There are so many possible variables here. You are assuming that both
exposures would have been the same on film.
That big a difference is not simply the number of elements. Even
assuming single coating and 15 elements in the zoom, that's 20 more
reflecting surfaces @ 0.5% = 5 0ght loss over the 135/2.8 or about
1/10 stop.
You are relying on four little pins (2 each) to convey info from the
lenses to the camera so it can calculate how much light will be coming
through when they are stopped down for exposure. Those on the 135 are
probably slightly imperfect due to manufacturing variance and use and
wear. Those on the Kiron may well be even further off if Kiron didn't
exactly reproduce the Zuiko relationship between aperture and pin
location. If they happen to be off in opposite directions in these 2
particular lenses, they could show quite a large difference. Notice
however, that all this has only an inexact relationship to the actual
light transmitted when the lenses are stopped down during exposure!
A further factor could be in lens design. I assume little corners cut
here and there in internal element sizes, etc. could make for
considerable weight, size and cost differences without affecting the
primary media lens test criteria of sharpness and contrast very much.
We tend to treat the marked and published numbers as gospel where, in
fact, they are all subject to ranges of variation.
If you are really concerned, the useful real world test is to take a set
of comparison images across the f-stop range on the same roll of
transparency film using manual mode. That will tell you all you need to
know about the true speed differences between these lenses. In auto
mode, all this mechanical stuff has no impact on correct TTL exposure
anyway.
Moose
Wayne Culberson wrote:
I bought a Kiron 80-200/4.5 recently from G. Reese, ....
I was shooting with it yesteday, and noticed it reads about 1 to 2 stops
slower than the Zuiko 135/2.8, when both set 135mm at f/16 and pointed to
the same scene (using OM2s in auto mode).
Do all zooms do this, or just the ones I own?
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