In summary
The Canon RF 10-20mm f/4L IS STM lens has the widest angle of view so far for a zoom lens made for a ‘full-frame’ camera – and it’s also the widest lens with both autofocusing and in-built optical stabilisation. This makes it both special and quite challenging to use effectively.
There are a few 10mm lenses made by third-party manufacturers like Laowa, Rokinon and Samyang, but most are manual focus primes. Their main advantage is significantly lower prices.
Canon’s RF 10-20mm f/4L IS STM lens is pricey for a lens with limited applicability, however its excellent build quality and good performance will justify the investment for photographers who really need such wide coverage.
Full review
Announced in October, 2023, the Canon RF 10-20mm f/4L IS STM lens is designed for photo and video professionals and encompasses an angle of view of up to 130 degrees at the 10mm position. Weather-resistant construction, high flare resistance and a customisable lens function button on the barrel make it ideal for photographers who need ultra-wide coverage when recording stills or shooting movies. This lens comes with a built-in lens hood and at the 20mm position its front element withdraws into the lens barrel, increasing the hood’s efficiency.
Angled view of the RF 10-20mm f/4L IS STM lens showing the built-in lens hood and outwardly bulging front element. (Source: Canon.)
Unlike most lenses with flatter front elements, the RF 10-20mm f/4L IS STM lens can’t accept screw-in filters. Instead, there’s a filter holder for gelatin filters, which need to be cut-to-size and inserted behind the rear lens element. Templates are provided in the downloadable instruction manual (available from Canon’s website) for cutting filters for both full-frame and cropped-sensor cameras.
The optical design of this lens consists of 16 elements in 12 groups and includes three aspherical elements, three standard UD (ultra-low dispersion) elements and one Super UD element to minimise chromatic aberration and distortion throughout the zoom range. Canon’s Air Sphere Coating (ASC) and multi layer Super Spectra Coating (SSC) have been applied to suppress ghosting and flare and ensure consistent colour reproduction and sharp details.
A fluorine coating on the front surface effectively repels dust, water droplets, grease, and dirt, making the lens easy to keep clean and reducing the need for repeated wiping. It also has an anti-reflective effect that helps to ensure clear images.
The nine-bladed iris diaphragm creates a circular aperture for smooth and pleasing bokeh. Peripheral Control Image Stabilisation (IS) technology offers five stops of optical stabilisation, which increases to six stops in combination with an in-body stabilised camera, like the EOS R5 II we used for our tests. The new IS technology can also minimise wide-angle ‘fluttering’ at the peripheries of the frame
This is the first Canon L lens to feature a lead-screw type ‘STM’ stepping motor to drive autofocus, which should provide quiet AF when capturing video. The AF system also includes a position sensor so the lens can return to the previous AF setting when the camera’s power is cycled off.
The minimum focus is 13 cm when the lens is focused manually or 28 cm when it is used in AF mode, where reproduction is roughly half life-size at the 30mm focal length.
Who’s it For?
The RF 10-20mm f/4L IS STM lens is designed and priced for professional users who need to record extreme ultra-wide shots or videos. Although it weighs roughly half of the weight of Canon’s previous ultra-wide zoom, the EF 11-24mm f/4 L for DSLR cameras, it’s still quite a large and heavy lens. It also offers slightly wider coverage, making it a better option for photographers and videographers who shoot landscapes, cityscapes, architecture, real estate and weddings.
Wide coverage combined with a relatively compact size and minimal focus breathing make this lens a good choice for videographers. (Source: Canon.)
This lens is also a good choice for astrophotographers who specialise in wide-angle starscapes. The wide field of view is not entirely distortion-free as you’ll quickly see a significant amount of perspective distortion if the lens is pointed upwards or downwards.
In addition, even when the lens is held perpendicular to the scene, objects towards the edges of the frame will appear stretched as a result of the extreme field of view. While the obvious barrel distortions at the widest angles of view can be managed by in-camera corrections, the spread of perspective is an inherent factor in ultra-wide-angle lenses.
