens to make any aperture adjustment and then re-wrap the lens. Try doing this repeatedly in the course of a shooting session without disturbing the focus or camera positioning – it is difficult, if not impossible. An automatic lens solves the problem. Just adjust aperture using the control dial. No need to unwrap and re-wrap the lens any more!
And now to the main issue most people are asking me about – image sharpness and coma. The Rokinon lenses have a well deserved reputation for being lenses that minimize coma (I’ll use coma here as a proxy for coma and astigmatism). Even when shot wide open, the Rokinons perform exceedingly well in the coma department, much better than the Sigma lenses, which need to be stopped down to achieve the same coma performance. HOWEVER, the Sigma lenses have a well deserved reputation (confirmed on test benches) for incredible edge to edge sharpness throughout the Art Series lineup. Refer to DxO testing and MTF charts to compare the difference between the Rokinon and Sigma lenses. The Sigma lenses win hands down in this regard. While I admit to not having experience with many high-end lenses such as Zeiss or Sony GMaster lenses, I have been truly astounded with the overall sharpness of my Sigma lenses. The sharpness extends from edge to edge, not just in the center, as in the Rokinon lenses, which also appear to be softer in general than the Sigma lenses.
With regard to the Sigma 20mm f1.4 specifically, I have a different view than most when comparing it to the Rokinon 24mm f1.4. When shooting a single frame image (non-pano), if I crop the image to the same field of view as the Rokinon 24mm, the coma affected stars are cropped away, but I gain the advantage of the stunning sharpness of the Sigma compared to the Rokinon, while achieving essentially the same field of view. When I shoot for a panorama, as long as I provide sufficient overlap (I overlap 50%) the coma affected stars are not included in the stitched image, except in the extreme outer edges, which is easy to correct in post processing.
And my last observation regarding the Rokinon vs. Sigma debate. In daytime shooting, the Sigma lenses produce results that are rated among the best lenses. And they are fully automatic, to boot. By giving a little forethought to how I use the lenses at night, I can get the best of both worlds (daytime and nighttime) with the Sigma lenses.
I believe that the photographer should take into consideration the type of night photographs they wish to create in making the decision on which lenses to acquire. My own style is evolving to emphasize the foreground, and reduce the sky as the main element of the image. In this regard, the Sigma lenses excel, especially as compared to images I have taken with my Rokinon lenses. Please feel free to comment with your own experiences with these, or other lenses. I’d love to hear from you!