Minolta CLC Metering System

by Dick Sullivan

The Minolta proprietary CLC (Contrast Light Compensation) metering system was introduced in the 1960s as a new approach to obtaining correct exposures for high contrast scenes. It consists of two CdS cells mounted on little prisms that in turn are cemented onto the front and back upper faces of the pentaprism. The locations of these two CdS cells correspond exactly to the hot spots 'A' and 'B' in Figure 6. The CdS cell at point 'B' is more sensitive than the one at 'A'. Since the CdS cells are wired in series with one another, an average of the two is reflected in the effective reading. Since most scenes contain a bright sky above and a more subdued subject in the foreground this mixing of the two areas is supposed to keep the sky from overpowering the foreground. The obvious question is, "What if I shoot something that has no sky or I shoot in portrait (vertical) format?". Well, the difference in the two CdS cells is low enough (about 1 stop) to produce good exposures regardless of scene content.

Figure 6

Many of the old SRT series cameras, suffer from a condition known as "cell separation". The CdS cells are cemented to the smaller prisms and the pentaprism with balsam cement that deteriorates with time, heat, moisture and rough handling. This cement is the same type, used to bond lens elements into groups. If the cement darkens, crystallizes or comes loose, your metering system will be way out of calibration. You should check the CdS cells by using a small bright light source. If you can obtain two meter peaks at points 'A' and 'B' in your viewfinder, all is well. If you do not detect these peaks, in all likelihood one of your CdS cells has become separated or is bad. Since the CdS cells themselves, rarely go bad, it is most likely that the balsam cement has deteriorated. This condition is not fatal, but is somewhat difficult to repair. Most good camera shops will have repair personnel that can clean and re-cement the CdS cells. It won't be cheap, but if the rest of your SRT is in good shape it might be worth it. The alternative is to use a hand held meter, or another camera to obtain the correct settings.

I have successfully repaired this separation on several of my SRT cameras by cleaning the separated prism surfaces with acetone and re-cementing them with clear epoxy. This, of course, requires disassembly of the camera top assembly, which may be beyond the skill level of some. I hope to cover the details of this repair, for the more adventurous, in a later article.

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