If this can be tolerated (or taken advantage of by creative photographers), this lens is great for shooting in tight spaces and capturing interesting viewpoints. Sharpness is improved when the lens aperture is stopped down and the lens is surprisingly flare-free, even when the bright light source is inside the frame.
The relative compactness and low weight of this lens make it easy to pack into a camera bag and therefore good for travellers. Weather-resistant construction enables it to be used in mist and very light rain, making it a good choice for location shooting.
Build and Ergonomics
While much is made about the relatively compact size of this lens, it’s still quite large for a wide-angle lens. The constant fast (f/4) maximum aperture and in-lens image stabilisation contribute to its size, as does the optical complexity.
That said, the new lens also has the advantage of more up-to-date technology plus the short flange distance of the RF mount. This enables the path of light through the lens to be straighter, reducing the potential for aberrations.
As noted above, the front element, which has a diameter of roughly 55 mm, bulges outward a little further than the shallower parts of the lens hood rim. Immediately behind the hood is a narrow red branding ring, followed by the 10 mm wide, programmable control ring, which has click stops to permit precise adjustments to the selected function. (The ring can be de-clicked by the Canon Service Centre – for a fee.)
The focusing ring is located behind the control ring and separated from it by a 2 mm wide section of the barrel. It’s approximately 14 mm wide with a narrowly-ridged rubber-like grip band. Because focusing is driven from the camera, this ring turns through 360 degrees when no power is supplied.
Immediately behind the focusing ring is the zoom ring, which is about 26 mm wide, with a 22 mm wide rubber-like grip band that dips inwards in its middle, which makes it more comfortable to handle. A 4 mm wide smooth section at the trailing edge of the ring carries index markings for the 10mm, 12mm, 14mm, 16mm, 18mm and 20mm focal lengths, which are lined up against a white mark on the fixed, 27 mm wide section of the barrel that leads to the metal lens mount.
Slider switches for the focusing mode (AF/MF) and image stabiliser (ON/OFF) are located around the left hand side of this section of the outer barrel where they’re easy to reach, just above a programmable lens function button. The last couple of millimetres of the barrel are set-in and made from metal. The index mark for mounting the lens is located here.
A narrow rubber gasket surrounds the lens mount to keep out moisture and dust. Inside it and covering the rear element is the gelatin filter holder. The supplied lens cap fits over the built-in hood and is held in place by two pinch clips. An RF rear cap and soft carrying Pouch (LP1219) are also provided.
Performance
Our Imatest tests showed the review lens to be a good performer on the EOS R6 Mark II camera supplied for our tests. Not surprisingly for such a wide-angle zoom lens, we found edge and corner softening at all focal lengths with both JPEGs and CR3.RAW files, which were converted in to 16-bit TIFF format with Adobe Camera Raw, our preferred raw file converter.
The highest centre resolution was obtained at the 16mm focal length, one-third of a stop down from the maximum aperture, with the lowest at the 10mm focal length. Interestingly, our test results show consistent results across a wide range of focal length and aperture settings until diffraction kicked in at around f/16, as shown in the graph of our test results below.
Because chromatic aberration is corrected in the camera – as are vignetting and distortion – we carried out these tests on converted raw files. The results showed lateral chromatic aberration remained entirely within the ‘negligible’ band, which is better than expected for such a wide-angle zoom lens. In the graph of our test results below, the red line marks the border between ‘negligible’ and ‘low’ CA.
Vignetting could be seen at f/4 in uncorrected files across all focal lengths, which was expected for this type of lens. It was most obvious at 10mm but remained visible at all focal lengths when the lens was stopped down to between f/5.6 and f/7.1.
