From: "Skip Williams" <skipwilliamsom@xxxxxxxxxxx>
> Subject: Re: [OM] Two technical questions
>
> 1. The in-front-of-the-lens converters are just concentrating the light
> rays. Tradititional converters are extra lens elements that change the
> focal length and focal ratio, thus the light reaching the film. (I hope
> that didn't sound too lame.)
A rear converter doesn't really change the focal length. It's a diverging
element that behaves like a Galilean telescope eyepiece and simply magnifies by
two. Because its object is virtual (ie the real image from the prime lens) it
provides a real image. The length of the element is designed such that the
magnified image is still at the film plane. The loss of two stops for a 2X
converter arises because the total image area is increase by a factor of 4.
The distinction is an important one in macro work when you add extension. For
example if you add 50mm of extension to a 50mm lens with your 2X converter
between the two then the combination does indeed behave as though it is a 100mm
lens with 50mm extension and you get a magnification factor of 0.5X. However
if the 2X converter is placed between the extended 50mm lens and the camera
body this is no longer the case. The lens plus extension yields a
magnification factor of 1X which is doubled to 2X by the converter.
David
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