400D/50D AF comparison

[neuroanatomist]

Welcome back - things have changed around here, just a few days ago the forum backbone was changed, so we're all still getting used to it.

Unfortunately, I can't offer any first hand experience with either body, but I can offer some specification-based observations. The 400D has 9 AF points, only the center point is a cross type (which can detect features in multiple orientations, making it more likely a focus lock will be achieved). The 50D also has 9 AF points, but all of them are cross-type. Also, the center point on the 400D is more precise and accurate with f/2.8 or faster lenses, because one of the two lines in the '+' shaped AF point is f/2.8 sensitive. The 50D also has enhanced function with f/2.8 or faster lenses, but instead it's a dual-cross point with an f/2.8-sensitive 'x' superimposed on the f/5.6-sensitive '+'. That means the 50D center point is more likely to get a lock with the f/2.8 part of the sensor. Basically, the 400D uses the same AF sensor used in the 20D and 30D. The 40D got a new sensor, and that has been reused in the 50D and 60D (i.e. it's still current, but then so is the T3i/600D's - it's the same one in your 400D).

As for AF speed, the 400D uses Digic2 whereas the 50D uses Digic4 - two generations faster, and the AF speed and AI Servo AF tracking will be better because of that.

One more point - and this one is critical if you have any 'fast' lenses. The 50D offers autofocus microadjustment (AFMA), a feature that lets you calibrate the AF system for each specific lens. With f/2.8 and faster lenses, or even slower lenses if they're telephoto and used near for close subjects, AFMA is a big plus. Any time that the DoF gets thin, any small focus errors are more evident - being able to correct those by using AFMA means a lot more in-focus shots. AFMA was one reason I went from a T1i/500D to a 7D, and I'll never buy another camera that doesn't have AFMA.