Bit of Help With Extension Tube Calculations

Hi - Firstly I have read the stickies and found them very helpful but I do need some further info.
1) Am I correct in working out that the Canon EF25 Extension tube will reduce the minimum focusing distance of the 400mm f/5.6 from 3.5m (11.48ft) to 2.46m (7.87ft)?
2) I'm assuming extension tubes have no glass elements so do not reduce image quality?
3) Will autofocus and all metering be retained with my 30D
4) If that's all ok where can I get the best price on one in the UK - tbh £100.00 seems a bit excessive to me!!!
Thanks as always.

1) Sounds about right, but I couldn't be certain. I experienced the same kind of distance when I tried tubes on my 100-400

2) Correct.

3) Metering will be retained, AF is problematic - most people using extension tubes use manual focus and physically move the camera in and out to focus. The manual focus ring turns into a semi-zoom with the tubes on.

4) I paid £109 for mine in a shop, and with the fun I've had with them, it's worth it. Cheaper can be had on eBay though, or Kerso (look in the Market section for contact details)

OR you could get the Kenko Extension Tube Set (36+20+12mm) for ~$180/£90 + shipping @ B&H...

Would someone mind sharing the formula for determining the effect the extension tube will have?

M = E / F (M=magnification; e=extension tube length; f = lens focal length

This applies when the lens is focused at infinity. So 25mm extension would permit 50mm lens to register a 0.5x image at the sensor or film... a true 1" object would cast an image that is 0.5" tall on the sensor or film. Additional use of the focusing helical threads would permit additional magnification, depending upon the amount of extension built into the lens focusing threads.

Thanks for the help guys.

Is there a way to calculate depth of field when using extension tubes?

DOF in close-up photography is affected only by the area of view (magnification) and the aperture in use...the focal length does not matter! Falloff of sharpness is more accentuated with longer FL, falloff of sharpness is more gradual with shorter FL.

I have seen a table that will give DOF at various magnifications at f/11 for two different sized COC, and other apertures can be calculated from that...change aperture by two stops, DOF is 0.5x or 2x from f/11.