Canon RF 10-20mm f/4L IS STM review: the ultimate ultra-wide zoom

The Canon RF 10-20mm f/4L IS STM is the full-frame mirrorless successor to the Canon EF 11-24mm f/4L USM for DSLRs, which was launched back in 2015. 

Far from just a quick mirrorless update, the Canon RF 10-20mm F4L IS STM makes full use of the RF mount – which enables the rear element to sit much closer to the camera sensor, opening up a realm of innovation in optical engineering. Indeed, this is the world's first rectilinear zoom lens with a 10mm focal length and autofocus – making it, on paper at least, one of the best wide-angle lenses for those who work at the extreme end. 

We were able to compare our beta sample to the EF 11-24mm that it replaces, and looking at them side-by-side the new lens is considerably more compact and portable with a much neater cylindrical footprint (the older EF lens has a more awkward shape, which could be more difficult to load into a kit bag). 

Weighing just 570g it’s also considerably lighter, yet packs in new features like a Control Ring, custom button, and all-new tech like Peripheral Control Image Stabilization – impressive!

The new lens only zooms to 20mm at the long end, rather than 24mm on the prior model, though it does go slightly wider to 10mm rather than 11mm. This is a welcome tradeoff, as most people looking to buy this optic will likely have access to a 24mm focal length (with a lens like the Canon RF 24-70mm f/2.8L or Canon RF 24-105mm f/4L). Having a slightly wider focal length and cutting down on size and weight is a smart choice.

Canon RF 10-20mm f/4L IS STM: Specifications

Canon RF 10-20mm F4L IS STM: Key Features

Thanks to the advantages afforded by Canon's RF mount, it's been possible to improve the RF 10-20mm in pretty much every conceivable way over its EF predecessor. It has been made much lighter and more compact, while also being able to squeeze in the latest tech, like its 5-stop Image Stabilizer, Peripheral Control IS technology, a Control Ring and a custom button.

It's also Canon's first L-series lens to feature a stepping motor (STM), known for its smooth and quiet operation, making it ideal for recording video. In another first for Canon, it's also the company's first optic to use an STM positioning sensor. This means that the STM autofocus motor doesn't have to reset all the way, each time the camera is turned off, which makes it faster to get up and running with little delay. 

A filter holder on the rear element is featured for mounting 35 x 20mm gelatine filters, which we'll come back to later. The lens also boasts a built-in plastic hood, and comes with front and rear lens caps and a soft case for storage.

On the lens barrel you'll find the large focus ring for full-time manual focusing, a Control Ring that can be set to adjust a number of key settings, an AF/MF switch, and a custom button. There's also a switch to engage the optical Image Stabilization (IS), which offers five stops of stabilization on its own or six-stops when used with a compatible EOS R body with in-body image stabilization (IBIS).

Canon RF 10-20mm f/4L IS STM: Build & Handling

One of the most notable differences between the new lens and its predecessor is its portability and compact design. Weighing just 570g, it is 610g lighter – that’s a weight reduction of over 50%! And with dimensions of 83.7 x 111mm, it’s also about 35% smaller in footprint – an astonishing improvement!

This is a welcome change and makes it much easier to fit into a camera bag, as ultra wide-angle zooms are generally quite awkward shapes and can take up a lot of space. 

Due to the lens being a slimmer design and much better balanced (the old EF 11-24mm feels really front-heavy by comparison) we can see the RF 10-20mm being a much better choice when it comes to video, lending itself to gimbal use nicely such as on the DJI RS 3 Pro Combo. It could even be a good choice for drones (if yours is powerful enough to support a full-frame mirrorless body and a lens).

Handheld video should be impressive too, though it wasn't something we were able to extensively test in our short time with the beta model. When shooting handheld, users will be able to take advantage of the new peripheral Image Stabilization which corrects for wobble at the corners of the frame caused by the IBIS system when capturing video. In short, Canon has created an algorithm that calculates the amount of distortion occurring at the sensor level; the lens IS is then employed to compensate for it.

