Newb here... General Advice

Doug Pardee wrote:

WilbaW wrote:

Doug Pardee wrote:

The AF system in the XT/350D is designed for apertures of f/4 and higher.

Interesting, I haven't come across that before. Where did you get that info?

I don't remember the exact details... it was a long time ago. There were Canon specs about how accurate the 7-point AF system used on the first two Digital Rebels was supposed to be, and the specs were simply too loose for fast lenses. And there was a lot of grumbling that the nifty fifty didn't autofocus reliably at f/1.8 on the XT/350D.

That only stops with the 50/1.8 STM.

Starting with the XTi/400D, the later Rebel models had much better AF systems, including higher-precision AF at the centerpoint with lenses of f/2.8 or faster. Well, except for the XS/1000D, where Canon dragged that obsolete 7-point AF system back out for one more model.

How much higher precision in the later models? The 9-point AF system was introduced on the 20D, and Canon said that the 20D's dual-precision center AF point "achieves up to 3 times the standard focusing precision when used with EF Lenses featuring maximum apertures larger than or equal to f/2.8."

From p.12 of the XTi/400D White Paper (PDF), emphasis added:

"The Digital Rebel XTi has the same 9-point AF sensor and AF unit as the EOS 30D, including the sophisticated dual-precision cross-type sensor in the center AF position. This represents a very substantial upgrade from the Digital Rebel XT. The XTi is the first entry-level EOS Digital camera to have an enhanced precision cross-type, center AF point compatible with f/2.8, a feature unique in its class. The XTi has some significant AF performance improvements over the Digital Rebel XT. For example, the XTi has the same wide-area focusing as the EOS 30D. With an f/2.8 or brighter lens, the center AF point detects focus with the f/2.8 light flux. The base line of the center AF point's vertical-line sensitive sensor is twice as long as that of an AF point compatible with f/5.6. This makes focusing detection more precise."

Here's a thread of interest: https://www.dpreview.com/forums/thread/1750662

Thanks a bunch for the thorough research. It all makes sense to me.

The XT/350D's AF system works okay at f/4 and above. Let's make that at f/3.5 or above, to allow for the kit lens. If you open up wider than that, there's no guarantee that the focus will be precise enough.

I'd bet the AF sensors in the 300 and 350D have an f/5.6 rangefinder base (as are all "standard precision" AF sensors in everything else, AFAIK), rather than f/4 as "designed for apertures of f/4 and higher" might suggest, but I know what you mean when you say that.