As part of my wide-angle TL120 project
(installing 55mm Mamiya lenses at a fixed focus for use at
f/22), I've been thinking about metering.

Correct exposure depends on the correct balance of:
Amount of light
Shutter speed
Film speed
Aperture size

In the TL120, light level is measured with a sensor in the
prism. Without the 3D World prism installed, there is no
in-camera metering available. The coupling of the prism to
the meter may be electrical (as it is in my older TL120) or
it may be optical (as in my newer) but the prism is an
integral part of the system and without it, the meter will be
meaningless.

Film speed is sent to the meter circuit from the dial on the
top of the camera. The same goes for shutter speed. The
trick is telling the meter circuit what aperture the camera
is using.

The stock lens assembly has a variable resistor ring with a
pair of contact wipers. As you rotate the aperture ring, the
wipers are moved to different places on the variable
resistor. The resistance across the wipers is measured and
that tells the meter circuit what aperture you have chosen.

Since my camera will be used at f/22, I should be able to
solder in a single set resistors and the meter will be ready
to go. The trick is to find the right resistor values. To
do that, I sat down last night measured the behavior of my
lens. Here's what I found:

There are three connections on the finder-lens. My older
camera has spring-loaded contacts. My newer camera has a
soldered connection. The center-most contact we'll call (A),
The middle will be (B) and the outside (C).

The resistance between (B)-(C) is always 6.09K-ohms. The
sum of the resistances between (A)-(B) and (A)-(C) is always
6.08K-ohms.

stop A-B A-C
22 0.81 5.28
16 1.10 4.98
11 1.70 4.39
8 2.29 3.79
5.6 2.89 3.19
4 3.49 2.59
2.8 4.09 1.99

(All of the above values are in K-ohms.)

My initial thought was to stick in two resistors (.8K and
5.3K) and I'd be done. But the stock TL120 uses a f/2.8
finder-lens and my wide-angle TL120 uses a f/4.5 finder-lens,
the sensor in my prism is going to see 1 1/3 stop less
light than it expects (because it was calibrated for the
f/2.8 lens). If I believe the meter, my images will be over
exposed by 1.3 stops.

Here are my ideas. I can use the f/22 resistor values and
advance my film speed setting by 1 1/3 stop. Or, I could
interpolate the resistor values for f/14 (which is 1 1/3 stop
less than f/22) and use that with the actual film speed setting.

Reality check please.

Am I going the correct direction with my adjustments?

Is adjusting for the difference in viewing lenses this simple
or am I missing something important?
________________________________________
John Thurston
Juneau, Alaska
http://stereo.thurstons.us

--- In MF3D-group@yahoogroups.com, John Thurston wrote:

>
> As part of my wide-angle TL120 project
> (installing 55mm Mamiya lenses at a fixed focus for use at
> f/22), I've been thinking about metering.
>
> Correct exposure depends on the correct balance of:
> Amount of light
> Shutter speed
> Film speed
> Aperture size
> Film speed is sent to the meter circuit from the dial on the
> top of the camera. The same goes for shutter speed. The
> trick is telling the meter circuit what aperture the camera
> is using.
>
>
> Since my camera will be used at f/22, I should be able to
> solder in a single set resistors and the meter will be ready
> to go. The trick is to find the right resistor values. To
> do that,

Hi John, you were right at the outset but started to stray. The
metering circuit is a wheatstone bridge null balance circuit, very
standard stuff. You do not need to compensate for 1+1/3 stops for the
simple reason that the light coming through any lens @ f/22 is the
same, whether it's a f/2.8 or f/4.5 or f/11 max aperture. Use the
resistor values for f/22, or a 6.1K potentiometer with one end tied
to the wiper. I assume you are going to tuck this inside. By using a
potentiomer rather than the 2 resistors you can make small
adjustments without having to unsolder and change resistor values.

Good Luck with it mate. Cheers, Peter

--- In MF3D-group@yahoogroups.com, "Peter Thomson"
wrote:

>
> --- In MF3D-group@yahoogroups.com, John Thurston wrote:
> >
> > As part of my wide-angle TL120 project
> > (installing 55mm Mamiya lenses at a fixed focus for use at
> > f/22), I've been thinking about metering.
> >
> > Correct exposure depends on the correct balance of:
> > Amount of light
> > Shutter speed
> > Film speed
> > Aperture size
> > Film speed is sent to the meter circuit from the dial on the
> > top of the camera. The same goes for shutter speed. The
> > trick is telling the meter circuit what aperture the camera
> > is using.
> >
> >
>
> Hi John, You do not need to compensate for 1+1/3 stops for the
> simple reason that the light coming through any lens @ f/22 is the
> same,

Duh! What was I thinking, SLR talk. No, you're right on the latter,
as the lens is at max aperture for metering at all times. You do
compensate for the 1+1/3 stops. But I still recommend the
potentiometer use.

Cheers, Peter