>>> And that's only the labeled f/stop. The actual f/stop is only
valid at
>>> infinity. The effective f/stop get numerically larger as the
extension
>>> increases.
AG:
>Doesn't that depend on methodology of focusing? For example, most
>modern lens designs use internal-focusing which decrease focal-length
>as you focus closer, whereas older designs operate by extending the
>entire lens mass forward which keeps the focal length the same.
Yes--- for pure extension this applies:
Effective Aperture = Lens Aperture x (1 + Magnification / Pupil
Magnification)
For the Z. 135 macro the PM=1 so it is eady--at 1:1 lose 2 stops. Big
Foot loses 1.5 stops with some slight FL shortening and I don't what
else. The cost is loss of working distance.
I don't know exactly how the internal focusing works as far as
effective aperture and have never seen any explanantion.
Some Nykons directly report the real aperture--nice for manual flash.
I still some tables somewhere.
And at 1:2 f45 on the Z. 135 has been quite soft in my hands
too--perhaps OK at lower mags but haven't tried in quite awhile.
Mike
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