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[OM] Re: olympus-digest V2 #1620

Subject: [OM] Re: olympus-digest V2 #1620
From: Mark Marr-Lyon <o9938156@xxxxxxxxxxxx>
Date: Thu, 4 May 2000 22:38:41 -0700
Eric Pederson <epederso@xxxxxxxxxxxxxxxxxxxx> wrote:
[snip]
>To wit, how are the depth of field indicators on Zuiko lenses calculated?
>Basically, they need to arbitrarily decide on a value for the Circle of
>Confusion. Is this 30 microns for all lenses (I read somewhere that this
>is fairly standard in the 35mm format, but I can't recall the reference
>or estimate its reliability)? 

I don't know how they calculate the d.o.f, but I'd just like to say
that the Circle of Confusion is my all time favorite scientific name
I've ever run across!  Specifically, I like the Circle of Least Confusion,
which is the smallest diameter circle containing all the focused light
rays.  I've always somehow wanted to get myself into the circle of least
confusion...

>Since the allowable value for the circle of confusion should vary with the
>anticipated enlargement (and to a lesser degree film speed/graininess), it
>would be useful to be able to reestimate d.o.f. for different values of
>the c.o.c. from the lenses as well. Is there a way to interpret e.g. the
>f8 mark as giving the f8 depth of field at 30 microns, but the f16
>indicating the same f8 depth but at ~100 microns? 

I would guess that this would be a difficult calculation, since it could
possibly depend on a lot of things, like where the diaphragm is and what 
aberrations come into play at what apertures, etc. Basically, it is why 
the bokeh is different between different lenses.  Maybe a few test rolls 
with a target at various distances and apertures could tell you what you 
find to be an acceptable d.o.f for your given lens?

>Similarly, any tricks for extended lenses (especially with the 14 and 25
>mm rings)? I find that my sense of d.o.f. is almost always off from the
>finished product with my very amateur macro work (mostly with the 90mm f2
>portrait lens which does a decent job pretending to be a macro lens)

If that's the Zuiko 90/2, then by all accounts it definitely does far more
than pretend to be a macro lens.  That's only what I've heard, since I'm 
not
fortunate enough to own one.  Are your experiences with it different?

>I'll happily summarize off-list replies, but this group seems to do
>everything on-list... :-)
>
>Thanks in advance!
>Eric

Mark.

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