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Re: [OM] Tubes etc. was 90mm macro

Subject: Re: [OM] Tubes etc. was 90mm macro
From: "Terry and Tracey" <foxcroft@xxxxxxxxxxxxxxxxxxxxxx>
Date: Fri, 5 Feb 1999 20:55:48 +1100
>Another thought, The 25mm tube loses 1 stop.  The 2x converter loses 2
>stops.
>Are there lenses that are optimized for extension tubes?
>


Magnification is lens focal length divided into extension. 50mm lens with
25mm xtn, focussed at infinity is 1/2 life size. 50mm macro focussed on
close (inbuilt 25mm) + 25mm tube is life size.

My Tamron 90mm focusses to 1/2 life size. 45mm extn. Add a 25mm tube I have
about 3/4 life size. The longer the focal length, the less the effect of
tubes. But I always carry a 25mm tube, no matter what the lens.

M=xtn/focal length.

Light loss.

Effective aperture = aperture times (magnification +1)

So if I'm at f8, at 1/2 magnification, my effective aperture is f12 (1 stop
loss). This is true for a symmetrical lens, which most macros are (or close
to).

A symmetrical lens is one that the aperture size when measured with a ruler
is the same from the front and back of the lens.

Foxy



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