At the time there were "extinction" light meters. The idea
was to close it down, as stated in the article linked, until the
shadows were not visible. The problem, of course, is that the eye
is not constant in its sensitivity so an extinction meter is not
at all accurate. However, they were cheap, at least compared to
photocell meters, which actually measure light. Note that a spot
photometer, like the SEI meter, is not an extinction meter,
rather it has a fixed intendity spot of light from a lamp that is
matched against a variable spot transmitted thorough the meter.
They eye is very good at matching brightness where the comparison
samples are adjacent. These meters are very accurate or at least
repeatable where an extinction meter is not either.
An old trick with a view camera was to use the lens stop to
find the setting that would loose detail in the shadows on the
ground glass. If a lens capable of very small stops is used this
is possible under many light conditions. It is common for old
view camera lenses to stop down to f/45 or even f/64. This trick
was used as a way to judging exposure since it is essentially an
extinction meter. However, since the eye varies in its
sensitivity with the ambient light level this technique really
gives only a very approximate idea of light intensity. This is
exactly the technique used with the Rolleiphot to judge exposure.
OTOH it will give a good idea of depth of field and may be a
useful accessory for that.
On 8/4/2020 6:37 AM, Eric Goldstein wrote:
https://www.photo.net/discuss/threads/rolleiphot-how-does-it-work.129040/
https://www.flickr.com/photos/phollectormo/14920781660
Eric Goldstein
On Tue, Aug 4, 2020 at 9:31 AM wayne pinney <tennjed@xxxxxxxxxxx> wrote:
Bought one of these. It was cheap; in good shape; intriguing.---
Can find absolutely nothing on how it is supposed to be used, other than a
stray comment or two indicating that it was meant to be a depth of field
preview, and possibly an exposure aid, to be used on pre-bayonette Rollei.
Anyone know of a source of information on use of the Rolleiphot?
Best,
Wayne---
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