The autofocus stepping motor (STM) should be a boon for video use, too, as these motors are notably smoother and quieter during operation. It also means that focus whirr shouldn't be recorded by your internal or external microphone. In our testing we found the autofocus to be both very quiet and snappy.

Despite being smaller and lighter than its predecessor, the Canon RF 10-20mm squeezes in lots of new physical features. Many of these are found on the lens barrel, such as the Control Ring (for fast access to exposure settings like ISO), a custom button and a switch for the Image Stabilizer. That said, the focus distance window found on its predecessor has been omitted this time around.

The rear gelatine filter holder is a welcome addition, though it's fairly basic and limiting. While neutral density and color filters could be handy to rear-load for video use, filters like ND grads, variable NDs and polarizing filters will be more tricky.

Lee Filters make a specialist SW150 filter holder and series of filters designed for ultra-wide lenses, such as the EF 11-24mm and other ultra-wide optics like the Sigma 14mm f1.8 DG Art. While Lee has yet to announce any plans to make its SW150 system compatible with the new Canon lens, it seems likely given time. This would enable a full range of filters to be used, opening up more options for landscape photographers.

Canon RF 10-20mm: Photo Performance

When it came to autofocus the STM impressed as focusing was quick, smooth, and quiet, making it ideal for both stills photography and video. As you'd expect from one of Canon's top-flight L-series optics, all the rings on the lens barrel felt smooth and the switches clicked nicely into place – and they were also easy to find and engage when you have an eye to the viewfinder.

When it comes to image quality the Canon RF 10-20mm f/4L IS STM boasts an advanced rectilinear optical design. You'll usually find lenses with an ultra-wide focal length of 10mm (full-frame) to be a fisheye with bags of barrel distortion. 

A rectilinear design, however, makes this optic special, as vertical lines of buildings should remain straight, making it a good choice for architecture and landscape photography. Of course, you pay a premium for this, with Canon's new lens costing $2,299 compared to a lens like the 7Artisans 10mm f/2.8 manual focus fisheye which costs just $139.

While a focal range of just a 10mm difference doesn't sound all that impressive on paper, you might question if this is worth it over just a super-wide prime. However, when you can actually see the difference it makes in photos and video then it is much more apparent how big of an effect this small focal length can achieve. Below are the same images shot at 10mm and 20mm to show the full range of the lens.

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One issue I did note with our sample was that there was a strong vignette present in the corners of RAW files when shooting at the widest 10mm setting, though this was corrected with the in-camera JPEGs. 

There is a lens profile to correct this in Adobe Camera Raw, Lightroom, and Canon Digital Photo Professional, so it isn't really an issue, but I think this is something that we can expect on all wide-angle lenses going forward as lenses get smaller and digital corrections get much better. In Lightroom looking at the RAW images, I could not tell that any corrections had even been applied.

I don't remember the vignetting or curvature of straight lines to be notable when I was framing up, so the in-camera software is likely correcting this to make the live view display look more like the processed JPEGs. These corrections should also hopefully be applied to any video footage.

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Below you can see the RAW file from our R5 on the left and the in-camera processing in the JPEG hard at work on the right, to pull and stretch any vertical and horizontal lines to make them straight. It also pulls the most egregious vignetting found in the RAW file out of shot. 

The lens is very sharp, I tested this at the same time as the considerably cheaper Canon RF-S 10-18mm lens and the results were incredible, with the 10-20mm lens being clearly visibly superior. Unsurprising when the 10-20mm lens features sixteen elements in five groups, with three glass-molded aspherical (GMo) elements, one of which is UD, as well as a Super UD element to boot. It also boasts Super Spectra, Air Sphere (ASC), and Sub Wavelength (SWC) coatings.

