G R O A N.
I oversimplified a little. 1-11 has a clear central spot (with cross hairs)
in a ground glass screen, while 1-12 has a larger clear central spot (with
cross hairs) in a clear fresnel screen
1-5/6/7 have a (focal-length specific) microprism in a clear fresnel screen.
Thus the "clear" screens are not plain "glass", but have a fresnel pattern
on them to ensure even bright coverage.
I hope that was . . . illuminating.
--
Piers
-----Original Message-----
From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
Of ScottGee1
Sent: 03 May 2005 22:19
To: olympus@xxxxxxxxxx
Subject: [OM] Re: Simple question about 'clear' screens
Piers, that really clarifies the use of the completely clear ones, thanks!
I assume that the ones with microprisms (1-5, 1-6 and 1-7) use that focusing
aid and the rest of the screen remains, well, clear.
Thanks!/ScottGee1
On 5/3/05, Piers Hemy <piers@xxxxxxxx> wrote:
>
> I interpreted Scott's question to be a reference to the completely
> clear screens - 1-9, 1-11, 1-12.
>
> 1-9 is the odd one, designed for use with endoscopes. It is UNUSABLE
> with a photographic lens to my understanding.
>
> 1-11 and 1-12 are completely clear screens for macro and micro use, to
> get as much visible image as possible in the dim conditions which are
> usual in such applications. The result is that the screen provides an
"aerial image"
> which can appear to be in focus at all times. To focus using those
> screens, you first have to "tune" the camera focusing system to your
> eyesight. On models with built-in dioptric adjustment, do this by
> taking off the lens, then adjusting the dioptric until you can see
> clearly the cross hairs on the screen. Then with the lens fitted,
> focus by keeping the cross hairs in focus, and adjusting the
> camera/subject position until the image is also in focus.
>
> --
> Piers
>
> -----Original Message-----
> From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On
> Behalf Of Simon Worby
> Sent: 03 May 2005 21:27
> To: olympus@xxxxxxxxxx
> Subject: [OM] Re: Simple question about 'clear' screens
>
> ScottGee1 asked:
>
> > From the archives my understanding is that one cannot focus with the
> clear
> > area, but rather the focusing aid is used to focus - correct?
>
> The focussing aid (split prism / microprism) is not *needed* to focus,
> but it can be helpful. However, without anything (ground glass or
> focussing aid), focussing is impossible.
>
> What you are doing when you focus, is to get the image on the
> focussing screen sharp and in focus. If it's clear glass you can't see
> the image on the focussing screen as there isn't one; hence it needs
> to be at least part ground glass with or without a focussing aid.
>
> The focussing screen is at the same distance from the subject as the
> film will be when it's exposed to the light. Hence you are making sure
> the image, when it's projected onto the film, will be in focus.
>
> To the eye, due to the various lenses, the focussing screen appears to
> the eye to be about 6 foot away, though in reality it is but an inch or so
away.
> (It's not possible for normal humans to focus down to an inch!)
>
> > <snip> Also, how much brighter/clearer are the clear screens than a
> comparable
> > 1 series matte screen? 2 series?
>
> I think a stop or so. But, IMHO, they are almost impossible to use
> except for specific purposes. Unless you have a specific purpose in
> mind, I would recommend a ground glass screen of some description, with a
focussing aid.
> Some photographers like not to have the focussing aid (as per 1-4 or
> 2-4) but personally I think they make life hard work.
>
> Regards,
>
> Simon
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