T3D Re: More macro quesions,... using slide bar

[john bercovitz <bercov@xxxxxxxxxxx>]

> What are the pros and cons of using extension tubes,  
> and also the same for using plus lenses instead.

The usual lens is designed for a range of magnification 
from 0.1 to 0.  That is, you can go from 10 to 1 reduction
on out to infinity.  If you try to use it at 1 to 1, it 
gets pretty screwed up (I've tried it).  A plus lens 
makes the lens think it is looking at infinity.  Plus 
lenses are designed with closeup work in mind unlike the 
usual prime lens.  In addition, as you noted, the plus 
lens doesn't change the focal length or the entrance 
pupil significantly so you get to use the same f/stop.  
I don't think plus lenses are perfect but they seem to 
give me much better results than an extension tube on 
the usual lens.  Best just buy a macro lens.  I got
mine used and they weren't too too dear.

John B
http://www.angelfire.com/ca2/tech3d/index.html


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