A MUCH EASIER solution would be to use Back Button Focusing (BBF), which allows you to leave the camera in AI Servo nearly all the time. With BBF you're in full control of when the AF system operates and when it doesn't. You can start it up and acquire focus continuously, tracking an active/moving subject through the viewfinder and tripping the shutter when you want to grab a shot or take a short burst. Or, if you want to focus and recompose, use the AF On button to acquire focus where you want it, then lift off the button to stop AF, recompose and take your shot. You also can use a button on, button off, button back on technique to "dodge" obstruction that come between you and the subject while panning.
Another benefit of using AI Servo is that many modern zooms are varifocal or partly varifocal, meaning they don't maintain focus when zoomed. When you use One Shot with these, any time you change the focal length you have to consciously refocus the lens (doesn't matter whether you're using BBF or not... you still must lift off the button, then reapply it to refocus). However, if you are in AI Servo (possible thanks to BBF), the AF system will instantly and automatically correct for any loss of focus when a zoom is varifocal.
A minor drawback of using AI Servo all the time is that AF "confirmation" doesn't work in that mode. It actually makes sense, since in AI Servo the focus is continuous and never "locks", so there's nothing to "confirm" with the green LED or the "beep" (if you have it enabled). I don't know about 90D, but my 7D Mark II's display "AF" on the focus screen any time the AF is active. While that's not quite a "confirmation" that focus has been achieved, it does let me know AF is active. (As a side note: I enable the "beep" on my cameras. Since it only works in One Shot mode, this serves to remind me if I'm in that focus mode when I pick up one of my cameras and start shooting. Helps me avoid using the "wrong mode" with active, moving subjects. When I'm shooting sporting events, it's not uncommon for me to hear other people's cameras "beeping" at them... and I know that a high percentage of their shots will miss focus.)
With BBF, I hardly ever need to use One Shot. I shoot a lot of sports and some wildlife, leaving the camera in AI Servo and using BBF 99.5% or more of the time. Been doing that with my SLRs and DSLRs for at least 20 years. About the only time I switch to One Shot is when shooting a posed portrait, a macro/close-up or landscape/architecture. I shoot those a lot less, and don't always change modes anyway, so some of my cameras probably haven't been out of AI Servo in months... maybe even years.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2), EOS M5, some other cameras, various lenses & accessories
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