You have to give it to Sigma, the company is making some superb, unique lens options for all types of cameras. The 50-100mm f/1.8 DC HSM Art for Nikon F, Canon EF, and Sigma SA APS-C cameras is one of the oddest, with a fast constant aperture and a short telephoto to medium telephoto range equivalent to 75-150mm. Designed as an ideal complement to the 18-35mm f/1.8, this lens is designed to replace three primes with 35mm equivalents of 85mm, 100mm, and 135mm without sacrificing a fast aperture. Fortunately, Sigma was able to really come through with outstanding performance, even wide open, with crisp details and few to no aberrations.
I honestly wasn’t expecting much when I was given this lens; a zoom lens with a constant f/1.8 aperture seems like a fantasy, and had the skeptic in me ready to pounce. In order to test it, I put it on my a7R II with an electronic adapter and set it to APS-C mode. Besides the convenience of being able to shoot this lens with my regular camera, this configuration also offers the advantage of letting me try the lens in full-frame mode—a common question nowadays, since many users are adapting lenses to multiple formats. Luckily, the lens functioned nearly perfectly with the adapter, allowing for fast, accurate autofocus. The focusing system did make more noise than many of its contemporaries and seemed to “hit” the ends when reaching minimum and maximum focusing distances. No cause for alarm, but just lacking the finesse of a native Canon zoom.
A hefty lens
One other thing to note is the size. Photography and physics have always gone head-to-head when it comes to lens design. You can only make a lens so small before you seriously compromise on performance. Sigma went the less compromising route and, as a result, you have a hefty lens. It weighs 3.3 pounds and measures 6.7" long, so it's not exactly a lens you just toss in your bag for a day out. This also means it weighs more than a couple of primes in the same range, so you are really going to need to justify having a zoom for this purpose. A more conventional 70-200mm f/2.8 would be similar to this lens in size and weight, but with double the range. Sure, you may lose a stop of light, but if you are looking for a telephoto zoom the extra reach can be extremely helpful. A bonus is that most 70-200mm lenses are compatible with full-frame cameras, should you upgrade later.
Anyway, back to the optics. Sigma did nail this lens design, making performance a top concern, and threw in all of the company’s bells and whistles, including a series of low dispersion glass elements, a high refractive index element, and Sigma's Super Multi-Layer Coating. Also, the f/1.8 aperture will be greatly appreciated for its ability to throw backgrounds beautifully out of focus with an exceptionally sharp center frame. At f/1.8, there is a small amount of vignetting, which is not surprising, given the design, but worth mentioning. Flare is well controlled and it doesn't seem like there are any significant advantages or disadvantages to either the 50mm or 100mm ends.
Fast focusing and sharp performance
The lens is sharp, which seems to be a major focus of Sigma's revamped Global Vision lineup. It is fully compatible with the USB Dock for firmware updates and focus adjustment. I didn't need to use it with the α7R II's on-sensor focusing, but having used the software in the past, I can say it is worth having. As you stop down, performance obviously improves by increasing sharpness and decreasing vignetting. Distortion is, for all intents and purposes, non-existent, which makes sense considering the focal length. Normal to medium telephoto lenses are generally quite good when it comes to minimizing distortion.
Now, one omission of the lens is image stabilization. This isn’t a huge concern, since many primes in this range wouldn't have it, but many users would have likely appreciated it for a 75-150mm equivalent zoom because the obvious comparison would be a 70-200mm f/2.8. The tripod foot, much like the 150-600mm Contemporary lens, is extremely small. It may work, but it is tougher to fiddle with, and the smaller fit would be less effective at steadying the lens. A nice feature of the tripod collar, however, is the stops every 90 degrees, so you can lock-in the proper positions easily.
The rest of the lens feels solid. It feels good in the hand, has decent balance, and the focus and zoom rings are quite smooth. It is a bit large for the limited range, but it does the job well. Sigma doesn't state that there is any significant weather sealing, which I find to be a major weakness for this type of lens. Users are going to have to be cautious if they decide to pull it out on a rainy day.
Where the image circle ends
And, for those who are curious, here are some samples of what the image circle looks like on a full-frame body. At 50mm there is strong vignetting where the image circle ends, but there is a good bit of workable space vertically, making it useful for getting a little extra height in some images. At 100mm, it will cover the entire frame with some noticeable vignetting in the corners, but nothing Photoshop can’t help fix. It is nice to know that if you need a little extra on the edges you can use a larger area of the sensor. The corners do fall off pretty badly on full frame but, for a little extra space, the extra pixels can be extremely useful.
A note for video shooters: the lens is not perfectly parfocal, but comes very close. I could see this being used for documentaries quite often. Videographers will likely greatly appreciate this lens because it covers Super35 and provides additional depth-of-field control.
After using the 50-100mm, I was pleasantly surprised by the sharpness and control over aberrations. However, I struggle to see where many photographers would need a single large zoom over two or three compact primes. Photographers working in environments where lens changes are dangerous to the camera or where speed is critical may find this to be a lifesaver, but otherwise I personally feel a couple of nice primes would be a much better, and lighter, option. As a companion to the company’s 18-35mm f/1.8, it is a great choice, and it makes sense for users looking for a two-lens setup to fill all their basic needs.