topic Re: AF question in EOS DSLR & Mirrorless Cameras

AF question

Maria — Mon, 05 Jun 2017 13:47:43 GMT

I am looking at upgading from a 6D to a 5D Mark iii.... when looking at the specs for the 5D iii I noticed it said, "5 central dual cross-type points (f/2.8 diagonal)"... the 6D has 1 cross type at f/5.6 .....but what does that mean?

Does it mean that for example on the 6D that that 1 center AF point is only a cross type and reliable at f5.6 or higher (ie f8, f11 etc) and the 5D iii is only cross type at f/2.8 or higher (not at say f1.8 or f1.4???)???

Re: AF question

ebiggs1 — Mon, 05 Jun 2017 14:34:28 GMT

It basically means it can focus in both vertical and horizontal. Most focus point can't do that.

Re: AF question

Maria — Mon, 05 Jun 2017 15:02:46 GMT

thanks! where does the aperture that is listed come into play? ie why is it listed as part of the cross type info? does the cross type not work at larger apertures than what is listed?

Re: AF question

TCampbell — Mon, 05 Jun 2017 15:52:01 GMT

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.

Re: AF question

Maria — Mon, 05 Jun 2017 16:14:03 GMT

thanks!

"it's the opposite" - that answers my question!

Re: AF question

ebiggs1 — Mon, 05 Jun 2017 17:52:39 GMT

One additional point, the aperture is just a threshold. In other words it is where Canon believes you get the best results.