Re: [OM] Zooms at Dawn - the 50mm shootout - Teaser

Subject: Re: [OM] Zooms at Dawn - the 50mm shootout - Teaser
From: Moose <olymoose@xxxxxxxxx>
Date: Mon, 19 Apr 2010 22:48:09 -0700
On 4/19/2010 6:54 PM, Chuck Norcutt wrote:
> Ah, yes.  I forgot about that.  But is it only zooms or any lens that works 
> via internal focusing?

Undoubtedly someone has gone to the trouble of designing an internal 
focusing prime with close to constant effective focal length, but it's 
got to be tricky and expensive to do. OK, maybe nobody has built one;  I 
just want to cover my bases.

In any case, most of them do change focal length as focused closer. That 
was one factor in my choice of the Tamron 90/2.8 macro, which uses a 
conventional long helix to focus. The Canon 100/2.8 macro is IF. So as 
one focuses closer, the working distance gets even shorter than would be 
expected with conventional design.

With any lens at 1:1, the the focal plane to subject distance is 4x the 
focal length. The Canon 100 mm, instead of 400 mm, has a close focus of 
1:1 @ 310mm, for an effective FL of 77 mm.

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