How good is the RF 100-400 for Sports?

NorthernMike wrote:

Thanks! That post by birdbrain was quite helpful. They certainly do seem to like it, and there are some great shots in there. One of the things I was worried about was whether the narrow aperture would be limiting for faster moving scenes, but there are afew beautiful shots of birds in flight, so it's probably not going to be a concern outdoors.

This slower lens will struggle more than a faster one in very low light, where the actual light available makes it hard for the AF system to see in real time, but in very bright light where the bottleneck is motor and communication speed, it may easily surpass faster EF lenses.

Also, keep in mind that a smaller aperture is sometimes helpful for focus, since the deeper DOF means that when you first point a lens at the next subject, it is more likely to be focused enough for the AF system to immediately recognize objects like faces and eyes, and work on focusing on them. I have this zoom, and I have the R5 and EF lenses of 400/4, 400/5.6, and 400/6.3, and unless the light is low enough to need the faster EF lenses, the RF100-400 generally gives less focus problems. If you start adding TCs, then the AF may start to favor the faster EF lenses when ambient light flux on the sensor gets very low and becomes the main AF speed bottleneck. The RF100-400/5.6-8 can take the RF 2x, but the AF speed drops a lot more than it does making the 400/4 into 800/8, even with moderate light sometimes.

Of course, the body plays a large role in AF ability, too.

You might want to consider how much background blur/separation you want, because despite any AF ability, a smaller aperture dictates a deeper DOF. Blurring the background more requires either getting closer to the subject, or using a larger aperture or pupil (actual size; not necessarily the f-ratio per se).