The “cutting edge” in photography doesn’t always come with the most expensive gear. It is possible to make a high-quality lens for a lower price than ever. Canon’s RF100-400mm F/5.6-8 IS USM is a good example.

Qualitative Construction

The Canon RF100-400mm f/5.6-8 IS USM full frame telephoto zoom lens measures 3.13 inches by 6.48 inches (7.95 x 16.46 centimeters). When zoomed up to 400mm, the length is 9.75 inches (24.76 cms). The lens weighs in at 6.4 ounces (635g) and is therefore lighter than the standard zooms.

The lens was very light and small, so I spent a lot of time hiking in the National Forests at Mammoth Lakes. It is easy to carry around and doesn’t weigh much.

Canon RF 100-400mm f/5.6-8 IS USM lens mounted on Canon EOS R3.
Canon RF 100-400mm f/5.6-8 IS USM lens.

The lens has a rubberized zoom and focus ring and a customizable plastic control ring. It also features a lock switch that can be activated at 100mm, an A/MF focus mode switch and a switch to turn off or on the lens image stabilization.

The rings are extremely smooth, and I have had worse experiences using telephoto lenses twice the price. The zoom ring has a 45-degree throw. It is stiff and smooth, so I didn’t have to worry about zoom creep or accidental shifts.

One time I mistakenly set the zoom to 340mm, thinking I was shooting at 400mm. This was a rare event, but I have also used other telephoto lenses where the zoom is too loose. Canon deserves credit for doing it right. Although I would have preferred the lock switch to work at 400mm, it was unexpected that it would be working when I entered this review.

Canon RF 100-400mm f/5.6-8 IS USM lens at 100mm.
Canon RF 100-400mm f/5.6-8 IS USM lens at 100mm.
Canon RF 100-400mm f/5.6-8 IS USM lens extended to 400mm.
Canon RF 100-400mm f/5.6-8 IS USM lens extended to 400mm.

The RF 100-400mm is a non-L-series Canon Lens. It does not have weather sealing. The mount doesn’t have a rubber gasket for sealing the lens to the camera body. I don’t believe any of the rings or switches offer extra protection from moisture and dust. Although the lens is in good working order, I am curious about how long it could last without any sealing.

The box does not include a lens hood. A compatible Canon ET-74B is available for an additional $45.

Canon RF 100-400mm f/5.6-8 IS USM lens.

Image Quality

It is the quality of the lens’ images that will last, regardless of how high-quality it operates. First, I tested the sharpness at 100mm and 400mm. The lens can be opened wide at f/5.6 at 100mm. This is sufficient to ensure sharpness. However, the center sharpness of the lens is at its best with f/9.

For maximum depth of field, I noticed that the apertures between f/22 at 100mm and f/32 at 100mm were too soft. The lens is wide open at the 400mm end at f/8. I found maximum center sharpness at f/8, and then I stopped down to f/10. But, honestly, the differences between f/8 & f/10 were subtle. 400mm was too wide for me to see f/20 down to f/45.

Next, I examined vignetting using in-camera compensation. The 100mm end shows a significant improvement, with only a one-third stop at f/6.3. After that, vignetting is mostly cleared by f/9. At 400mm, there was a significant improvement with two-thirds of the stop down at f/10 and a nearly even exposure at f/14. The effects of vignetting will be minimized if lens compensation is turned back on in camera or let RAW processing software handle things during import.

A $650 lens with this focal range would seem to have a high price. I assume that there is a lot of ghosting and fringing. Surprisingly, this doesn’t appear to be the case. It is noticeable that there is more purple and green fringing at 100mm than at 400mm. However, it is well managed across all areas. Although subtle ghosting is not a problem when shooting with the sun in frame, it is noticeable.


To eliminate as much variation as possible in autofocus testing, I used the RF 100 to 400mm f/5.6-8 IS USM mounted onto the Canon EOS R3, Canon’s top autofocus performer. The RF 100-400mm has a nano ultrasonic motor (Nano USM), for autofocus. I found it almost silent. The lens performed flawlessly in simple shooting situations where it was asked for a move from one focus plane to the next. It is very quick to adjust its focus and track general movements throughout the frame.

400mm | f/8 | 1/250s | ISO 3200
359mm | f/8 | 1/400s | ISO 1600

Two scenarios required extra care after I had shot with the lens.

First, the EOS R3 lens paired with the EOS R3 could not focus birds in low light (AF-C) using Servo AF (AF -C). The reliability of the EOS R3 was improved in certain cases by switching to One-Shot (AF-S), but it wasn’t always that easy. It happened in situations that I didn’t expect to be troublesome, but I have never used an autofocus lens with an aperture greater than f/8 before. One instance was when I tried to photograph a bird in a tree at noon, but it was too dark. Despite the fact that the situation was not complicated, I had to manually focus the shot.

Another problem is tracking speed for fast-moving subjects. My dog would sometimes run towards me and leave behind a trail of photos. I could still see the EOS R3 tracking the subject accurately in the viewfinder. The RF 100-400 doesn’t have the autofocus precision or quickness required to reliably keep up with these difficult cases.

400mm | f/8 | 1/2500s | ISO 1000
400mm | f/9 | 1/2500s | ISO 1600

Telephoto Zoom for Everyday Photographers

The RF 100-400mm is the culmination of many years of Canon lens development. It’s easy to forget about, so I’m glad I got to try it. It is small and light, which makes it difficult to dismiss the optics. The maximum apertures at 100mm and 400mm are very slow, but it costs $650, which is the lowest price I have seen for a full-frame zoom telephoto zoom in this range, even including third-party brands. These are not confidence-builders, but Canon seemed to be able to identify the intended audience for this lens.

Conceptually, I like the “budget” of RF 100-400mm because it is a combination of what is available and what would be difficult or costly to implement. It’s better to do it this manner than the alternative, which would have meant poor or unacceptable performance that would have brought down the entire experience.

200mm | f/11 | 1/320s | ISO 100

What are the Alternatives?

Canon’s rapid release of long telephoto RF mount lenses is amazing to me. The RF mount is still relatively new. Canon also has a higher-end RF 100 to 500mm f/4.5-7.7L IS USM lens available, alongside the RF 100 to 400mm f/5.6-8 IS USM.

Canon RF 100-575mm f/4.5-7.7L IS USM lens is an L-series lens which eclipses 100-400mm almost in every way. With its extra 100mm, it has a wider zoom range at the far end. It’s slightly faster, with a third-stop increased maximum aperture at both the wide and the telephoto ends. However, these benefits have a drawback: the lens is heavier and larger than the RF 100–400mm.

The RF 100-400mm comes with a heavier tripod collar, while the RF 100-40mm does not have one. Additional lens controls include a focus limiter and three stabilization modes. The zoom ring can also be adjusted to adjust the tightness. It is an L-series lens and has a weather-resistant construction that features more seals and attention paid to materials. This lens has better optics and reliable autofocusing. However, it comes at a staggering $2,900.