20D AF at f/8???

[kiwimichael]

According to Canon: The high-precision center AF point is compatible with f/2.8 lenses, while the other 8 points are sensitive to f/5.6

While I understand the sensitivity of the other 8 points I'm not sure what the high-precision center AF point means. I always assumed that higher f numbers mean less light and more trouble focussing. I never knew there was an issue with f/2.8 lenses and AF.

My real question tho, is can the 20D AF at f/8.0? (even if only with the center AF point)

Cheers
Michael

[timmyquest]

I'm almost certain it will not.

[Roger_Cavanagh]

Just to avoid any confusion: the AF limitation of f/8 means maximum aperture of f/8. The EF 400/5.6 will work quite happily whatever aperture setting is used, but stick a 1.4x TC on it and the max aperture becomes f/8 because you lose a stop and you'll lose AF.

Canon have disabled AF deliberately. It's not a fault. They think that AF becomes too unreliable on cameras like the 10D. On series 1 cameras, which have better AF, AF will will with an f/8 max aperture, but I think not beyond.

There is a workaround that involves taping over some of the lens contacts that keeps AF working, but there are mixed reports about how well it works.

Regards,

[kiwimichael]

Hi, I realise that f/8 will not AF on the 10D but I'm wondering if it will work on the 20D?

My old EOS 3 had a similar arrangement (albeit with many more AF points) whereby the center AF pints were compatible with f/2.8 lenses. On this camera the the centre AF points could AF one stop slowerr than the rest of the AF points.

Has anyone tried AF at f/8 on the 20D?

Cheers
Michael

[Jon]

Canon specifically says "f/5.6" for the 20D. If they don't shut down AF on a max. aperture of f/8 or slower, you won't be able to rely on the results anyway.

[CyberDyneSystems]

No... the 20D will not AF with an f/8 max ap.

[Malaxos1]

I don't get it. I tried shooting smaller than f8 and it worked. I even was able to AF at f32. Are you saying it won't focus at anything smaller than f8 (if that is so then something is wrong with my camera, in a good way for once), or are you saying that it will focus but even though it is set to f32, the lens is really stopped down to f8?...Dean

[Roger_Cavanagh]

I don't get it. I tried shooting smaller than f8 and it worked. I even was able to AF at f32. Are you saying it won't focus at anything smaller than f8 (if that is so then something is wrong with my camera, in a good way for once), or are you saying that it will focus but even though it is set to f32, the lens is really stopped down to f8?...Dean
Dean,

See my previous post.

Regards,

[Malaxos1]

So Rger, this is only with the use of a TC?

[Roger_Cavanagh]

So Rger, this is only with the use of a TC?

TCs cause a 1 or 2-stop reduction in max aperture gets reduced. I think there are some f/8 lenses, so these wouldn't AF, TC or not.

All cameras focus with the lens wide open, which is why large aperture glass tends to focus better and faster whatever stop is being used.

Regards,

[scottbergerphoto]

There is confusion on this thread between the aperture in use and the maximum aperture of the lens. The lens has its aperture wide open until you depress the shutter button or the DOF preview button. Your 20D can AF with a lens whose largest available aperture is f/5.6 or larger. It doesn't matter what aperture you take the picture at. You can have your f stop set at f/32 and as long as the maximum aperture of the lens is 5.8 or better, the camera will AF. A TC reduces the the maximum aperture of the lens by 1(1.4X) or 2(2X)stops.
Regards,
Scott

[shiningstardv]

Thanks Scott, that clears a lot of confusion up.