DEAR RICHARD ET ALIA,
As I still do some work as a director of photography on film/video productions
and teach film/video production, I can state that video/cinema lenses intended
for professional production are still calibrated in T stops. The local
production houses' lenses are calibrated in T stops and the Barbados Film and
Video Association has a lovely set of brand new, coated prime lenses for their
Black Magic 4K cameras, all calibrated in T stops.
Further, depth of field tables for cinematographers are also calibrated in T
stops and meters and/or feet. I just viewed some tables for Zeiss prime cine
lenses.
CHEERS!
BOB
From: "`Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
To: "pure-silver@xxxxxxxxxxxxx" <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, August 18, 2016 3:44:57 PM
Subject: [pure-silver] Re: Large Format Metering at the Groundglass
In the days before lens coating became common it was a routine to measure
transmission of lenses. In the motion picture industry lenses were sometimes
marked with "T" or transmission stops. The more glass-air interfaces a lens has
the greater are its losses. Modern multi-coated lenses generally have quite
high transmission so that f/stops can be pretty accurate but a through the lens
metering system will still compensate for any loses. TTL is pretty easy to do
in a 35mm SLR but becomes more difficult in other types of cameras. A TTL meter
will also compensate for changes in transmission for a zoom lens at different
focal lengths as well as bellows draw compensation.
There have historically been a number of meters with ground glass probes.
General Radio Co. made a strobe meter with a ground glass probe in the 1950s.
Most of these (maybe all of them) measure at a spot on the GG. They can be used
like a spot meter to measure lighting ratio and image contrast at a distance.
Sometimes quite useful.
If coupled to the outside of the ground glass the probe is affected by the
transmission of the glass which can vary with its texture and the angle of
incidence of the light from the lens.
Its not difficult to determine the iris settings on a lens as far as agreeing
with the f/stop since the stop is just a geometrical ratio of the size of the
entrance pupil to the focal length at _infinity_. The entrance pupil is the
size of the stop diaphragm as seen from the front of the lens. It is affected
by the magnification of the lens components in front of the stop. If a point
source of light is placed at exactly the infinity focal plane it will project
an image of the entrance pupil out of the front of the lens. A translucent
screen, like a piece of ground glass, placed over the lens will show this
circle if light. Its diameter is the effective diameter of the stop. Dividing
the focal length by this diameter will give the stop which can be compared with
the marks on the lens. For fairly long lenses a view camera will work as a sort
of optical bench which will allow a simple measurement of true focal length,
the locations of the entrance and exit pupils and the locations of the nodal
points. For calibration of stop scales the marked focal length is usually close
enough. I think I have posted the complete procedure for measuring these
parameters to this list in the distant past, they may be in the archive
somewhere.
The problem with T stops is that they are not correct for calculating depth of
field. I think they were popular for only a couple of years around the early
1950s. Even single coated lenses, if not too complex, have transmissions good
enough so that T stops are not very useful. However, they insured exposure made
with different lenses would be the same, important for motion picture purposes.
On 8/18/2016 11:28 AM, Richard Lahrson wrote:
Hi, the Sinarsix does not meter though the groundglass but rather at
the image plane. In Ansel Adams revised "The Camera" there is a
picture of it on page 66 where it is cited as a useful device for checking
the aperture markings of lenses as they can often be off.
I set up lights and grey card to check the 120mm/8 Nikkor SW. The
last mark is f/64. But it stops down a full two stops more. It's good
to know. I"ll be checking the apertures of my other large format
lenses as well.
Best. Rich
On Thu, Aug 18, 2016 at 7:51 AM, Luis Miguel Castañeda < octabod@xxxxxxxxx >
wrote:
BQ_BEGIN
Sinar had an attachment for gossen meters, but iirc it metered in the inner
side of the ground glass. The only advantage for such cumbersome methods in
general photography was the independence of bellow factor
El 18 ago. 2016 4:47 p. m., "Robert Hall" < robert.g.hall@xxxxxxxxx > escribió:
BQ_BEGIN
Would you imagine the need to calibrate it based on the particular ground glass
you have in the camera?
Robert Hall
www.RobertHall.com
www.RobertHall.com/workshops
www.facebook.com/robert.g.hall
On Wed, Aug 17, 2016 at 3:16 PM, Richard Lahrson < gtripspud@xxxxxxxxx > wrote:
BQ_BEGIN
Hi folks! For some time, I"ve wanted to meter through the camera
at the film plane for large format. I have a Luna-Lux SBC (silicon blue cell)
meter with the accessory fiber optics probe. The probe attachment
is good for measuring small objects via reflected light (though a spot
meter, which I also have is better). The probe attachment also works
with the older Lunasix. The probe is supposed to work with measuring
at the groundglass. I tried it, but the problem with keeping the probe
on the glass, holding the meter and keeping the light out with the
dark cloth,,,you get the picture. I gave up.
There are three other alternatives: the old Sinarsix, the newer meter
probe also from Sinar that attaches to a couple of more modern
meters. Both these alts measure selectively. There is also
a Horseman(?) attachment which measures 4x5 but full frame.
The Sinar models measure an area 7X9mm. The probe can be moved
anywhere in the 4x5 area. I did not buy the newer Sinar probe largely
because of the whimpy cord connection for the probe.
I bought the original Sinarsix and got all the accesories including the
2 extra dials for reciprocity failure, the manual and the 5x7 cassette
with case and keys! Took a chance on the eBay with no returns.
The only missing item is the 8x10 intermediary frame that allows
use of the 5x7 cassette on the 8x10 format. If anyone happens to
have the intermediary frame lying around, let me know!
The unit fired up after removing the mercury battery and replacing it
with the Wein air cell! The problem with the old Sinarsix is the
Cadmium sulfide cell which can retain a memory if you are not
careful.
The Sinarsix takes the guesswork or math out of bellows extension
and will handle filters up to 2X exposure and check for light falloff
with wideangle lenses and camera tilts and swings. The manual is
informative and clear. I'm still working my way through it!
Rich
BQ_END
BQ_END
BQ_END
--
Richard Knoppow dickburk@xxxxxxxxxxxxx WB6KBL
