Considered members of the “holy trinity” of essential lenses, these two premium zooms provide fast speed and, between them, a wide range of focal lengths, from a moderate wide angle of 28mm to a medium telephoto of 200mm. Adding a wide-angle zoom completes the lens trinity. In Part One I looked at just such a lens, the Canon RF 15-35mm f/2.8.
Here in Part Two, I test two other top-of-the-line Canon L-series zooms, the RF 28-70mm f/2 and the RF 70-200mm f/2.8. For many photographers, such a trio of lenses covers the majority of shooting situations, day and night.
For astrophotography, the focal lengths of these lenses make them suitable for nightscape shooting, tracked constellation portraits, Milky Way star fields and large deep sky targets. Venture beyond a focal length of 200mm and you enter the territory where small refractor telescopes or dedicated astrographs can outperform camera lenses for astro work, usually at much lower cost.
But for shorter focal length subjects, these two premium lenses are superb. They are part of a new generation of zooms for mirrorless cameras that offer speed and image quality matching or exceeding fixed focal length “prime” lenses. The old wisdom that zooms are worse than primes is being overturned.
Checking the Mechanics
Both lenses are made for, and fit only on Canon’s R-series mirrorless cameras. Each is a new design notable for its speed. In fact at f/2, Canon’s RF 28-70mm is unique. No other company offers such a fast maximum aperture in a zoom lens covering the wide-angle to short telephoto range. Even Canon never had such a lens for its DSLR cameras.
By comparison, the RF 70-200mm f/2.8 is similar in optical specs to its Canon predecessor for DSLRs. However, the RF version’s new design makes it five centimeters shorter than the older lens. While short and easy to store when set to 70mm, the lens extends by seven centimeters when zoomed to 200mm. The 28-70mm also zooms externally, but by only two centimeters at 70mm.
When using these lenses aimed up at the sky my concern was their zooming mechanism slowly creeping down, ruining long exposures. I saw no such slippage. The lenses stayed put even after several hours being aimed vertically.
Both lenses are fully weather sealed, so should not suck in dust or moisture when zooming. Only a long term test would prove if that promise holds true.
At 1200 grams (2.6 lbs), the RF 70-200mm is 200 grams lighter than the older Canon EF mount version. But at 1400 grams (3 lbs), the 28-70mm is a hefty beast for such a short focal length zoom. That’s the price to pay for its extreme speed of f/2. That, and its huge front lens that requires costly and hard to-find 95mm filters. The 70-200mm accepts more common and affordable 77mm filters.
Both are auto-focus lenses, with the 70-200mm also offering image stabilization (IS), a feature useful for handheld daytime use. The manual “focus-by-wire” ring proved precise to use. At infinity, both lenses are parfocal, maintaining focus throughout the zoom range, while also maintaining a constant aperture.
Testing the Optics: RF 28-70mm
For each lens I tested samples I had on loan for a weekend, with only one partially clear night for my testing. While sufficient to pass judgement about optical performance, I was not able to take a full suite of “beauty shots” to show off each lens.
I primarily judged how sharp stars appeared at the center and corners of a full-frame Canon EOS Ra camera. The RF 28-70mm is spectacular, even wide open at f/2. At 35mm, its best focal length, stars appear as nearly perfect pinpoints at the extreme corners, and show only slight astigmatism at 28mm and 50mm. At 70mm slight coma is evident, but requires pixel peeping to see.
At the longest focal length where chromatic aberration usually shows itself, in this case at 70mm, I saw only a tiny degree of blue false color fringing on bright stars.
However, vignetting is prominent, with the corners at f/2 two stops darker than the center at 28mm and 1.5 stops darker at 70mm. Stopping down to f/2.8 lessens the extent of vignetting. However, as I show here, even at f/2 Adobe’s lens correction automatically compensates for most of the uneven illumination.
Testing the Optics: RF 70-200mm
Despite its fast f/2.8 speed, the RF 70-200mm is remarkable for what it does not show – any chromatic aberration (CA), which usually plagues longer lenses. Even at f/2.8 stars show negligible false color halos (longitudinal CA) or rainbow-like dispersion (lateral CA), either in the center or at the corners of the frame.
When used wide open lesser telephoto lenses often exhibit some degree of spherical aberration which bloats stars. Not so this RF model. Even at f/2.8 stars are tack sharp at the center of the frame. At the extreme corners stars show a low level of astigmatism similar at all focal lengths, again apparent only under pixel peeping.
The major flaw of the RF 70-200mm is vignetting. The darkening of the frame corners is mild at 70mm, but becomes oddly severe at 200mm. While the corners are only 1.5 stops darker than the center, the vignetting affects the entire outer 50 percent of the frame, not just the corners. Automatic lens correction in Lightroom or Camera Raw only partly compensates for this high degree of uneven illumination. Shooting long deep-sky exposures might require flat field frames.
With its fast f/2 speed the RF 28-70mm will be great for nightscapes, as well as for tracked constellation portraits and Milky Way starfields. I’m tempted! Fitting it with filters would be a challenge, however.
I find the RF 70-200mm less seductive. The severe vignetting at its longest focal length would be annoying to deal with. And yet, it is sharper and with less chromatic aberration wide open than my older Canon 200mm f/2.8 L, a prime lens that has to be stopped down for best quality.
The elephant in the room is their high cost. For just astrophotography it is hard to justify either lens, however tempting their speed and sharp optics might be.
But if you can find good reason to use them for daytime shooting – landscapes, portraits, weddings, sports and wildlife, areas where their accurate auto-focus and fast speed might be ideal – then either lens can be a perfect choice despite the price. Indeed, some non-astronomy reviewers have called these the best zooms they’ve ever tested, and reason enough to switch to Canon R cameras!
Canon does offer less costly alternatives in their RF 24-70mm f/2.8 and RF 70-200mm f/4. Along with Canon’s new RF 14-35mm f/4, the trio of slower zooms makes a more affordable “holy trinity” for Canon R cameras. While all will work well for astrophotography, the extra stop of speed offered by the faster versions is something I suspect you will always long for and wish you had bought!
Plus: Superbly sharp optics, with both usable at maximum aperture.
Minus: Massive size and weight for the 28-70mm; vignetting on 70-200mm; cost!
Canon RF 28-70mm f2 L USM
Canon RF 70-200mm f2.8 L IS USM