Rectilinear distortion was also found at all focal lengths, ranging from very obvious barrel distortion at 10mm and 12mm through to slight barrelling at 20mm. Both vignetting and distortion are well addressed by the in-camera corrections and also easily corrected when converting raw files into editable formats.
Backlit subjects were handled particularly well, although we found a few flare artefacts when a bright light source was within or just outside of the frame, especially with wider angles of view. Fortunately, there was no apparent loss of contrast due to veiling flare, even when the light source was inside the frame. The review lens produced particularly fine, 18-pointed sunstars around bright light sources when stopped down to f/22.
Autofocusing was generally fast and accurate and the built-in stabilisation system worked well for all our test shots. Close focusing is limited to 25 centimetres throughout the zoom range and provides interesting perspectives at the widest angles of view, as shown in the Samples section.
Despite the fast maximum apertures across the zoom range, it was difficult to obtain soft backgrounds in close-ups, even at the widest aperture settings and with the 20mm focal length. Angular distortions were more eye-catching at shorter focal lengths, where distant areas of the scene were only slightly softened at f/4 and while some background softening could be seen at 20mm at f/4, bright background highlights often appeared as donuts.
Conclusion
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SPECS
Picture angle: 130 to degrees diagonal
Minimum aperture: f/22
Lens construction: 16 elements in 12 groups (including 3 aspherical, 3 UD and 1 Super UD elements); ASC and SWC coatings plus a fluorine coating on the front surface
Lens mounts: RF mount
Diaphragm Blades: 9 (circular aperture)
Weather resistance: Dust- and moisture-resistant sealing
Focus drive: Stepping motor (lead-screw type)
Stabilisation: In-lens optical; up to 5 stops (6 stops with IBIS coordinated control)
Minimum focus: 25 cm
Maximum magnification: 0.06x at 10mm, 0.012x at 20mm
Filter size: Rear gelatin filter holder
Dimensions (Diameter x L): 112 x 83.7 mm
Weight: 570 grams
Standard Accessories: Front and rear caps, soft lens case
Distributor: Canon Australia; 1800 021 167
TESTS
Based on JPEG images captured with the Canon EOS R6 Mark II camera.
Based on CR3.RAW files recorded simultaneously and converted into 16-bit TIFF format with Adobe Camera Raw.
SAMPLES
Vignetting at 10mm f/4.
Vignetting at 12mm f/4.
Vignetting at 14mm f/4.
Vignetting at 16mm f/4.
Vignetting at 18mm f/4.
Vignetting at 20mm f/4.
Rectilinear distortion at 10mm.
Rectilinear distortion at 12mm.
Rectilinear distortion at 14mm.
Rectilinear distortion at 16mm.
Rectilinear distortion at 18mm.
Rectilinear distortion at 20mm.
10mm focal length, ISO 200, 1/80 second at f/4.5.
20mm focal length, ISO 200, 1/80 second at f/4.5.
10mm focal length, ISO 200, 1/13 second at f/7.1.
20mm focal length, ISO 200, 1/13 second at f/7.1.
Sunstar at 10mm focal length, ISO 200, 1/200 second at f/22.
Sunstar at 20mm focal length, ISO 200, 1/400 second at f/22.
Close-up at 10mm focal length, ISO 200, 1/80 second at f/4.
Close-up at 20mm focal length, ISO 200, 1/80 second at f/4.
Stabilisation test; 13mm focal length, ISO 200, 1/3 second at f/7.1.
10mm focal length; ISO 200, 1/640 second at f/10.
10mm focal length; ISO 200, 1/160 second at f/10.
14mm focal length; ISO 200, 1/320 second at f/8.
14mm focal length; ISO 200, 1/100 second at f/5.
20mm focal length; ISO 200, 1/200 second at f/16.
10mm focal length; ISO 200, 1/500 second at f/11.
Rating
RRP: AU$4199
- Build: 9.1
- Handling: 8.9
- Image quality: 9.0
- Autofocusing: 9.1
- Versatility: 8.5
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