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Newb here... General Advice

Started Jun 5, 2017 | Discussions thread
Doug Pardee
Doug Pardee Veteran Member • Posts: 9,920
Re: XT/350D quirks

WilbaW wrote:

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.

Right, they are f/5.6.

[I know you know the following -- this is for other readers who might not know.]

It can be confusing talking about aperture and AF because of the two separate contexts:

  • the maximum f-number the AF sensors are designed to work at (f/5.6 normally, f/2.8 for the high-precision sensor) -- this is what you'll occasionally find in AF specifications, and
  • the minimum f-number the AF sensors can be expected to accurately focus -- this is rarely mentioned, in part because it's rather variable, and in part because it's a limitation most buyers are blissfully ignorant of.

The two are intertwined. A wider AF sensor requires lower f-numbers to work, but its wider base allows more accurate rangefinding for better focus at lower f-numbers.

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