PinholeR5 wrote in post #19177128
I used to think that it was just as good to crop in post. However, I noticed that when I'm using Eye AF and the subject is small in the frame that it will have a hard time locking focus and when I switch to 1.6 crop, since the subject is larger in the frame, the AF system does a better job of nailing focus. I haven't tried this in the RF system since I don't have the 100-500 or RF TCs. I've used this technique with the 100-400 II and the Sigma 150-600C, mostly with the 1.4TC. I've done it with the Sigma and the 2x TC but not often. It generates a bit of a "lack of power" feel in post because you can't really crop anymore since it is "pre-cropped"(which annoys me) but the focus is better and the result, I believe, is better than simply doing all the cropping in post.
One a side note. One of the downsides I have noticed of using the crop mode is that since my current camera setup doesn't really indicate when I'm in crop mode (or the indicator is there and I simply don't notice it) I'll often forget that I put the camera in crop mode causing me to take many shots without realizing it is in crop mode. I keep asking myself "how didn't I notice the field of view change"? But, it does happen. Sometimes I only do a few shots without changing lenses in a session and only notice I was in crop mode when I'm in post. That is really annoying. I have to figure out how to fix that as well as mapping the crop mode to one of the buttons so I can switch in and out quickly. I know many folks have done this, I've just never gotten around to doing it

IMO, the best way to implement crop mode would be to integrate it into the the lens' zoom ring, where it clicks at max zoom, and beyond that, it engages digital zoom in steps. That way, you always have the optics working for you, but get the more useful EVF view and smaller files when you don't need the entire sensor, and you can always "zoom" to a comfortable FOV for tracking the subject. Prime lenses could have an added digital zoom ring (or use the control ring).
There may be a hardware reason for one fixed crop factor for video, but for stills, there is no practical reason not to have any size crop factor that you want.