T3D Re: Focal length

[Tom Deering <tmd@xxxxxxxxxxx>]

On 9/20/98, john bercovitz wrote:
>Tom Deering writes:
>
>> I'd like to continue to use John's formula, but I want to make sure I
>> understand the concept of focal length.  If I place a 45mm lens on a
>> 100mm bellows, do I now have a 145mm lens?   Is there a different way
>> to calculate focal length?
>
>Well, I somewhat tongue-in-cheek refer to the concept as the "operating
>focal length".  That phrase would horrify any optical engineer because
>there is actually only one, fixed, focal length for a (non-zoom) lens.
>To really clarify what's going on, I think the first thing to put at your
>T3D website is a paper on perspective (it's in Adobe Acrobat).
>
>To answer your question, the maofd spreadsheet will calculate the 145 mm
>from the 45 mm and the distance to the subject.  It will use the 145 mm
>after that.

First I feel like a bright student, but then I'm a complete dolt.  Could
you say that again?  I'm not getting it. Can I just add the bellows length?
That seems like the obvious answer, but I don't want to trip over some
unseen obstacle.

By "operating focal length" you mean the actual focal length when foccused
at a particular point, as opposed to the focal length printed on the barrel?

Cheers,

Tom



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