I posted another thread the other day talking about the meters in my mechanical SLRs that want 1.35v batteries. Figured I'd start a clean one that will hopefully end up in Google searches in the future when people have similar questions. I know I have only been able to make any progress on these cameras thanks to people like members of this forum (and others) who have documented their projects.
First things first. I am using a Reveni Labs Spot Meter (1st gen; I see there's a Mk II available now) as my baseline. I've always had results as expected with this meter when shooting medium and large format with zone system development, and when I first bought it to replace my Minolta Spotmeter F, it matched perfectly, within less than 1/10th of a stop in every scenario where I tested it. As my metering target of known brightness, I am using the following setups:
But before I did that, I did some baseline tests to see where my built-in camera meters landed. Since both meters should have something approximating a center-weighted-average metering pattern, I made sure to fill the entire frame with the panel or section of illuminated wall that was at an even and known EV value for each camera meter reading. No other light was present in the room. Measurements were taken using a 50mm f/1.8 lens on each camera.
Using the same 1.4v zinc air battery in both cameras, (with voltage confirmed at just north of 1.35v by my multimeter), results are as follows:
We can see that the FTb was very close to dead on, and also quite linear. Lucky! The OM-1 was off by ~1/3 of a stop at the low end, and the overexposure increased steadily as brightness grew. This suggests not only zero calibration being off, but something nonlinear happening in the circuit, or possibly even something wrong with the CdS cells.
After successfully soldering in a 1N6263 schottky diode to the FTb circuit, I installed an Energizer 357 battery (voltage confirmed at 1.575 volts using my multimeter), and repeated the tests.
The FTb now displays basically the opposite problem as the OM-1: It gets fairly close at high EVs, but underexposes progressively more as EV goes down. It would seem the diode I installed is not dropping the voltage as much as I hoped, or as linearly as I hoped.
My next step is to try calibrating the FTb meter using this thread as a guide. It seems a good candidate with multiple points of adjustment available to hopefully straighten out the response a bit.
I will also try the unmodified OM-1 meter circuit with the silver oxide battery. Maybe it will end up working perfectly and I can skip the monkeying around in the camera's innards and just shoot it.
I have some lingering concerns about the validity of the readings with my LED light source. It's an attractive option because it gives me fine tuned control over the light output, and I can do my tests at night when I actually have time for such things. But I may go out on an overcast day and meter a gray card in a few different lighting situations to see if the results are significantly different. The vast majority of my B&W shooting is done outdoors in overcast conditions, so that would theoretically be the best scenario for testing.
First things first. I am using a Reveni Labs Spot Meter (1st gen; I see there's a Mk II available now) as my baseline. I've always had results as expected with this meter when shooting medium and large format with zone system development, and when I first bought it to replace my Minolta Spotmeter F, it matched perfectly, within less than 1/10th of a stop in every scenario where I tested it. As my metering target of known brightness, I am using the following setups:
- EV 15: An LED video light panel by Neewer, set to 100% brightness and 5600k color temperature. Neewer claims a CRI of 97 for this light, which isn't stellar, but I hope is sufficient for my purpose. The spotmeter gives me identical readings without deviating even 1/10th of a stop at all areas of the illuminated panel.
- EV 12: Same LED panel, but set to 12% brightness. The math checks out here and the spot meter confirms that it is dead on at EV 12.
- EV 8: The same LED panel, but instead of metering off the panel (shining it into the meter), I am using it to illuminate a large section of blank white wall. Brightness and distance to wall is adjusted until the spot meter reports an evenly-illuminated EV8 on a large section of the wall. Color temp is still at 5600k.
But before I did that, I did some baseline tests to see where my built-in camera meters landed. Since both meters should have something approximating a center-weighted-average metering pattern, I made sure to fill the entire frame with the panel or section of illuminated wall that was at an even and known EV value for each camera meter reading. No other light was present in the room. Measurements were taken using a 50mm f/1.8 lens on each camera.
Using the same 1.4v zinc air battery in both cameras, (with voltage confirmed at just north of 1.35v by my multimeter), results are as follows:
| Instrument | Measured EV w/ EV 8 Target | Measured EV w/ EV 12 Target | Measured EV w/ EV 15 Target |
| Spot Meter (Baseline Reference) | 8.0 | 12.0 | 15.0 |
| Canon FTb | 8.5 | 12.0 | 15.5 |
| Olympus OM-1 | 7.6 | 11.0 | 13.5 |
We can see that the FTb was very close to dead on, and also quite linear. Lucky! The OM-1 was off by ~1/3 of a stop at the low end, and the overexposure increased steadily as brightness grew. This suggests not only zero calibration being off, but something nonlinear happening in the circuit, or possibly even something wrong with the CdS cells.
After successfully soldering in a 1N6263 schottky diode to the FTb circuit, I installed an Energizer 357 battery (voltage confirmed at 1.575 volts using my multimeter), and repeated the tests.
| Instrument | Measured EV w/ EV 8 Target | Measured EV w/ EV 12 Target | Measured EV w/ EV 15 Target |
| Spot Meter (Baseline Reference) | 8.0 | 12.0 | 15.0 |
| Canon FTb w/ diode modification | 9.5 | 13 | 14.5 |
The FTb now displays basically the opposite problem as the OM-1: It gets fairly close at high EVs, but underexposes progressively more as EV goes down. It would seem the diode I installed is not dropping the voltage as much as I hoped, or as linearly as I hoped.
My next step is to try calibrating the FTb meter using this thread as a guide. It seems a good candidate with multiple points of adjustment available to hopefully straighten out the response a bit.
I will also try the unmodified OM-1 meter circuit with the silver oxide battery. Maybe it will end up working perfectly and I can skip the monkeying around in the camera's innards and just shoot it.
I have some lingering concerns about the validity of the readings with my LED light source. It's an attractive option because it gives me fine tuned control over the light output, and I can do my tests at night when I actually have time for such things. But I may go out on an overcast day and meter a gray card in a few different lighting situations to see if the results are significantly different. The vast majority of my B&W shooting is done outdoors in overcast conditions, so that would theoretically be the best scenario for testing.
Last edited:
