Formally, Canon doesn’t have a “prosumer” (mid-grade) grade lens lineup like Sony with its G class. However, they’ve started adding a few lenses that could well be. One of them is the Canon RF 28-70mm f/2.8 IS STM. Obviously, it has one characteristic that stands out from Canon’s lower end products – its large maximum aperture of f/2.8. Another one is the price tag of $1100 USD / 1200 EUR. That’s hardly budget money and more comparable to the Canon RF 24-105mm f/4 USM L IS. So let’s see what you could get for your hard-earned bucks here.
What you’ll immediately notice upon first contact is the fact that this is a retractable zoom lens that has to be turned from its transport to working position by moving the zoom ring to the 28mm mark. Such a mechanism is more reminiscent of low-end lenses, although, to be fair here, it’s not a showstopper. It’s easy enough to “activate” the lens. You don’t have to press a button in conjunction with turning the zoom ring from its transport position. Still – it’s “weird” given the price tag. On the positive side, this design allows for a very compact transport mode. It’s 3cm shorter than the RF 24-70mm f/2.8 IS USM L. The build quality is otherwise fine, although more in line to what you’d expect from a consumer-grade lens – which it simply is, of course. The used materials are of good quality and the control rings operate smoothly. There’s even some degree of weather sealing. Canon wouldn’t be Canon if they included a lens hood. They do so with their L-class lenses, but this isn’t one of them, obviously. For a $1100 USD lens, this is a bit pathetic, though.
Coming back to more positive aspects – the AF is based on Canon’s stepping motor (hence the STM in the lens name). It is both very fast and silent. Manual focusing works by-wire, of course. Another feature is the image stabilizer, which alone is rated at 5 stops and 7 stops when combined with the camera’s IBIS (if available). Canon even lists a 7.5 stop efficiency “in the image center” only, which is an odd marketing stunt, really.
| Specifications | |
|---|---|
| Optical construction | 15 elements in 12 groups (2x UD, 2x aspherical) |
| Number of aperture blades | 9 (rounded) |
| min. focus distance | 0.27m (max magnification 0.24x) |
| Dimensions | φ 92.2 x 76.5 mm |
| Weight | 495g |
| Filter size | φ67mm |
| Hood | petal-shaped, bayonet mount, OPTIONAL |
| Other features | Image stabilizer (5.5 stops, 7 stops w/ IBIS), weather-resistant |
Distortions
The RAW distortion level that we can observe below is not great, but it’s not terrible either. At 28mm, there is a heavy barrel distortion of 4.3% which wouldn’t have been all that unusual during the SLR era either. Neutral distortion is reached around the 35mm mark, from which point onward the distortion type switches to pincushion. They are mild at 40mm, but increase to medium distortion at 70mm.
With activated auto-correction the issue is reduced to a negligible degree. Funnily, the pincushion distortion at 70mm is somewhat overcompensated to a mild barrel distortion.
Vignetting
While the RAW distortions aren’t too bad, the RAW vignetting is – which is not unheard of in consumer-focused Canon zoom lenses (…). At 28mm f/2.8, the image corners are almost black (without distortion correction). At f/8, vignetting is still at 2 EV (f-stops). The vignetting is less extreme at 40mm f/2.8 and 70mm f/2.8, and acceptable from f/4 here.
So full autocorrection is a good idea. This reduces the vignetting to a maximum of 0.8 EV (f-stops) at f/2.8, and it’s negligible from f/4. Please do note, however, that this comes at the cost of increased sensor noise due to the required signal amplification.
MTF (resolution) at 45 megapixels
The Canon RF 28-70mm f/2.8 IS STM has the weirdest resolution characteristic that we’ve seen from a zoom lens in a long while. In the low to mid range, the lens can rival prime lenses with an outstanding broader center quality and a very good outer image field. At 70mm, the broader center is still superb, but the border/corner quality falls apart. The border quality is still good at f/2.8 but the corners are downright poor. For decent corners, you have to stop down to f/11 (!) here, but it’s at least not impossible.
The centering quality of the tested sample was good. The field curvature is low.
