Extension Tubes - how many is too many?

I have searched through the forums but did not locate an answer to a question I had. Currently I have an EF 25 extn tube which I like to use on my 100-400mm. I would like to get even closer and add another tube, but I don't know how many tubes I can add before I go too far and the lens will no longer focus. I went through the lens documentation at the Canon website but again, I could not find an answer.

Is there a method to calculate how many tubes you can add before focus becomes impossible for each different lens?

Theoretically, you can probably focus down to the front element. Will you have enough light, without it shining into the lens, and will your DoF be unusable? I don't know. But you might find this of use:

Suppose you have a lens of focal length F which can focus down to distance X when mounted on your camera. Suppose you use an extension tube of length L. Then you can focus down to the distance given by the formula
F(XF +XL - Lf)/(XL -LF +f^2)
In words, you do the following. First multiply the shortest distance by the focal length, the multiply it by the tube lenth and then multiply the tube length by the focal length. Add the first two numbers and subtract the third. Then multiply the result by the focal length. That gives you the numerator.
Next take the product of the shortest distance by the tube length and the product of the tube length with the focal length, both of which you already calculated, subtract the second from the first, and then add the square of the focal length. That gives you the denominator. Divide the numerator by the denominator, and you are done.
Here is an example. Let the focal length be 50 mm, the shortest distance 200 mm and the tube length 25 mm. Then with the tube, you can focus down to
50 x (200 x 50 + 200 x 25 - 25 x 50)/(200 x 25 - 25 x 50 + 50 x 50)
which comes out to 110 mm.I wish this were simpler than it is, but I don't see any useful way to simplify it further.

I found it here:

http://photo.net/nikon​-camera-forum/00IZwD

The longer the focal length of the lens, the more tubes you could add and still focus (manually). However, you'd better have very strong, rigid tubes or risk having the camera or lens (depending on where the tripod mount is) fall when that long stack of tubes snaps from the weight at the end of it.

Your main problem will be the decreasing aperture; if you add tubes equal to the focal length to any lens it'll cost you two stops, and get pretty dim. Where it gets tricky is where the lens is internal-focusing (like the 100 macro), which actually changes effective focal length as you focus closer. IIRC, at 1:1, it's really about a 70 mm lens (and at around f/5.6).

I've used my full stack of Kenko tubes (12, 20 and 36 mm) on my 300 f/2.8 and 500 f/4 with no particular problem, although adding a TC to the stack does make AF problematic. The 100-400 would probably handle 40-50 mm of tubes with no AF problems.

If you want to figure the numbers, figure the physical aperture is fl (focal length)/f-stop. Take that number and divide the (fl+tube length) by that physical aperture value to get the effective f-stop. If it gets much past f/6.3 (f/9.6 if you've got the latest firmware on your Mk III), you'll lose AF.

My simple solution after buying some Kenko tubes (12, 20 and 36mm) was to put each of them, and combinations of them, on each of my lenses and measure the effect on focussing distance.

Broadly, for wider lenses, the maximum focussing distance reduces rapidly with increasing tube length. The effect is less for longer lenses. This is the result that you expect from the calcuations in the previous post (EDIT: Moose's post). From memory, multiple tubes (i.e. longer than 36mm) were practically useless for anything less than about 85mm (but I would have to check my measurements to be sure).

The effect on dedicated macro lenses is much less. I can use all 3 tubes on my 100L without any change on minimum focussing distance - the working distance is reduced by the physical length of the tubes, but still usable.

I did not want to buy another extension tube if I could not use it but now I think I can fiqure out what will work. Thanks to all for the information.

I bought the Kenko 3 piece automatic ext tube set to let me get the best effects for the $$