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Re: [OM] IF-lenses and focal length - On the plus side

Subject: Re: [OM] IF-lenses and focal length - On the plus side
From: "Daan Kalmeijer" <daank@xxxxxxxxx>
Date: Fri, 24 Mar 2000 14:24:43 +0100
>>>>I have read that at closest focus, the actual focal length of the
>350/2.8 is only 270mm!<<<
>
>And GReese posted some time back that at min-focus, the 50/2 is actually
>at 40mm, so I think these two examples would qualify to say that IF lenses
>do change FL. How or why is left for the other Zuiks :)
>
>/Acer V

IF (macro) lenses have a plus side, against the disadvantage of having a
shorter focal length than you payed for. Because they focus closer by
shortening their focal length, they do get a larger aperture. (numerical
aperture = aperture diameter / focal length). This results in less loss of
light while doing macro work. The 50/2 really becomes a 40/1.6  !!!!
Because the 50/2 isn't a real IF lens (it does change in length while
focussing) this 1.6 aperture isn't enough to compensate for the light loss
at 1/2 magnification, but it does help.

John Shaw explains the pro's an con's of IF lenses in his Closeups in Nature
book. He is in favor of IF lenses for macro or close-focus use.

Daan



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