Canon’s RF 15-35mm f/2.8 forms the wide-angle end of a trio of lenses that many photographers consider the “holy trinity” of zooms needed to cover most shooting situations. In addition to a wide-angle zoom, the trinity is made up of a “normal” zoom (24mm to 70mm) and a telephoto zoom (70mm to 200mm). To complete the trinity test, in Part Two I look at Canon’s RF 28-70mm f/2 and RF 70-200mm f/2.8.
The Zoom Advantage
I’ve avoided zoom lenses up to now, preferring the traditionally sharper optics and faster speed of fixed focal length “prime” lenses for the high demands of astrophotography. However, the high quality of Canon’s RF 15-35mm provides good reason to switch to a zoom. Though costly, the RF 15-35mm can replace several wide-angle primes, making for a more compact kit of gear when traveling. Note that this lens fits only on Canon R-series mirrorless cameras. It will not fit, nor focus, on a Canon DSLR.
Zooms also offer the flexibility of framing and composing nightscape scenes more easily. For those reasons, a wide-angle zoom with a range from 14mm to 24mm or to 35mm has long been a favorite choice of nightscape photographers, with f/2.8 lenses from Nikon, Sigma, Sony and Tamron being top picks.
Canon’s RF 15-35mm was introduced in 2019 as one of the flagship L-series lenses for Canon’s then new mirrorless R cameras. Like its predecessor, the 16-35mm L Mark III made for Canon DSLRs, the new RF lens offers a constant speed of f/2.8 throughout the range, but with a new optical design providing a welcome extra millimeter of wide-angle reach.
Testing the Optics
The severe test of any lens is to shoot starfields with the camera on a tracker to eliminate star trails which might mimic aberrations. Stars should appear pinpoint sharp and free of colorful halos both in the center of the frame and out as far as possible to the extreme corners.
Remarkably, the RF 15-35mm performs at its best at 15mm, usually the most demanding focal length for aberrations. As I show here, at the extreme corners stars show a small degree of astigmatism elongating their images. Compared to my Rokinon 14mm SP, their premium wide prime, the Canon RF isn’t quite as sharp to the corners. So here the prime outperforms the zoom.
At 35mm, stars show more tangential and sagittal astigmatism than at 15mm, flaring stars at the corners into winged shapes. However, compared to my Canon L-series 35mm prime at f/2.8, a lens I’ve long valued for its sharpness, the Canon RF zoom is better. The zoom beats the prime.
It is at the mid-range of 24mm where off-axis aberrations appear their worst, exhibiting what looks like coma beginning about 50 percent from center to edge, but still well controlled at the corners.
Indeed, even at 24mm the Canon RF looks sharper than my old 24mm Canon L-series prime and, as I show here, outperforms my Sigma 24mm Art at the corners, even with the Sigma prime stopped down to f/2.8. Again, zoom beats prime.
Chromatic aberration of any type is negligible at any focal length. The flaw the RF lens does show in abundance is vignetting, with darkening of the corners worse at the shortest focal length of 15mm, where the light falloff is two stops at f/2.8. The extent of vignetting lessens at f/4, but the extreme corners are still down by two stops compared to the center.
Automatic lens correction in Lightroom or Camera Raw compensates for the vignetting, boosting the brightness of the corners, though always at the risk of more noise if the night scene is dark and underexposed.
Checking the Mechanics
The lens is parfocal. I found focusing first on bright stars with the lens set to 35mm made it easier to precisely focus. Zooming out to a wider focal length maintains focus.
This is an autofocus (AF) lens, also equipped with image stabilization (IS) for daytime use. While Canon R cameras are sensitive enough to allow autofocusing on bright stars, I prefer to trust my eye over the electronics and to manually focus.
As with most new AF lenses, that’s done via “focus by wire.” The manual focus ring controls the ultrasonic motors (the USM in the lens name) which perform the focus motion, even under manual control. There is no focus scale on the lens to allow pre-setting the focus to infinity. The process works well, allowing precision movements. But focusing does need to be performed each time you use the lens.
The body is made of “engineered plastic,” for light weight. The RF 15-35mm weighs 840 grams (1.85 lbs), roughly 300 grams lighter than either Sigma’s 14-24mm Art or Tamron’s 15-30mm SP, which each require an EF-to-RF adapter to use them on a Canon mirrorless camera body, adding yet more weight and length.
Unlike other zooms of this range with bulbous front lens elements, the RF 15-35mm can accept circular front-mounted filters, though large ones 82mm in diameter.
The lens extends outward by 1.5 centimeters when zoomed, but oddly is longest when at its shortest focal length. The RF lens is fully weather sealed, promising that no dust or moisture will get into the lens despite its externally moving barrel.
The major downside is that, like Canon’s other premium RF lenses, the 15-35mm is costly. It won’t be for everyone, even those committed to the Canon R system.
As of this writing, we have yet to see Canon or third-party lens makers come out with many (or any!) lower-cost wide-angle primes for the RF lens mount. Among the few exceptions are from Samyang/Rokinon – their 14mm f/2.8 auto-focus AF and cheaper 14mm manual focus MF – and the manual 15mm f/2 from Venus Optics/Laowa.
However, this fast Canon RF zoom can do the job of several primes, in part justifying its high price. At least that’s how I rationalized its purchase! That, and selling off older lenses.
If you are looking for one lens to rule them all for nightscape photography, I can recommend the RF 15-35mm. While Canon does offer a less expensive alternative in their new RF 14-35mm, its maximum aperture of f/4 makes it less suitable for most astrophotography.
Plus: Fast speed; sharp images across the frame, especially at 15mm.
Minus: Aberrations most evident at mid-zoom; some vignetting; cost!