Review of the Incredible Canon RF 10-20mm f/4L IS STM

Do you remember the Canon EF 11-24mm f/4L USM that was released in 2015? It was a one-of-a-kind lens and a large and heavy one. Canon released an RF version, and it is not only smaller and lighter, it offers an even wider field of view. This is the review of that lens; the RF 10-20mm f/4L IS STM.

The Canon EF 11-24mm f/4L USM lens offered an amazing field of view without any significant barrel distortions. But it was a large lens with an enormous front lens that gave it an impressive appearance. Although it was well-balanced on a full-frame DSLR, it was heavy.

This has changed with the RF version of this lens. This one’s small and lightweight. It even offers a wider field of view, but it’s just one millimeter. However, this makes it the world's widest field of view for a rectilinear lens.

How It Looks

It’s rather amazing how Canon was able to reduce the size of this ultra wide-angle compared to its predecessor. The shorter flange range probably has something to do with it. The RF 10-20L measures 83.7 x 112mm and weighs only 570 grams, which is less than half the weight of the EF version.

It has a built-in lens hood, although its effectiveness can be doubted with such a field of view. More importantly, it protects the spherical front lens, which doesn’t protrude from the lens hood. The lens barrel does move while zooming. At 20mm, the lens barrel is positioned deeper, which increases the efficiency of the lens hood.

The RF 10-20L has a zoom ring, obviously, a focus ring, and the programmable control ring. There are two switches, one for the focus and one for the stabilizer. The lens also has a programmable button.

The minimum focus distance is 25 centimeters. This offers a maximum magnification of 0.12x at 20mm and 0.06x at 10mm. It makes the lens unsuitable for real macro. It’s not what this lens is intended for in the first place.

The shape of the front lens makes the use of filters impossible. Canon has provided the ability to use gel filters instead, located at the back of the lens. If you like to use graduated filters with this lens, you could choose a special adapter and a 150mm filter system, like the one manufactured by Haida.

The Field of View

The diagonal field of view at 10mm focal length is 130 degrees. As a number, this doesn’t give any idea how wide it truly is. Although the lens isn’t a fisheye by any means, it feels close. When compared with a 16mm field of view, it becomes clear how much more can be captured with a 10mm focal length.

This wide field of view has some major consequences for the appearance of vertical lines. Although it seems to be without a lot of distortion, there is an extreme amount of wide angle perspective distortion if the lens is pointed upwards or downwards. Besides these wide-angle distortions, objects towards the sides of the frame will become stretched. A car or television will become twice or three times its length. This is not because of this lens but a natural effect of an extreme field of view.

The wide angle perspective distortion will make vertical lines converge towards an imaginary distant point. This effect can easily be corrected in post-processing, but it will result in a reduced field of view and a significant loss in resolution. The only alternative for these converging lines is the use of a tilt-shift lens.

I have compared the corrected image with the RF 10-20L lens with the result from a TS-E 17mm f/4L lens. Even though a lot of correction is necessary, the RF 10-20L still offers a much wider field of view. But you will lose half the resolution in the process.

If the lens is kept perfectly horizontal, no perspective correction is needed. It makes the lens perfect for use in tight spaces. Just keep in mind the stretching at the left and right side of the frame towards the edges.

The Image Quality

No matter how wide the field of view is, if it results in a significant loss in image quality, the lens will be less useful. In the center of the frame, the quality is often good or even perfect. Defects will appear only at the edges of the frame, especially the corners. For an ultra wide-angle, it's difficult to maintain that quality.

Just like any modern ultra wide-angle lens, the RF 10-20L also strongly relies on in-camera lens correction. This becomes apparent when the correction is turned off. I have done so while photographing a brick wall from approximately 50 centimeters away. It shows an extreme barrel distortion at 10mm focal length, and it almost feels like the lens hood is visible in the frame. Naturally, this is less of a problem with a 20mm focal length.

With the lens correction turned on, it clearly shows how much improvement is achieved at 10mm focal length. The barrel distortion is nearly gone, and the vignetting is removed. This lens correction has some negative effect on the image quality in the extreme corners. The image is much softer at those corners, but the sharpness is improving when stopped down.

Coma and Flares

The RF 10-20L is perfect for extreme wide-angle landscape photography. In that case, it’s important to have no flares if the sun is in the frame. Although I didn’t capture a sunset or sunrise, the extreme field of view made it easy to capture the sun in the landscape during the winter. I haven’t noticed any flares at all, which is impressive. I’m sure there will be situations where flares will occur, but I didn’t encounter these yet.

More important are coma distortions. Especially for night sky photographers, these distortions in the extreme corners can ruin a perfect point like a star. I have captured a night sky, and even in the extreme corners, the stars show almost no coma distortion.

Conclusion

It’s easy to say I’m truly impressed with this lens. The lens has good image quality and doesn’t show a lot of flares or coma, as far as I noticed during my review period.

The small size and being lightweight are important factors. Compared to the alternative, the huge Canon EF 11-24mm f/4L USM, it has improved a lot. This lens is easy to take with you.

There is image stabilization available, which offers a maximum of six stops of stabilization. In theory, this allows you to shoot up to 8 seconds handheld. I haven’t tried this, but it allows for a lot of flexibility. Canon mentioned how the system compensates for a wobbly image in the corners due to stabilization, which is common in these extreme focal lengths.

Although the RF 10-20L offers a shorter focal length, the difference won’t be noticeable unless you compare it next to 11mm. But it does make a huge difference compared to the regular 14mm, 15mm, or 16mm focal lengths.

I reckon the RF 10-20L is perfect for the landscape photographer that loves an extreme field of view or the real estate and architectural photographer that works in tight spaces. It’s also perfect to capture a night sky.

If you need that much field of view, this lens is a no-brainer. Just be careful of the wide-angle perspective distortion.

What I Like

• Small, compact, and lightweight
• Image stabilization up to six stops
• Programmable button
• Silent STM autofocus
• Extreme field of view
• Image quality is good
• Almost no coma distortion
• Not sensitive to flares
• Lens cap is firmly secured
• Gelatin filter holder
• Nice star forming when stopped down

What I Didn’t Like

• Depends completely on lens correction, especially at 10mm
• Impossible to use the 100mm filter system
• Lens correction results in reduced image sharpness

Most of the remaining issues I have with this lens result from the extreme field of view itself, instead of the performance of this lens.

I want to thank Canon Netherlands for providing this lens for this review.