It's the opposite but this is confused by the fact that a "low" aperture value refers to a larger opening than a "high" aperture value. E.g. An f/2.8 aperture is allowing considerably more light to pass through the lens, then say, an f/16 aperture.
The aperture size is described as a ratio referred to as a "focal ratio" and it's the number of times the physical diameter of the opening (through which the light can pass) can be divided into the focal length of the lens. E.g. If you have a 100mm lens and the aperture diameter happens to be 25mm wide then that 25 divides into 100 exactly four times... so that's described as "f/4". But suppose the aperture opening were 50mm wide (twice as wide)... that would only divide into 100 twice or "f/2". So the larger physical opening has a lower focal-ratio.
Most camera's don't have working auto-focus at f-stops higher than f/5.6 (in other words they cannot have working auto-focus at f/8, f/11, f/16, etc.) but keep in mind that all lenses are "wide open" when focusing (in other words they use the lowest focal ratio possible for that particular lens). They only constrict the aperture blades at the moment when you take the shot (and by the at point the focus is already done.) So you can still shoot at say, f/16... using a lens capable of, say f/4 (at wide-open)... and it would work. It'll focus at f/4, constrict the aperture blades to f/16, take the shot, then retract the aperture blades back out to f/4 again.
The 5D III, 5D IV, 7D II, 1D X and 1D X II can also have working auto-focus at f/8 (or lower). All other Canon models are f/5.6 or lower to have working auto-focus.
Tim Campbell
5D III, 5D IV, 60Da