How does this compare to the previous EF version of the lens though? Well, to find out I set up on a tripod and shot an identical scene at f/8 with both the Canon RF 10-20mm f/4L IS STM and the old Canon EF 11-24mm f/4L USM to see how they compared. While only a 1mm difference at the widest setting, the RF 10-20mm does squeeze significantly more into the frame.

Results at f/8 we found to actually be quite similar, although the newer RF version had a slightly better level of sharpness across the entire frame, with just better edge and corner clarity in the new RF model.

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Canon RF 10-20mm f/4L IS STM: Sample Images

Canon RF 10-20mm f/4L IS STM: Video Performance

Video performance was very impressive with 4K footage shot using the Canon EOS R5 looking so sharp and clear. The autofocus throughout was also whisper quiet and very very fast, paired with Canon's excellent subject recognition and tracking algorithms I really didn't have to think much about the focus at all in the moment, and when checking through footage everything was pin sharp.

Image stabilization is especially good, even more so when combined with Canon's in-body image stabilization in the EOS R5. Only the R5 running the latest firmware will be able to make full use of the new Peripheral Control Image Stabilization function at launch. This is a clever bit of algorithmic technology that promises to eradicate the "Jell-O"-like wobble at the corners of the frame, which is often the result of using ultra-wides with in-body image stabilization systems.

Footage was smooth in panning and static shots, and even walking wasn't horrendous, although still not close to the level of a gimbal, it is better than a lot of other Canon lenses I have tried. The super wide angle of this lens also means using Canon's active image stabilization and its cropped footage is a lot more reasonable. 

Canon RF 10-20mm f/4L IS STM: Lab Results

We run a range of lab tests under controlled conditions, using the Imatest Master testing suite. Photos of test charts are taken across the range of apertures and zooms (where available), then analyzed for sharpness, distortion and chromatic aberrations.

We use Imatest SFR (spatial frequency response) charts and analysis software to plot lens resolution at the center of the image frame, corners and mid-point distances, across the range of aperture settings and, with zoom lenses, at four different focal lengths. The tests also measure distortion and color fringing (chromatic aberration).

Sharpness:

Center sharpness is excellent, at all tested focal lengths, even wide open at f/4. Corner sharpness appears much softer, however ultra-wide lenses need to be positioned very close to our large, flat test chart in order to maintain correct framing. This means the corners of the chart are much further away from the lens relative to the center, causing them to fall outside the lens's focal plane, thereby reducing the corner sharpness scores.

Fringing:

Fringing is noticeable at all focal lengths and apertures, though it's not severe.

Distortion:

Canon seems to have abandoned any efforts to reduce optical distortion in its RF-mount ultrawide lenses, instead relying on in-camera firmware corrections. With this turned off, the RF 10-20mm f/4 exhibits catastrophically bad barrel distortion at 10mm, and it's still pretty awful at 12mm. Thankfully this is very effectively ironed out by enabling in-camera distortion correction - an option which really shouldn't be turned off.

Canon RF 10-20mm f/4L IS STM: Verdict

The Canon RF 10-20mm f/4L IS STM improves on the EF 11-24mm f4L USM in every way possible – and while I won't go as far as calling it affordable, but it won't cause such a big dent in your wallet either. If you were considering buying the older EF version, getting this new version is a no-brainer, and for those who are considering upgrading, it is well worth it.

While our sample did show some vignetting at the wide end throughout the aperture range in the RAW photos, Adobe Camera Raw will be able to correct this effectively and it shouldn't be an issue. This is also negligible with in-camera JPEG processing stretching it out of shot when correcting the vertical and horizontal lines. The camera can also impressively relay the corrected image in the viewfinder, so I could frame my shots perfectly.

The RF 10-20mm lens is notably sharper across the board compared to its predecessor, I tested the Canon RF-S 10-18mm at the same time as this, and it is night and day between the quality of the two, so you definitely are getting what you pay for here! Its impressive results extend into video as well, with solid image stabilization even at full width, and great 4K footage. 

If you are a pro who needs the widest angle possible, then this is the best Canon wide-angle lens so far!