I’ve put this page up in response to a few queries I’ve had regarding the metering accuracy of the 139.
As with any camera, or any other manufactured item, the 139 is built to within certain tolerances and is tested to perform within certain limits. The limits set for the meter are wider than might be expected with acceptable errors up to ±1.1EV in some circumstances. The limits vary across the metering range with tighter limits in the middle of the range but, even then, the limit is ±0.6EV or nearly ⅔ of a stop.
Here’s the graph from the 139 repair manual showing the limits over the metering range of 0 - 18EV.
These tolerances should be borne in mind if checking the metering of your camera against another. Also remember that the meter or camera you are comparing against may also have errors. If, for instance you were to compare two 139s against each other and one was set near its lower limit and the other near its upper limit, there could be over 1 stop of difference between them, even in the middle of the metering range, but they would still be within tolerance.
What matters most, of course, is the results obtained: whether the image on film is correctly exposed or not. There are other factors to consider here including lens aperture accuracy and shutter speed accuracy and, not least, development accuracy. These are all variables that will affect the results. The only one of those which is relevant to discuss here is the shutter accuracy. The limits for the 139 shutter are shown in the table below taken from the repair manual.
The largest acceptable error is at 1/1000 with a tolerance of ±0.44 stop. Generally the limits are narrow enough to not cause problems themselves but could still add to any metering errors to give an overall exposure error.
These tolerances are not untypical in my experience and my recommendation to anyone using any film camera is to find your own personal Exposure Index (EI) for the camera/film combination you wish to use. You may find it necessary to adjust the ISO a little to get your preferred results but this has been a recognised practice for as long as meters, built in or otherwise, have been available.
If though, having read this far, you believe your 139 is genuinely not metering correctly, it is possible to adjust it. There is a potentiometer accessible under the right front (looking from the front) leather cover. Some 139s have two access holes in which case it’s the potentiometer under the top most hole. The adjustment is very sensitive and a very small adjustment will change the metering significantly but it can be done. Note too that the potentiometer may be sealed which can make it, initially, difficult to move. If necessary the whole leather can be removed and the plastic cover removed to get better access to the potentiometer but the camera base plate and lens mount trim also need removal first. There’s more information on my DIY page describing removal of the covers.
Footnote: Nearly all Contax 139 cameras, and some other models too, over expose unless they have been recently adjusted. This is due to deterioration of the metering cell. More information on this can be found on my blog at https://contax139.blogspot.com/2019/02/does-your-139-over-expose.html