I'll start by saying, "in-depth" is relative in this case. The Canon RF 100-500mm F4.5-7.1 L IS USM Lens and Canon EF 100-400mm f/4.5-5.6L IS II USM Lens are two of the best telephoto zoom lenses ever made. As long as the lens will mount on your camera (the RF lens requires a Canon EOS R-series camera), there is no bad choice to be made here. Overall, these lenses are similar in so many regards that it is a stretch to create an "in-depth" comparison.
Let's start by looking at the name. Obvious is that the RF lens has a considerably longer focal length range. The 400mm vs. 500mm difference is illustrated in the RF 100-500mm Lens review. The 500mm focal length is very noticeably longer than the 400mm focal length, and I can think of no reason to not prefer the longer range. That is, no reason as long as the maximum aperture remains similar at 400mm.
We know from the name that the RF 100-500's max aperture is narrower at the 500mm end, and the unusually narrow to-f/7.1 spec gives us pause. The Dual Pixel CMOS AF system in Canon's R-series cameras autofocus lenses with narrow apertures very adequately, making very narrow openings quite usable, but we still want to know if the aperture opening size was being sacrificed at the 400mm focal length to keep this lens compact and light.
What is the Canon RF 100-500mm F4.5-7.1 L IS USM Lens's maximum aperture opening at 400mm? The early answers were:
Answer 1: When the camera is set to 1/3-stop increments, the maximum 400mm aperture reported to the camera is rounded to f/6.3.
Answer 2: When the camera is set to 1/2-stop increments, the maximum 400mm aperture reported to the camera is rounded to f/5.6.
While the max aperture does not change in these modes, the rounding of the actual opening size does, and the true aperture is likely between these two numbers, holds for only a short range of focal lengths, and the 1/3-stop difference in noise made visible by an offsetting 1/3-stop ISO change will matter to few. For the RF lens, Adobe software says that 400mm is f/6.2 (and that 500mm is 7.0).
Here is a comparative look at the max aperture step-down by focal length for these two lenses:
|Canon EF 100-400mm L IS II||100-134mm||135-311mm||312-400mm|
|Canon RF 100-500mm L IS||100-150mm||151-253mm||254-362mm||363-471mm||472-500mm|
While the RF 100-500 holds f/4.5 modestly deeper into the focal length range, the EF 100-400 L II holds a modest advantage at the longer focal lengths. Still, the differences are minor.
Back to the focal length range available: both lenses accept extenders, enabling the native focal length range to be significantly lengthened. Noteworthy is that the RF lens will not zoom wider than the marked 300mm native focal length while an extender is mounted. Extenders are primarily used for obtaining a focal length not native to the lens, meaning the 300mm wide end limitation is not important. The RF 100-500 better avoids the need for extender use, but being able to zoom out is sometimes helpful when an extender is mounted, such as when finding a subject in the viewfinder, and being able to fully zoom out makes the lens significantly more compact. Very noticeable is that the 100-500 range can be extended substantially longer than the 100-400mm range, up to 1000mm vs. 800mm. The max aperture of 500mm with extenders becomes rather narrow, though as we just looked at, the difference at the equalized 400mm focal length is minor.
Let's look at the image quality comparison next. The RF lens has less lateral CA in the periphery at 100mm and might be slightly sharper at this focal length at f/4.5. That is the only even somewhat noteworthy difference I see in the shared focal length range. Comparing the image quality from these two lenses at f/8 will hurt your eyes.
Of high interest to many of us is the RF 100-500mm lens at 500mm vs. EF 100-400mm with 1.4x at 560mm comparison. In this comparison, the RF lens is sharper, especially in the center of the frame. The EF lens leapfrogs the RF lens focal length, but the RF lens has a wider aperture. Mount 1.4x extenders behind both lenses and the EF lens appears to hold a very slight edge. Keep in mind that the comparison is now 700mm f/10 vs. 560mm f/8, not especially apples-to-apples. I'm not a fan of the 2x extender behind either of these lenses, but they perform very similarly in the 1000mm vs. 800mm comparison.
The RF lens shows fewer flare effects in our testing (having one less lens element is likely advantageous) and has modestly more peripheral shading with a wide-open aperture at 100 and 200mm.
Looking at specs and measurements, the Canon RF 100-500mm F4.5-7.1 L IS USM Lens vs. Canon EF 100-400mm f/4.5-5.6L IS II USM Lens comparison shows the RF lens slightly longer and noticeably lighter.
As illustrated in the image included with this article, today's length equation is:
EOS R5 + Canon RF 100-500mm L IS Lens = Canon EOS 5Ds R + Canon EF 100-400mm L IS II Lens
The EOS R5's extended viewfinder rises modestly higher than the 5Ds R's viewfinder, but this increase is advantageous in use.
Let's change the RF vs. EF equation to weights:
In ounces: (26.0 + 48.2) - (32.8 + 56.1) = 74.2 - 88.9 = -14.7 oz
In grams: (738 + 1,365) - (930 + 1,590) = 2,103 - 2,520 = -417g
Overall, the RF lens and R camera combination gives us an additional 100mm of focal length range in a similar-sized package that weighs nearly a pound (over .4 kg) less. Primarily, this line summarizes the significant differences between these lenses.
The RF lens has a control ring, providing additional functionality. The EF lens requires a Canon Mount Adapter EF-EOS R to mount on an R-series camera, and an adapter with a control ring is available to equalize this difference. With the RF lens control ring positioned where the EF lens's switches are located, the RF lens switches required a move and are now located between the zoom torque adjustment ring and the focus ring. The RF lens focus ring and zoom rings are modestly smaller in length, and the focus distance window is omitted on RF lenses.
The EF lens tripod foot is removable (and has a lower profile), while the RF lens's entire tripod ring is removable. I prefer the RF design for stability.
The RF lens has a slightly higher maximum magnification (0.33x vs. 0.31x), but both lenses have a great capability in this regard. The RF lens's image stabilization system is rated at 5-stops vs. the EF lens's 4-stop rating.
The RF lens paint color is whiter (the new white) than the EF lens's color, and RF lens hood is primarily white, while the EF lens hood is black. The RF lens zoom torque adjustment ring is now fully ribbed, and the RF lens barrel holds a more constant diameter over its length. Both lenses are well-built and fast-focusing.
The RF lens costs more, but neither lens is inexpensive. If paying this much, the difference is relatively small. Factor in the cost for an adapter for the EF lens to mount on an R-series camera, and the net difference is even less.