Do T-Rings fit over a lens or do they replace the lens?

[Arcturian]

The T-Ring that you attach your camera to, does it fit on with a lens, making you able to control the vary the zoom in your images? if it replaces the lens that seems kind of restrictive, does that you mean you can only take pictures at the default magnification of your scope?

Look forward to hearing from you guys, cheers

[kalasinman]

Half replaces the lens and has the same mount and has threads, the other half fits in where the eyepiece normally goes on the telescope and has threads so you can screw the two halves together. There will be no lens or eyepiece used and the focusser of the telescope will focus the camera.

[RikM]

It replaces the lens and you are left with the native focal length of your telescope unless you add Barlow to increase it or a focal reducer to decrease it but these are not without hassles. Many people find it easier to have several telescopes of different focal length.

[Stu]

You can also do eyepiece projection which allows you to vary the magnification. I have an extension tube which fits to the focuser and holds the eyepiece. It has a T thread on the end to allow the camera to be fitted

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[Scott]

what Rik said .

just think of your 'scope as a fixed f/l camera lens.

You can increase the number of effective lengths by

1) barlows or reducers

2) more 'scopes

3) More cameras          

Edit:-

4) what Stu said                                                                      

[ronin]

What is the camera?

If it is a T-ring for a DSLR then it replaces the lens, you can get some "T-rings" (don't think they are actually called T-rings, but simialr) that by whatever means attach a camera and lens to the scope. However these tend to be for smaller cameras and ones on which the lens cannot be removed.

If the lens is off then it is prime focus imaging, if the lens is on then it is afocal imaging.

There is no magnification in prime focus imaging, you simply get an image size that is dependant on the scope focal length and the angular size of the object being imaged.