so whats the solution?
A few things: CdS meters were not perfectly linear even when the cameras they came with were new. There are a few ways you can handle this. Typically, as has been mentioned, the non-linearity shows up when the light is lower. If you use a roll of Portra 400 and set the meter to 200, your photos in good light might be slightly overexposed and those in poor light should be about right. Color print film has some lattitude and that lattitude should take you the rest of the way. You can get a hand held light meter. You can adjust the ASA dial at the beginning of a day's shooting based on a known accurate meter. This would be for using a 1.5 volt battery in a camera made for a 1.35 volt battery. Most people no longer shoot slide film. It is expensive and has very little exposure lattitude. If you want to shoot slide film, look for a camera which does not have a CdS meter. These would include the Minolta X-700, Canon EF, Nikon N90S, Canon New F-1 etc. Presumably you are looking to get some enjoyment from your picture taking. Don't worry that your camera's meter is off by 1/10 of a stop. The perfect doesn't have to be the enemy of the good.
Minolta's first 35mm SLR was the SR-2 of 1958. It introduced the SR mount. That same badic mount is found on cameras even after Minolta stopped making them. Manufacturers in China and possibly also Japan sold them in the U.S. under the Vivitar and other names. The SR-2 did not have a built-in meter. Clip-on meters were later made for SR series cameras. The first Minolta interchangeable lens SLR with a built-in meter was the SR-7. This camera had a meter cell on the outside of the body. It did not read light through the lens. As such, the Auto Rokkors were not meter coupled. What was auto about them? They had auto diaphragm operation.
The SRT 101 of 1966 had built-in through the lens metering. This required a tab to transmit the aperture setting to the camera. I have seen a few odd cases where someone attached a meter tab to an older SR lens. The SR mount changed again in 1977 with the addition of the MD tab. Some of the early MC lenses, like the 58/1.4, were carry-overs from the previous Auto Rokkor designs. Minolta did not make important changes to the lens coatings until the introduction of the MC Rokkor-X lenses in 1973. You can use Auto Rokkors on later cameras with stop down metering. To slightly complicate things further, in the early SR era, Minolta took some lenses from the earlier rangefinder models and put SR mounts on them. These lenses do not have auto diapgragms. For regular picture taking, any of the MC Rokkor-X, MD Rokkor-X or plain MD lenses will get you the best results.
A resistor would not work nor even a diode like in some adapters. They drop a fixed amount from the silver oxide battery but silver oxide while relatively more stable than alkaline it's not as stable as mercury and thus the voltage drops during the life of the battery. Modern metering circuits work with a range of voltage and thus doesn't affect their accuracy. A regulator would work but a regulator would drain the battery.
so whats the solution?
later production runs of the 102 (and earlier 101) models do not have mirror lock-up
A germanium diode - unlike a resistor, is ideal for it's fixed voltage drop of 0.3V and minimum current. Here's instructions if you're DIY type although it uses a Schottky diode (0.4v drop) -> Converting the SRT to use 1.5v cels
There is a rokkorfiles website which will explain the differences between the different series of Minolta manual focus lenses.
Was this just marketing then and the meters were actually not good enough for slide film?
Yes it would drop 0.3v but the silver oxide battery must provide 1.5V for its entire useful life but it doesn't. Although not as bad as alkaline the silver oxide battery voltage do drop during its useful life. A voltage regulator which would regulate the voltage to 1.35V regardless of how high the supply voltage is would work. But you have to put the power switch ahead of the regulator chip otherwise it would drain the battery.
so whats the solution?
You may not need a solution, but there are lots of approaches you can try.
I'd start by reading a gray card at f16 on a sunny day with a 625A 1.5v battery and see how far off from sunny f16 your meter is.
Right I found the Canon EF, Canonet QL17 GIII, the Olympus 35-RC work fine with alkaline batteries. The SRT-101's don't so I don't use the meter in the SRT.
Yes it would drop 0.3v but the silver oxide battery must provide 1.5V for its entire useful life but it doesn't. Although not as bad as alkaline the silver oxide battery voltage do drop during its useful life. A voltage regulator which would regulate the voltage to 1.35V regardless of how high the supply voltage is would work. But you have to put the power switch ahead of the regulator chip otherwise it would drain the battery.
This first thing I do when I get in my car is check how much gas is in the tank. Doesn't everybody? I guess not.
Anyway, it's a good idea with any meter -- in-camera or not -- to run a simple sunny f16, or similar test for accuracy every so often. While many cameras & meters have battery checks, they don't check for accuracy.
Correct, the diode is not a voltage regulator. It's purposes is to simply start the meter at the voltage it was intended for. Over the course of it's use, the battery voltage will drop as is normal.
This first thing I do when I get in my car is check how much gas is in the tank. Doesn't everybody? I guess not.
Anyway, it's a good idea with any meter -- in-camera or not -- to run a simple sunny f16, or similar test for accuracy every so often. While many cameras & meters have battery checks, they don't check for accuracy.
speaking of battery test lights, with the diode would the battery test light come on at what voltage?
It starts the meter. So 1.5v coming through from modern battery, what does the diode do with the xtra voltage? It must be regulating it if only 1.35v is getting through.
The germanium, silicon and Schottky diodes each have a fixed voltage drop. A voltage regulator is intended to maintain the voltage output to a level. Zener diodes are a "poor mans" regulator as opposed to more complex designs.
Who needs a meter when you can take advantage of most color and b&w film's extreme latitude . . .
I see, so it does regulate the voltage from being too high. Then once the battery gets weaker to below 1.35v then what happens? The battery test function would show the needle not going into the pass test box. I think this is the ideal solution actually.
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