Please note that the MTF results are not directly comparable across the different systems!
Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure of sharpness. If you want to know more about the MTF50 figures, you may check out the corresponding Imatest Explanations.
Chromatic Aberrations (CAs)
Lateral CAs mostly hover around 1px on average at the image borders. This is still moderate.
Bokeh Fringing / LoCA
LoCAs, aka bokeh fringing, aka axial CAs is a color fringing effect on the Z-axis. It shows up with a purplish tint in front of the focus point and a greenish tint behind – and it’s nearly impossible to fully correct in post.
The Canon RF 28-70mm f/2.8 IS STM produces some color fringing at f/2.8 as you can see below. However, it’s mostly gone at f/4 already.
There’s no detectable focus shift when stopping down.
Bokeh
The primary reason for preferring a fast standard zoom lens over something like a 24-105mm f/4 is shallow depth-of-field photography, so let’s check out the bokeh.
Out-of-focus highlights are fairly nicely rendered near the image center. There are only slight traces of “onion-rings” (from the aspherical elements) and the outline of the discs is also comparatively mild.
When looking at the highlight rendering across the whole image field, the disc shape remains intact across most of the image field. Only the corner discs show obvious “cat eyes” at 70mm f/2.8. Stopping down to f/4 improves the corner discs a little and they are restored at f/5.6.
The rendering quality in the focus transition zone is a bit … meh, unfortunately. The more critical background blur is nervous (shown to the left below). The foreground blur (to the right) is very smooth.
Sun-Stars
Sun stars are an aperture effect that occurs at strong light sources – such as streetlamps in nighttime city scenes. This effect develops slowly the more you stop down. As long as the aperture is near-circular, there isn’t much going on – the bright light is just rendered as a “blob”. The first rays start to appear at f/8, and they are quite nice from f/11 onward. The rays are a bit “fan-like” but the result is still fairly pleasing.
Competition
As of the time of this review, Canon users can only shop native full format RF zoom lens within the Canon lineup (or fall back to an EF adapter solution). The Canon RF 28-70mm f/2.8 STM (to the left) has 2 cousins that are more or less within its scope – the much, much more expensive RF 24-70mm f/2.8 IS USM L (center) which is, unsurprisingly, also the superior choice – or the Canon RF 24-105mm f/4 IS USM L (to the right) which is comparable in price. Of course, there’s the speed and range difference – and that’s a matter of taste. Optically, the 24-105mm f/4 is more consistent, whereas the 28-70mm f/2.8 has an advantage at its wide to mid-range setting. We’d prefer the more conventional mechanical construction of the L lens.
Sample Images
The Canon RF 28-70mm f/2.8 STM IS is a somewhat strange combination of qualities that you can find in professional and budget lenses. This doesn't mean that this concept doesn't work. If you can live with a few compromises, the lens can deliver great results. Optically, it is nothing short of impressive in the low- to mid-zoom range. The center quality is still great towards the long end, but the corners are weak here. If you use the 70mm for portraits, this won't matter, but it's not ideal for landscape or architecture photography. The lens relies heavily on image autocorrection. In RAW images, the distortion and vignetting is high, but then again - image autocorrection is a fact of life now anyway.
The same multiple personalities also show up in the lens' construction. Canon used lots of plastics, but overall the lens feels well-made - if it wasn't for the retractable zoom mechanism. There isn't much wrong about it, but this isn't a cheap lens by any standard. The lack of a lens hood is also hard to digest here. Canon did, however, implement some degree of weather-sealing. Further positive aspects are the very snappy AF, and the image stabilizer is quite efficient (although not mind-blowingly so).
If you absolutely need a fast standard zoom lens but can't afford the L-lens, Canon RF 28-70mm f/2.8 STM IS is a decent, albeit overpriced choice.
The Good
- Excellent quality in the low to mid range
- Quick AF
- Lightweight and compact
The Bad
- What happened at 70mm, Canon?
- Retractable lens design
- Relies heavily on auto-correction
- Overpriced
- Not including a lens hood at this price point is ...
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Optical Quality
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Build Quality
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Price / Performance