On Thu, 3 Dec 1998 20:43:33 +0100, "Ulf Westerberg" <ulf.westerberg@xxxxxxxxx>
jammed all night, and by sunrise was overheard remarking:
> >I´ve experiend the same, a very big devation at 200mm. I mentened this
> >in an older mail to the group.
> >I guess it´s at what angle the light rays hit the screen. More evenly
> >distributed or concentrated. The series 2 difuse the light less than the
> >old ones.
> Yes, Richard, the logic of this is that different focal lengths deliver
> different images on the screen. Compare to the OM2000 which cannot handle
> 600mm+ teles because of it's (too) small focus screen.
It's not the size of the focusing glass, but rather, the mirror.
This was an issue, back in the age of the dinosaur, with some of the
"standard" screw-mount cameras, since you could mix and match lenses and
bodies with impunity, and pretty much every "B" league player, and below,
made a screw-mount (Pentax-compatible, before their bayonetted OM clones
came along) camera. Virtually all writeups of these camera types
included the obligatory test with the monster 600-800mm lens, to check
for cutoff.
Telephoto cutoff is based on the angle of light rays leaving the lens
toward the ultimate focal point, which is either the film plane or the
glass. With a wide angle, the light from the lens is rapidly expanding
>from a (relative) point source toward the boundaries of the film plane.
With a telephoto, the light leaving the lens tends toward parallel, the
longer the lens. Once you get toward an infinite focal length (or close
enough), the unformed image at the mirror plane is the same size as
it'll be at the film plane. So with a too-small mirror, you only get
part of that image.
Once you got, for the most part, camera makers doing all their own
bodies, this wasn't likely to happen. Not that it couldn't -- but you
wouldn't expect Olympus to just arbitrarily change the size of the
mirror going from OM-1 to OM-4 or OM-10 or OM-PC or whatever. Sure,
incremental changes to the design (spot beamsplitters as in the OM-4, or
area splitters as in the OM-77), but the basic specs don't have a reason,
most of the time to change.
The OM2000, of course, was OEMed from Cosina, and they, of course, had
their own way of making this camera body, which shows up in a mess of
other lineups as well, I suspect because most of the companies out there
just don't see a gigantic market in making manual cameras anymore.
> This makes these screens totally useless for me, I regularly shoot with
> 200-300mm and up, even for static subjects.
Has anyone done qualification of the OM-2000, to figure out where the
mirror cutoff starts. As I recall from the screw-mount days, you
generally needed a 600mm or so to see it for certain. But of course, it
varies by body.
> Maybe this should go into the FAQ?
It should.
> And are the Beattie screens also affected in the same manner?
It's entirely a function of the OM-2000's smaller mirror, nothing to do
with the focusing screen. Not that a different screen can't cause some
other problems (such as changing prism-meter readings in OM-1s and OM-2s),
but they don't get involved in this.
As a separate issue, folks using long lenses can definitely benefit from
a better screen. The microprism and split prism focusing aids are
useless with long, small aperature lenses. And because of the small
aperature (necessary for cost, to make them portable, and to drive home
the only functional difference between "long lens" and "telescope"), a
brighter-than-usual screen is a big help. I have been thinking about
getting the all matte series-2 screen for use with my 500mm reflex lens.
--
Dave Haynie | V.P. Technology, Met@box Infonet, AG | http://www.metabox.de
Be Dev #2024 | NB851 Powered! | Amiga 2000, 3000, 4000, PIOS One
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