I just purchased two Canon extension tubes, 25mm each, for a Canon 100mm f/2.8L Macro lens. In the instruction manual, it is recommended to only use one extension, not two or more; otherwise, image quality may degrade.
Is this actually true? If so, I think I'm going to return them, as 25mm is not enough. I also noticed that the distance to the subject (with both tubes mounted) must decrease to be able to focus.
I'm not sure why they say image quality will degrade. There is nothing but air inside the extension tubes - all you lose is a few stops of light. Possibly some vignetting (darkened corners). 50mm worth of extension is certainly a reasonable amount to use.
You will have a very shallow depth of field, and very little latitude to focus with the lens itself, rather you move the camera closer/further from the subject to focus. With the shallow depth of field, it is very hard to get close, in focus and hold the camera steady to get a shot, so you'll want to use a tripod if possible.
It is normal that the subject distance will have to be very close to use the extension tubes - that's really the whole point - the extension tubes allow you to move the lens and focus much closer to the subject, to get more magnification.
Edit: now that you've added that you are using a 100mm lens, there is something else to consider. The magnification you'll get from adding extension tubes is the length of the extension tubes divided by the focal length of the lens. So if you use your two 25mm tubes with a 50mm prime lens, you'd get 50mm / 50mm = 1:1 ratio. If you use them with the 100mm lens, it's 50mm/100mm = 1:2 ratio - that's half the magnification you'd get from using them with the 50mm lens. The macro lens combination might be partially compensated by the fact that it may focus closer (and because the macro lens may have an effective focal length of only 70mm or so when at closest focus). So experiment if you have a 35mm or 50mm lens - you may actually get better results than using the tubes with the 100mm.
This page actually recommends lens reversal for going beyond 1:1 magnification:
http://dpanswers.com/content/prod_macro.php
Also note that beyond 1:1 the CFD increases, while if you use it on a normal lens, to get closer to 1:1 it decreases drastically), albeit less than the extension tube itself, meaning that the camera is further away, but the tip of the lens is closer.
Image quality is actually not affected, but you are projecting the image to be larger , which means that the quality issues that are present will be easier to see. Your COF and aberrations for example. like a tabletop projector at school which you move further away from the wall to make it bigger - but in return it is less focused and less bright.
It's not the number of extension tubes that matters (except the small matter with more contacts that Skaperen metioned in the comment), it's how far you move the lens from the camera body.
For example I have a kit of extension tubes containing a 12mm, a 20mm and a 36mm tube, combinable to 68mm of extension.
There is basically two things that happen when you add extension tubes:
You change the characteristics of the optics because the lens was built to give an optimal result at a specific distance from the camera. The more extension you add, the more distorsion you will get.
The longests distance that you can focus gets closer to the lens (as well as the shortest distance).
With enough extension that longest focus distance will be behind the front lens, so it's no longer possible to focus on anything outside of the lens. How much extension that is differs from lens to lens.
It depends on your lens model.
For example: If you put extensions that would change focal length so much, you won't be able to focus like non-extended situation, even you try anything with that lens.
Also absorbed light will decrease so it is also another negative side.
Addendum: a VERY long stack of extension tubes with a heavy lens on the end (eg if you took a 300mm long-focus lens and actually backed it with another 300mm worth of extension tubes) can overstress the lens mount on the camera or between extension tubes, causing temporary (elastic bending of stuff) or even permanent damage.
If such setups are to be tried, using other means of lens support not entirely depending on the camera are recommended, eg tripod collars or 15/60 LWS rigging.
Normally, the preferred device for really long extensions is a bellows and not discrete extension tubes; most bellows do have their own tripod socket for this reason.
