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Mark Newton · Joined: 09 May 2012 Posts: 1

Hi, I bought some half decent extension tubes(the electronic kind) and just cant get my lens' to fix a focus on anything? Am i expecting too much? i.e. trying to get too much magnification?

ruxpriencdiam · Posted: Thu May 10, 2012 4:02 pm

What are half decent extension tubes???

I have and use extension tubes without any problems.

Mike Price · Joined: 06 Jun 2008 Posts: 2920 Location: South Wales

I agree with Barry. The easiest way to get the lens to focus with a very close subject is to get a rough focus on manual then move the camera back and forth slowly until sharp focus is achieved. The tubes cut down light so sometimes AF can struggle to get a fix if there is not enough light on the subject. I use kenko auto tubes, but usually on manual focus.

Mike

PaulCowan · Joined: 24 Feb 2005 Posts: 4182 Location: Evolving

The reason the lens won't grab focus is that the camera body needs a certain brightness for the AF to work. For most Canon cameras, the centre point works at f/5.6 or brighter, the cross points require f/4 or better.

If you have, say, a 50mm f2.8 lens and you add 50mm of extension, the effective widest aperture becomes f/4. Every doubling of length halves the effective size of the aperture. If you are starting out with an f/4 zoom you are already at the limits for good AF and the addition of tubes may reduce the light so much that even the centre spot can't work.

Bear in mind, also, that even at the best of times the central AF point needs some edge contrast to grab hold of. If your cross points are inoperative and the centre point is homed in on something which lacks local contrast then it won't work.

It's worth remembering that the reduction of apparent aperture is a real effect, so if you set your aperture on the camera at f/16 on a 50mm lens and use 100mm of extension, then the effective aperture would be f/32. At f/16 on any camera the image will already be being degraded by diffraction causing softness, but by using extension tubes to increase that to f/32 you are certain to get a very mushy image. To avoid this, don't use a tube length that exceeds the focal length of the lens and don't stop down beyond the diffraction limit of the camera, that way you will never be more than one stop beyond the theoretical limit and few people notice the effects of diffraction at that level.

In any case, with macros getting the focus point exactly right is critical to a good composition and the correct focus is unlikely to coincide with the central AF point. So it is generally best to switch off AF and focus manually.