> I guess I should have posted the bits from Shaw about why you don't want
> to bother with the math. As Walt mentioned: "With lenses like the 90/2
> Zuiko and its ilk, your guess is as good as mine about where to measure
> from." you need too much data about some modern lenses to be able to
> make the math work for you.
well I spent a few hours with a sekonic l-358 in reflective mode showing
raw EV values and my bronica etrsi with the lenses set to T i.e. shutter
open all the time and the sekonic where the film back would be. I measured
the light loss vs infinity focus for all 6 of my lenses at every interval
marked on the distance scale. The results are STRANGE..
the 40mm wide angle (equiv of a 24mm in 35 film terms) has no measurable
light loss until 2m and then less than a 1/6 of a stop until 0.7m after
that is rises fairly rapidly to 1/3 of a stop loss at 0.4m
the 50mm wide angle shows simular behaviour to the 40mm except the 1/3 of
a stop loss begins at 0.5m
the 60mm has 1/6th of a stop loss at 1m and by 0.4m its half a stop
the 75mm behaves simularly to the 60mm but does not focus as closely.
the strange part is:
the 105 macro has half a stop loss at 1m which is far far higher than any
of the other lenses at the same distance. furthermore at 0.5m the light
loss is 5/6 of a stop which is much much higher than the any of the other
lenses and goes up almost expendentially from there! at 0.35m (1:1) the
loss measured was 1 and 2/3rds of a stop and not 2 stops as predicted.
(the sekonic is accurate to 1/10th of a stop and I repeated the results to
make sure)
Even stranger is the 100-220 zoom. AT ALL FOCUSING DISTANCES AND ALL DOCAL
LENGTHS THERE IS NO MEASUREABLE LIGHT LOSS!!! THIS INCLUDES RUNNING IT AT
105M and comparing with the macro lens! GO FIGURE THAT. I should mention
the 100-200 zoom is both internal focusing AND does not change physical
length when you change the focal length. Also the rear element does not
change position. To make things more strange at 220mm focal length focused
at 1m the bornica manual says the lens is 1:4.9 according to the maths a
magnification of 1:5 should give a light loss of 1/2 a stop! This is
definately measurable by the sekonic and far outside the margin of error
which I estimate to be plus or minus 1/10th of a stop.
Om content? as soon as I can I will repeat the experiment with my olympus
on B setting and my zuiko and tamron lenses.
CONCLUSION
The maths may work for bellows extension but they do not work for focusing
distance at all!
Regards
James
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