Canon 20D and EF12II, EF 25II

[my1947]

I am having problem to put the extension tube on my 20D.  It can't focus at all. Is my 20D need to send in for tune-up or it just not match the 20D and extension tube?

Thanks

Michael

[Bob Atkins]

Depending on the lens, you may only get focus at very close distances. For example with a 50/1.8 II mounted on an EF25(II) extension tube you can't focus more than about 9" from the front of the lens and with a 35mm the maximum focus distance drops to around 4".

What lens do you have and is the AF system trying to focus and failing or is it not trying at all?

[my1947]

Hi Bob,

I had tried on 24-70L, 85mm & 17-40L.  None of the lens I used can be focused at all.

Thanks

Michael

[Bob Atkins]

There's no incompatibility with the 20D.

With zooms at the short end you may not be able to focus at all (even manually) witha an extension tube (especially the 25) because the focal point may actually go INSIDE the lens and be behind the front element! At the long end you should be able to focus on something a few inches away. Even if focus is within the focusing range, you may get oscillation and the focus may not lock.

The 85 (1.8?) should be better. With my 20d, the EF25 tube and a 50/1.8 I can get pretty decent AF, though the range is very limited and you have to be at almost the right distance (a few inches away) for the focal adjustamnt range to be large enough to grab focus. However focus is positive and I don't see oscillation.

Canon don't recommend AF with short focal length lenses and extension tubes because it can become unreliable.

[my1947]

Thank you for the information.  This was my 1st experience with the extension tube.  I may sell it on Craigslist or ebay since I can't use it.

Thanks again.

Michael