Testing Exposure Meters
Cameras from the 1960s and 1970s used selenium cell meters. These types of meters were great for the simple reason that they didnt use any batteries. The selenium created its own electricity, much like a small solar cell. You will be able to tell if the camera you are testing has a selenium cell meter because the cell is quite large, about 1 cm by 3 cm. As well, the meter will usually have a small round plastic lens to help the camera concentrate light on the cell. When first developed the selenium cell was thought to be very high tech. Even some of the cheapest cameras on the market at that time had gray gridlines on them in some from of imitation.
The problem with a selenium cell meter is that the selenium degrades over time from humidity and sunlight. The cell will probably be worthless unless the previous owner of the camera stored the camera in a bag or it had a selenium cell light shade. An example of a camera with a selenium cell light shade is the Contaflex IV.
Another thing to keep in mind is that selenium is nonlinear. This means that you cant totally trust a camera with a selenium cell. However, you can still consider buying a used camera even if the selenium cell meter doesnt work. All you need to do is learn how to use an external meter.
In the 1970s cadmium-sulfide (CdS) became very popular when it came to meters. For example, the Pentax Spotmatic was well known for using cadmium-sulfide cells. The cells were very resistant when it came to damage from sunlight. As well, the cadmium-sulfide meter cell was much smaller than the selenium cell. If the used camera you are considering buying has a 5 mm lens in order to measure the aperture you can be assured that it is using a CdS cell. As well, if the camera is using TTL metering, such as the Spotmatic camera does, you know that it is also a CdS cell. The CdS cell meter doesnt deteriorate with time so this type of meter in a used camera is a good choice.
The bad side of the camera having a CdS cell meter is that the camera needs to have a battery. Most of the cameras in the 1970s used a mercury battery that was called the PX625. This type of battery is no longer being made so you will need to search for a replacement. Or you can consider modifying the camera so that it uses the more modern silver or alkaline batteries. This, of course, will be a lot more work.
If you are considering buying a used camera that uses a CdS meter you will want to test the camera fully. Try to take the camera outside and then change the settings to ASA/DIN 100 and make sure that the outdoor camera reading is about f/11 16 at a shutter speed of 1/100
or 1/125. Then go back inside. You want to make sure that the camera reading takes about four stops to drop completely.
There are some obvious reasons why you wont want to buy a used camera that uses the CdS system:
The battery is dead. Many times you wont be able tell if the battery is dead or if the electronics of the camera are not working correctly. A rule of thumb is that most mercury cells will last for about ten years.
The battery is leaking. You can check for battery leakage by opening up the battery compartment to see if there is any messing residue inside.
The analogue match-needle is easily damaged when the camera is dropped or banged. You can check for this type of damage by holding the camera upside down. The needle should stay steady and not start floating.
Take your time testing the exposure meter of the camera before you buy. You dont want to get the camera home only to find that you now have a problem with the meter cell.
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