Canon Is Breaking Records Yet Again With the RF 10-20mm F/4 L IS STM And We've Tried It!

The title of the widest AF-capable rectilinear lens on the market has belonged to Canon for a long time with their EF 11-24mm F/4 L USM. Well, the king has been surpassed with today’s introduction of the RF 10-20mm f/4 L IS STM. How is it? We’ve used it for a few days to tell you all about it.

Straight Shooter

The key word in the title is “rectilinear.” To sum it up, every single straight line shot using this lens, no matter how close, is going to stay straight with no barrel distortion. Even at the extremely wide end of 10 millimeters, the lens does not succumb to the typical fish-eye effect.

The design of the lens is pretty simple. It fits with the rest of the L series lenses well with its plastic outside, a trio of control rings, and a few switches. The first one toggles between AF and MF modes, and the second one toggles the image stabilization, which is a feature the EF 11-24mm lacked. Last but not least is a user-programmable FN button like the one we’ve seen on the RF 135mm f/1.8 L IS USM.

Even considering the focal range, the lens is by no means large, nor is it heavy — quite the opposite in fact. It is surprisingly lightweight, but it does not feel cheap. Being an “L” lens it is, of course, weather-sealed even with the fact the front element moves while zooming. It does, however, stay well within the bounds of the built-in hood, so it is adequately protected.

Fast, Sharp, and Silent

Unlike the predecessor with its ultrasonic motor, the RF 10-20mm f/4 L IS STM uses a stepping motor. This is generally preferred for video use due to its lower audible noise levels compared to USM motors. However, some photographers prefer the ultrasonic motor.

Considering the sheer wideness of the lens, though, autofocusing speeds are honestly not a cause for concern. The autofocus is fast and snappy, and in my two days of using it, it seemed accurate. Shooting at 10 millimeters with an aperture of f/4 gives plenty of wiggle room. Stopping down to smaller values makes the matter even easier.

In terms of sharpness, it goes well with the rest of the “red-ringed” RF lenses. The 10-20mm renders plenty of detail even with the high-resolution sensor in the EOS R5. Yes, the corners are stretched out a bit, but that goes with the rectilinear nature of the lens. Chromatic aberrations are minimal even when shot directly against a strong source of light. There was an occasional lone flare when aimed into the sun at dawn, but nothing terrible or unfixable.

Vignetting seems well corrected even wide open but that I probably wouldn’t credit the lens for. Software seems to be the real hero when it comes to vignetting. I managed to spot a little bit of LOCA (longitudinal chromatic aberration) but only when I focused at the closest possible distance using the wide end of the zoom (the close-up of a cat in the gallery below shows it in the out-of-focus window frame). Focusing further out pretty much gets rid of LOCA altogether.

Corrected Quite a Bit

After my first day out with the lens using a Canon EOS R5, I got my laptop out, imported the CR3 files into Lightroom, and immediately realized that this lens was a secret to many. Because even Adobe had no idea what lens I shot the images with. Thanks to that, I saw the raw files with no lens corrections whatsoever. Let me tell you, they are absolutely necessary. Have a look at the same file shot using an R5 at 10mm and f/8. The first one is processed using Canon’s own DPP, whereas the second one was processed by Lightroom, which had no corrections available at the time of writing this preview.

Finally Here and Worth the Wait

It has been less than a week since I had a conversation with a fellow photographer who was complaining about the lack of a serious wide-angle lens for the RF mount. Well, here we are. The new RF 10-20mm f/4 L IS STM is perfect for architecture, real estate, landscapes, and cityscapes, and I can even imagine using it at concerts and parties. It is clear that the f/4 aperture might be limiting to some, but the low-light performance of something like a R6 Mark II is stellar, and the weight and size benefits are clear. 

Canon has truly created something impressive and unique with this piece. Small, lightweight, fast, sharp, and silent. Everything you want in a lens. Many photographers now have to stop resorting to adapting the larger and heavier DSLR 11-24mm f/4 and get a more modern unit. 

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