Some Zeiss/Contax lenses under expose by half a stop!

This is a continuation of a previous post on how the Contax lenses communicate to the camera body in which I mentioned I was doing some investigation into some questions I had and also some forum posts that suggested some lenses had compatibility issues with some Contax bodies.

To deal with the second issue first - I've not been able to duplicate any of the problems I've seen mentioned that might suggest some incompatibility between some lenses and bodies. I suspect these problems were due to individual lenses or bodies that had one or more faults.

I have, however, determined that the information passed from the lens to the body differs between lenses and that means, in some cases, certain lenses will cause a half stop under exposure.

The questions I had were raised by references in a few different Contax body service manuals to a feature called 'theta compensation'. This is something that appeared to relate only to some lenses. I initially tried to discover what theta compensation was but have been unable to find anything. Even an email to Zeiss was unproductive with Zeiss saying all the information was passed to Yashica. So I took a different approach and started to look at individual lenses that appeared to have theta compensation to see if I could find a difference between them and other lenses.

I was able to identify two lenses in my collection that have theta compensation. I was able to do this because those lenses have a different tab that couples to the AE/MM switch on the body. The AE/MM switch on the body is usually referred to as the 'theta switch' in the service manuals and it has three positions. The first, normal, position indicates an AE lens is fitted, the third position indicates an MM lens is fitted, but there is a second, middle, position that indicates an MM lens with theta compensation is fitted. So lenses that have theta compensation have a narrow tab that only moves the switch to the middle position.

Here's a couple of pictures of two of my lenses that show the difference. I've outlined the tag on the lens in red and the switch lever in green so their shapes and positions are more clear. The first is a 85mm f/2.8 MM lens. Note the tag is pushing the switch all the way to the end of its travel.

The second is a 50mm f/1.7 MM lens. Note the tag on the lens is notched and it only pushes the switch half way.

Besides the 50mm f/1.7 MM, I also found the 18mm f/4 MM had the same shaped tag so that is the second lens I have that has the feature.

To try and ascertain if there were any differences between lenses with and without theta compensation, I set up a test rig using the mirror box from a 139. The mirror box has the lens mount attached and the various couplings to the lens and has a potentiometer (variable resistor) directly coupled to the aperture stop down ring of the lens mount. By measuring the resistance of the potentiometer with various lenses attached and at all apertures, I was able to plot graphs of resistance against the number of stops the lens was stopped down. Here's a picture of the rig and the results.

The X axis is the number of stops the lens is stopped down. The Y axis is the measured resistance.

What is clear from this is there are two distinct groups of results with one group shifted by half a stop from the other. Two of the lenses in the group shifted to the right are the 50mm f/1.7 MM and the 18mm f/4 MM which had previously been identified as having theta compensation. Surprisingly, the third lens in this group is a 50mm f/1.7 AE lens. This is surprising as the AE lenses were designed and manufactured before MM cameras were brought out and before there was any method of detecting that such a lens had been fitted to the body. As a check that this wasn't just a fault of the individual lens, I tested a second 50mm f/1.7 AE lens and got identical results.

The second test I did was to check the position of the maximum aperture lever for each lens. In the 139 the lever is coupled mechanically to the aperture display so there's nothing that could be measured other than physical position but it was clear that the lenses that exhibited the half stop shift in the aperture stop down lever, also had a half stop shift in the position of the maximum aperture lever.

Here's the inside view of the maximum aperture lever (arrowed). You can see the black marks on the chassis where I've recorded the position for different lenses.

My first thought was that these two half stop shifts would cancel each other out so there would be no effect on the metering but it's not that simple. In the 139, for instance, the maximum aperture lever plays no part in the metering, it is only used to display the correct aperture in the viewfinder. The metering is done purely from the potentiometer coupled to the aperture stop down ring. So, not unsurprisingly, lenses that have theta compensation under expose by half a stop when used on a 139. This is easily confirmed by metering a scene with a lens with theta compensation then again with a lens that doesn't. The half stop difference in the meter reading is quite obvious.

With other MM bodies I thought the exposure error might not happen but I tested an RX, RXII and a 167 and it does happen and in all metering modes.

Conclusions

In the 167MT service manual there are some tables that show the aperture associated with each position of the aperture stop down lever and the maximum aperture lever. The tables have two different values, with a half stop difference, for lenses with and without theta compensation. This effectively confirms my findings that the physical position of these levers differ on lenses with theta compensation. Or, put another way, any particular position represents a different value on lenses with theta compensation to ones that don't.

You would think, therefore, that the MM bodies would carry out a different exposure calculation depending on the position of the AE/MM switch. I have tried manually changing the position of the switch but it makes no difference to the meter reading. Either my expectations are inaccurate, or the bodies don't have the theta compensation feature fully implemented.

Clearly I'm not able to test every combination of lens and body in every metering mode - I don't have examples or the time - and it may be that the half stop change in metering only affects a few lenses, but the phenomenon definitely exists. Whether it's a problem is for individuals to decide. Does half a stop error in metering matter? In the days when I used to use transparency film, I would have said it does.

If anyone has anything to add to this I would be very interested to hear from them. If you have seen the issue with particular lenses then which ones and with which bodies and in what metering mode? Also, if you find any lenses that display the wrong aperture in the viewfinder when other lenses are OK, I would be interested in the same information. And, finally, if anyone has any idea what theta compensation is (specifically related to the lenses under discussion), please let me know.

(Edit)

There was one other test I did which I failed to mention above, probably because it produced no useful information at the time. However, on reflection, the fact it showed no difference between lenses with theta compensation and those without is relevant. The test was to attach the lenses to my A7, digital camera, and check for differences in meter readings. The A7 has no connections to the lens and simply measures the light passing through it. The result was there is no difference in the light transmitted between lenses with, or without, theta compensation. This is significant because, if the lenses with theta compensation did transmit a half stop more light, the apparent half stop under exposure would be correct and make all the above rather inane.

(Edit 2)

I recently discovered the 50/1.7 MM lens, when attached to an RTS body, will cause the aperture readout in the viewfinder to be half a stop out. So, for example, with the lens set to f/5.6 the aperture display will show the aperture as between f/4 and f/5.6.