Olin Lathrop wrote:
> Vic Fraenckel wrote:
> 
>>>But it works a lot better using a 'real' camera with extension
>>>tubes on the lens so that you get a better stand off distance.
>>
>>What are these extension tubes?
> 
> 
> Extension tubes move the lens out further from the camera.  This allows for
> a closer focusing distance. 

Which is what you need to get 18" focus if you are using a 'telephoto'
(64mm C mount) lens.

> Extension tubes do NOT allow the front of the
> lens to be farther from the subject, in fact the opposite.

Correct. The extension tubes are needed to get close focus.
I use the long focal length to get a good stand off distance.

There are also special lenses available called 'telecentric' ($$$)
which are specifically used for inspection purposes. They give
correct perspective (parallel lines) at typical inspection distances 1-2'.

R
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