old lenses on Canon A630

I've bought a lensmate adapter for my camera (A630).

http://www.lensmateonline.com/newsite/A620A610.html

Lovely; I can now use standard 52mm filters and hoods.

So far, so fun.

However, I would quite like to have a go at
"long-shots"; I note that in the present market
42 mm screwthread fit lenses are "rather cheap"

If I purchased a 52mm to 42mm step down adapter,
could I use such a lens? How would the two
lenses interact? How would focus work?

BugBear

bugbear <bugbear RemoveThis @trim_papermule.co.uk_trim> writes:

>However, I would quite like to have a go at
>"long-shots"; I note that in the present market
>42 mm screwthread fit lenses are "rather cheap"

>If I purchased a 52mm to 42mm step down adapter,
>could I use such a lens? How would the two
>lenses interact? How would focus work?

It won't work at all. The camera lens expects to focus on an object
out in the world, while the lens is trying to focus an image a short
distance behind the barrel of the lens.

The way this works on SLRs is that you remove the original lens
entirely, and replace it with the telephoto lens, while carefully
making the lens mount to image plane distance the same as the lens was
designed for. That's what "T mount" adapters do. This is called
"prime focus" coupling, and it also used for mounting SLR cameras on a
microscope or telescope after removing the eyepiece.

But you can't remove the main lens from a P&S camera, so this method
simply doesn't work with them.

The only method that *does* work to couple a P&S camera to something
like a telescope or microscope is to leave the eyepiece in place,
giving a virtual image projected at a distance of a few metres, then
hold the camera lens up to the eyepiece and have it focus on the
image.

If the camera's entrance pupil is near the front of the lens, it can do
a pretty good job of mimicing the eye that the eyepiece is designed to
couple to, and give good images. On the other hand, if the camera has
an entrance pupil that's deep inside the lens, it may not be able to
capture most of the light from the eyepiece, and you may get awful
vignetting. It depends on the camera.

Dave