I believe all the EOS flashes are compatible with the EOS-M, and vice-versa, per B&H website info.
Thanks for that - I wonder why they just didn't go with the 270EX2 for this camera - the new flash looks just like the 270ex2.
In shape, yes, but looking at the specs page reveals a much different story:90EX
Dimensions - Approx. 1.7" (W) x 2.0" (H) x 2.6" (D) / 44.2 (W) x 52.0 (H) x 65.0 (D) mm
Weight - Approx. 1.8 oz. / 50g NOTE: (assuming without batteries)270EX II
Dimensions - 2.6 x 2.6 x 3.0 in./ 65.8 x 65.2 x 77mm
Weight - 5.5 oz./155g (without batteries)
Much smaller, much lighter, still
powered by 2 AA batteries
(CORRECTION: 90EX is powered by 2x AAA batteries, 270EX II runs on 2x AA), can function as commander as well. No zoom head, and much lower power flash than it's bigger brother (GN 9 @ 24mm
vs. GN 22 @ 28mm). It is even less powerful than the built-in flashes on Rebels (GN 13 @ 28mm). Interesting offering, the 24mm and weight make it pretty unique.
EDIT: Also, check out the relative size of the hotshoe for scale, and note that the 90EX has a plastic shoe.
Does the EOS M camera have MFA? That would be a big factor for me for using it as a backup.
Doubtful, the 50D was lowest model to have it. Would be nice though.
Why would it need AFMA? It is basing the focus on phase and contrast on the sensor. Isn't the reason for AFMA that the AF module in DSLRs are not located on the sensor?
EDIT: Sorry didn't see that trowski already addressed this issue.