Hello,
I am looking at buying a telephoto lens for sports and some wildlife.
I shoot on Canon EF-M with an EF to EF-M adapter. I currently have a 24-105mm.
Is a 70-200mm enough reach? Or would it be better to get a 100-400mm to have no overlap of focal lengths and get a more compressed image?
Is f/4 good enough for a 70-200mm or is it worth the extra money for a f/2.8?
Sports and wildlife photography get difficult because shutter speeds in the range of 1/1000 to 1/2000 seconds are frequently required to capture action.
A 70-200mm f/2.8 is often recommended for indoor sports (basketball, volleyball, etc). The f/2.8 part is very important because indoor sports arenas are dimly lit. Their light level is similar to that inside your home.
For daytime field sports (football, soccer, etc.) and wildlife, the distances are much farther, and the sun provides lots of light. Thus, the lens focal length becomes very important. Outdoor sports and wildlife photographers prefer to use lenses in the 500mm range or greater.
Nighttime field sports are the worst. Even the professional >US$10,000 lenses are only f/4. From what I've seen, pro sports photographers don't hesitate to use whatever ISO is required to get the shot. Those terrific noise-free sports photos we see on websites only look noise-free because they're downsized and displayed only a few hundred pixels wide.

This RAW photo was taken at ISO16,000 by an APS-C camera, then converted to JPEG using Photoshop Sony's Imaging Edge software. No noise reduction has been used. It looks OK because it's been downsized to 600x400 pixels. A local newspaper website uses this size to display its sports photos. The noise in the original, full-sized, photo will give any photographer nightmares.
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Lance H