Can someone please explain what extension tubes do. If I were to put a 25mm extension tube between the Canon 20D and a 100-400mm lens, what level of magnification would be possible (1:2, 1:1 etc.)? And how is the calculation made considering other focal lengths?

the extention tube would allow the lens to focus closer, however it wouldnt focus to infinity anymore. i think you need quite a lot of mm of tube to get to 1:1. some people use them with thier 500mmL lens, to capture small birds which are closer than the normal closest focussing distance of the lens...im not sure how to calculate the magnification..

The Canon 100-400 lens goes down to 1.6m - how close do you want to get :-)

Always get confused between the extender and extension tubes - one doubles magnification and one shortens the focal length. One thing to bear in mind - if you strap on one of these devices your AF doesn’t work anymore - at lease in the case of the extender - as I found out with the very same setup. The upside being despite its bulk the lens is still very easy to handle when focusing manually.

if you still want AF you can tape over some of the electric contacts on the extender, that stop the camera knowing that the aperture is smaller than its suposed to aF at so it does anyway although it is slower. you can find more about taping the electrics at Dpreview.

Crikey - taping up a brand new $1400 lens? thanks for the info Philip but i'll have to think long and hard about that one :-)

Apparently the 1.4X extender works with the 100-400mm lens, but the 2X extender will not AF with a lens with less than a 2.8 aperture. I just read this info on a retail site, so not sure if that is correct? Good way to get us amatuers to dream about the 2.8 super telephotos and spend our money foolishly;-) Edit: Which focal length would give the highest magnification using the Extension Tube EF25 II?

not sure - got the 2 x extender which definitely doesnt work although you have to remember to switch off the AF on the lens as it still spins around trying to get it.

could always sell the house/children and get the 1200mm lens...

lol. I heard the extenders will dull the sharpness and the clearity of a photo. Is that true?

not used it so much so far but im going to have a play over the next few days (off searching for wild Golden Eagles) so i'll let you know....

I use a Tokina 2x because it doesn’t turn the auto focus off. I can’t do a direct comparison but the sharpness and clarity are said to be similar to Canons unit at a third of the price. You do lose a little sharpness but not as much as doing a 50% crop.

As far as dulling a photo, I'd be willing to bet that anything you use between the receptor and the subject dulls the photo, but you pretty much have to take what you can get when you can get it.