Extension tube question

[Loupey]

Here is an example of what extension tubes do.

I mounted a 36mm tube on my 70-200mm zoom. The first set is with the zoom set at the 70mm setting. You can see that changing the focus ring all the way from minimum to maximum yield no appreciable change. At this setting, one would have to move the entire camera/lens to do the focusing (this is where a macro focusing rail is so useful).

Now zooming to 135mm with the same 36mm tube, you can see that the focus distance is farther (because the ratio of extension to the focal length is smaller). In addition, you can now see the total range of focus going from minimum to maximum.

These are what you can expect if you put tubes onto prime lenses. The beauty of using them with zooms is that now you can change both the magnification AND focusing distance with the zoom ring.

The last image is a recent real world example at f/8, 95mm, 48mm total extension. DOF is very shallow.

A decent set of tubes goes for around $180. A single Canon tube costs about the same.