Extension tube question

[dmm96452]

What effect do they have on DOF with close, middle and distant focal planes?

[another view]

An extension tube just lets the lens that you're using it with focus much closer than it could without it. Some lenses get a little longer as they're focused closer, and the tube just adds to that length. With the tube you probably won't be able to focus to infinity as well.

Any time you get closer to your subject and don't change the aperture or focal length, the depth of field gets shallower. With tubes, you can get a really shallow depth of field even with a small aperture.

[dmm96452]

Thanks, AV.

[ally]

Do thay hurt image quality in any way? should it just be looked on like a tc? and how much are thay? lol Thanks ally

[dmm96452]

Do thay hurt image quality in any way?...

Judging by the quality of the shots posted in this thread I would say no, or at least very minimal.

...should it just be looked on like a tc? ...

No. An extension tube is just that... a tube. No optics in them at all. It just moves the lens away from the camera to allow you to focus more closely. A tc is used to increase the focal length of the lens it is attached to. They do have optics in them and do affect image quality a bit more do to the fact that you are putting more glass between the subject and the sensor or film. It will also effect your maximum apeture. For example a lens with a 2.8 max apeture and a 2x tc will now have a max apeture of 5.6.

... and how much are thay? lol Thanks ally

You can check yourself at your favorite place to get equipment but generally teleconverters are more expensive then tubes. Mostly for the reasons discussed above...one is just a tube and the other has optics in it.

[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.