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[OM] the elusive f/2

Subject: [OM] the elusive f/2
From: William Sommerwerck <williams@xxxxxxxxxx>
Date: Wed, 06 Dec 2000 07:40:48 -0800
"I was swapping email with an OM defector -- he was dumping his OM gear in
favor of another system.

He said, "All those fast primes are great. But none are [sic] actually f2. The
end of the aperture scale is marked with a white line that *precedes* [the
"real"] f2. You can use the camera's meter to corroborate this. An adjustment
from 2.8 to 2 by turning the aperture ring to its wide open position does *not*
gain a full stop. All my coveted fast primes were the same way -- the 21, 24,
28, 50 Macro, and 100. I couldn't believe it. It's one of the reasons I'm
getting out."

I replied to him thusly: "You're crazy. If Zuiko sold a lens with a misleading
maximum aperture, they'd be hung out to dry..." Of course, I then examined the
one f2 Zuiko that I own (85/2) and I saw that he wasn't crazy at all.

So, to all of you, I pose this question: What gives???"


I'm replying to the group (rather than just the sender), because this is a
question of general interest. Questions like these are why God sent me to
minister.

As I've repeated ad nauseum, there are Several Important Things not taught in
school. One is Analytical Thinking. Another is Learning By Doing. A third is
Not Believing Everything You See Or Read.

Let's start with Learning By Doing. The only way to *know* whether the full
aperture is really f/2 is to make a series of "equivalent" exposures and see
whether you get the same density. Did the defector do this? Of course not. It
would have been too much trouble.

Now let's apply Analytical Thinking. I popped a 28/2 on my 4T body and checked
the spot readings. Yup -- there's only a half stop between f/2 and f/2.8. Why
should this be?

I can think of one plausible answer. Wide-angle lenses -- especially fast ones
at full aperture -- have significant edge fall-off. The original OM-1 had a
full-frame averaging meter. In an evenly lit scene, the fall-off would cause
overexposure. The aperture ring is therefore adjusted to make the camera
"think" a slightly slower lens is mounted.

This still leaves the teles, which don't have little or no significant falloff.
I've left a message with Vince Marino at Olympus, but I doubt he'll be able to
call back any time in the near future (ie, the next decade). We'll see.


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