[rollei_list] Re: SL66 & focussing screens
- From: "Evan Dong" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "ejdong@xxxxxxxxx" for DMARC)
- To: "rollei_list@xxxxxxxxxxxxx" <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 26 Oct 2014 19:36:06 -0700
Hi Richard,
Thank You for the in-depth explanation. I use the original Rolleiflex plastic
screens in all of my Rolleiflexes. I had just (2) Maxwell screen which I
dislike for the lack of "Snap-to Contrast" just the older version plastic
Rolleiflex screen.
With the exception of the Hi D screen in my 2.8E3, all focusing screens I use
in my Rolleiflex are later plastic screen with either grid line with split
image finder or the large Spot magnifier.
Evan
On Saturday, October 25, 2014 1:50 PM, Richard Knoppow <dickburk@xxxxxxxxxxxxx>
wrote:
----- Original Message -----
From: "" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "ejdong@xxxxxxxxx" for
DMARC)
DMARC)" <dmarc-noreply@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Saturday, October 25, 2014 4:51 AM
Subject: [rollei_list] Re: SL66 & focussing screens
>I have the Hi D screen installed in my Rolleiflex 2.8 E3 in
>replacement of the original screen.
>
> The Hi D screen is as how Ferdi described it. I found it
> brighter than the original F screen and a tad less than
> the Maxwell screen. However the ability to have Contrast
> is better than the Maxwell.
>
> If you have someone that has or had all three screens to
> compare against in all lighting condition, I would highly
> recommend it. Each individual see things different...
>
> Evan
Keep in mind that the screen brighteners (for lack of a
better generic term) serve mainly to concentrate the light
from the screen at the eye when in its normal viewing
position. They can not increase the amount of light coming
from the lens but can direct the light at the corners and
margins toward the center. The light from a ground glass is
partially diffused, the amount of diffusion depending on the
fineness of the texture of the glass. Some goes straight
through at the angle at which it impinges on the glass, some
is scattered over an approximate hemisphere. With just the
glass there is a bright spot at the center which moves
around with viewing angle, that is, you can make a corner
sharp by moving the eye toward the corner but then the
center and all other areas will become darker. By
interposing a condensing lens in the path between either the
light coming from the lens toward the screen or the light
leaving the screen and traveling toward the eye the light at
the corners and margins can be deviated toward the center.
This makes the corners look brighter. If the condenser is
reasonably efficient, and if its focal length is correct,
the overall brightness of the image can be made quite
uniform. Further, the amount of light at the center is not
much affected so overall brightness stays reasonably high.
However, any movement of the viewing position away from the
intended spot will result in an overall darkening. This is
not a problem with most reflex cameras because the position
of the eye is relatively fixed but can be a problem where a
condenser lens is installed in a view or press camera.
Some screen brighteners are nothing more than extra-fine
ground glass or something like a thin layer of wax between
glass or plastic sheets. This increases the amount of
scattering which tends to even out the illumination but at
the price of lowering the overall brightness.
Some cameras have conventional condenser lenses, an
example is the finder in the old Kine Exakta. This is fine
where there is room. Most of the screen brighteners,
however, are Fresnell" type lenses. This has the advantage
of being thin and flat. It is actually a series of
concentric prisms which approximate a continuous lens. The
pattern of the "ridges" is visible superimposed on the image
but if reasonably fine does not disturb it much. Some
Fresnell lenses, such as the Rolleigrid, have a clear spot
in the center allowing direct viewing of the ground glass
surface for focusing.
The ideal position for a condenser is on the lens side
of the ground glass surface facing the ground surface, this
minimises the amount of scattering or internal reflection
from the surfaces, however, it is more often more convenient
to have the condenser on the viewing side of the ground
glass but here again the ridged surface is best facing the
ground surface. This eliminates having two flat surfaces
facing each other with the attendant problem of reflections.
The Rolleigrid is made so it fits in this way. The advantage
of having the condenser on the viewing side is that it does
not affect the position of the focusing surface. Other types
of screen brighteners may require adjustment of the
reference surface for the viewing system to maintain focus.
Note that where a camera has interchangable lenses the
focal length of the condenser lens must be correct or it
will not work properly. It must focus the light from the
screen toward the eye. Since the light striking the corners
of the ground glass will depend on the angle of the cone of
light from the lens it will vary with the focal length, and,
indeed, with the distance focused on. However, except for
very wide angle lenses a given condener will work over a
rather wide range of camera focal length lenses and focused
distances. This is why some cameras with combined condenser
and viewing surfaces such as the Nikon F have a variety of
finder inserts for different focal length lenses.
FWIW, I have Rolleigrids in all of my Rollei TLRs
except for an old Standard. They make a significant
difference in apparent brightness.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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