Quick lesson on extension tubes please (with the 300 f/4L).

I've just ordered the Canon 300mm f/4L IS lens & I've been reading about it's macro capabilities. I've read about using it with an extension tube/s but to be honest I've no idea how they work.....over to you guys

They are hollow tubes...

By moving the lens away from the filmplane (cmos) you are able to move the whole owrks CLOSER to the subject,. and thus get larger magnification.

they are a wonderfull tool as they allow you to use them with any lens to get closer to your subject,. and since they have no glass involved,. there is not detriment to your lenses image.

The 300/4 L IS is a great lens. Focuses fairly close by itself. Use a 12mm or 25mm extention tube and it focus even closer. You lose a little light but still have quality. ISO 400 in daylight, run it through Neat Image later. I use mine on a monopod (much easier and more protable than a tripod) to do butterflies and dragonflies.

Note that if you expect any lens to continue to provide any automation (even something as simple as auto diaphram), you need to get the much more expensive auto extension tubes. The manual cheapies are not worth it IMHO.

Probably you will not notice much difference mounting an extension tube on that long lens, but the loss of ability to focus at infinity. This is due to the fact that extension tubes add to the normal macro magnification factor of a lens a quantity that is near to L/F, where F is the focal length of the lens and L is the legth of the tube (when the lens is "focused" to infinity). So, with a 300mm lens, even if you use the three tubes of the Kenko set (68mm), for instance, you will add only a mere 0.23 magnification factor to the lens. I do not know if it will be enough for your needs.

As Oipaz points out extension tubes may not be a very usefull option with such a long lens.

Put simply , a 300mm lens needs 300mm of extension from it's infinity focus setting to be able to give 1:1 macro magnification. Obviously the lenses own focus system will get it part way there but add a measly inch or two of extension tubes and it's still nowhere near that far!

Shorter lenses need corespondingly shorter extension to get the same magnification ratio so tubes are much more usefull with them

I thought this lens was best used with the 500d closeup attachment screwed onto the front of the lens . not used it yet as my 300mm still has not arrived. after reading lots of threads this appears to be the best system, (for field work)