Re: Closeup lenses

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

Michael Gordon wrote:
> I do not believe there is a significant change in focal length using a
> closeup lens, since the main lens will be set at or near infinity, the
> closeup lens simply causes the 50 mm lens to "see" something nearer

That's the way I always thought of it up until I used these exact 
lens combination formulas.  It sure seems like there couldn't be a 
more valid way of thinking about it than what you're said here.  The 
two almost agree but not quite - they differ very slightly in their 
suggested results.  I wonder if I could differentiate between the two 
by experiment.  Such a small difference...  It would be great fun if 
I can find the time.

I use a closeup lens the way you imply: I set the lens at infinity, 
screw on the closeup lens, and move the camera until the object is 
in focus.  So I would compare the two approaches using this setup.  
The problem is that the way you're suggesting it is and the way the 
lens combination formula suggests it is result in the object being 
in focus at almost the exact same distance from the lens.  I wonder 
if I can even measure the difference reliably.  I need to reconfigure 
the experiment to be more sensitive, I think.

John B


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