Contax 50mm f/1.7 AE/MM differences

I've worked on both AE and MM versions of this lens before but have never done a direct comparison of the two. Recently I had reason to work on one of each at the same time so was able to do some direct comparisons of their construction. I thought it was worth recording what I found.

The two samples I compared had serial numbers 6504056 (AE) and 7380543 (MM). The differences noted may not be the same for all AE or all MM lenses.

Plastic/metal

The lens uses metal components for the focusing helical, plastic for the internal block that includes the aperture mechanism and carries the lens elements and a mixture of metal and plastic parts for the focus and aperture controls and front cover and dress ring. This is mostly the same in both lens versions except the focus ring on the MM version is metal but plastic on the AE version. The AE version I had showed why this change was probably made - it had cracks in the plastic around the screws that attach the ring to the focusing mechanism. This had caused the focusing ring to become loose and allowed the focusing helical to separate - which is how I came to have the lens. I have seen some cracks like this before so is clearly a potential issue.

Focus guides

Lenses of this general construction usually have one or more guides that allow the focus helical to move back and forth but stopping it from rotating. If these guides, or the slots they move in, become worn, slop or backlash can appear in the focus mechanism. Many lenses have one of the guides split so that its width can be adjusted to take up any excess. The AE version didn't have a split guide while the MM version did.

The slot in the helical component was also different in cross section which means the guide only contacts the helical at the edges in the MM version. What difference this would make I don't know.

Lens mount

The construction of the lens mount is completely different on these two lenses but I think the mount changed before the MM lenses were introduced so this isn't a AE/MM difference as such. The mount, of course, works in exactly the same way but the parts and their assembly is different.

I guess the question has to be, if the differences are so small, why is one lens suited to MM mode cameras and the other not. There may be other differences between the aperture mechanisms that are not really visible. The small constructional differences I could see between the two assemblies were not worth mentioning but maybe they do have some significance. The basic operation of both look identical.

Edit: I have, of course, passed over the infamous 'ninja star' aperture shape of the AE lenses. This is minimal on the 50/1.7 but it is present and shows the shape of the aperture leaves on the two lenses are different.

Zeiss -v- Yashica ML lenses - 200mm f/4

I've had the opportunity to do a side by side comparison of a couple of Zeiss and Yashica lenses. It's not the sort of thing I normally bother about but one of the lenses is the Yashica 200 f/4 ML C (compact) which has an excellent reputation and I was interested to see how it compared to its Zeiss equivalent. The other comparison is between the Yashica 50mm f/1.7 ML and its Zeiss equivalent. That will appear in another post (eventually). Sorry, won't be happening as I've sold the Yashica lens.

Zeiss                                     Yashica

The two lenses are very similar in size and weight. Here's the specifications for both:

	Elements/Groups	Angular field	Min. Focus	Aperture range	Size (mm)	Weight
Zeiss	6/5	12° 40’	1.5m	f/4 – f/32	66.5 x 122	550g
Yashica	5/4	12° 30’	2.5m	f/4 – f/22	64 x 113.5	535g

The Zeiss has a filter size of 55mm, the Yashica 58mm.

This is not a scientific comparison, just a few shots on each lens compared against each other. I attached the lenses to my Sony A7II and photographed the roof of a nearby house putting the corner of the roof alternately in the middle of the frame and in the corner of the frame. I took photographs at f/4, f/11 and f/22.

I also took photographs of some out of focus highlights and also of a blank wall to check for vignetting and another wall to check for distortion.

The last photographs of the wall (not shown here) showed both lenses have slight pincushion distortion with nothing to choose between them. What they also showed was the focal length of the  Yashica is about 95% of the Zeiss. This is contrary to what the specification suggests. I'm not able to measure the exact focal length but just did a comparison of the two.

Vignetting at f/4 was very similar with both lenses.

Yashica

Zeiss

The images of the out of focus highlights are similar.

Zeiss

Yashica

Here's the rest of the images to compare for sharpness and contrast. All the images are 100% crops.

Yashica centre f/4

Zeiss centre f/4

Yashica centre f/11

Zeiss centre f/11

Yashica centre f/22

Zeiss centre f/22

Yashica corner f/4

Zeiss corner f/4

Yashica corner f/11

Zeiss corner f/11

Yashica corner f/22

Zeiss corner f/22

Both lenses are showing chromatic aberration when used on my Sony A7II. More so towards the edge of the images as is usual. I don't think there's any significant difference between the two lenses though.

One area of difference between the lenses is their compatibility with converters. Zeiss made x1.4 (Mutar III) and x2 (Mutar II) converters specially matched to some of their lenses including the 200/4. These converters can't be used with the Yashica as they are mechanically incompatible. But other converters can be used including the Zeiss Mutar I x2 converter. I took some more photographs using the Mutar I and Mutar II converters. The images are 100% crops.

Yashica f/4 Mutar I

Zeiss f/4 Mutar II

Yashica f/8 Mutar I

Zeiss f/8 Mutar II

My conclusion is that there is negligible difference between the two lenses. Yes, you could say one is better than the other in some instances but it's not always the case that the Zeiss is better than the Yashica which is what might be expected. In normal use, I would be happy to use either based on these results.

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.

The Contax/Yashica Bayonet

This might seem a particularly boring subject to write a blog post about but it's intended to be the first of a few posts regarding the information that get's passed to the camera body from the lens. This has been prompted by some questions I had and which I'm in the process of doing some investigation of. Also, the suspected incompatibility between some lenses and camera bodies suggested by some forum posts but yet to be confirmed.

Rather than try and cover everything in one long post I thought I would split it up and start here with a general explanation of the linkage between lens and camera.

Besides the bayonet itself, there are, up to, four points where the lens and body couple together. Earlier AE lenses had three then a fourth was added to MM lenses to tell the camera a MM lens is attached. The diagrams show the points of contact on the body and the lens with a description of each below.

 

Aperture Linkage Pin/Lever (Aperture stop down lever). The pin on the lens rotates as the lens aperture is changed and couples to a lug (described as a lever in the diagram) on a ring that rotates around the camera lens mount and which, in turn, couples to a device that converts the position of the ring into an electrical signal. On earlier cameras this was done using a potentiometer (variable resistor) which was part of the metering circuit. On later cameras, the potentiometer was replaced by an encoder that outputs a digital code.

The value passed to the camera represents the number of stops that the lens has been stopped down from maximum. It doesn't represent an actual aperture value.

Aperture Scale Coupling Pin/Lever (Maximum aperture lever). The 'pin' on the lens may actually be a lever or a shaped lug depending on the lens. It is in a fixed position that represents the maximum aperture of the lens. It couples with a lever on the body to transfer this information to the camera so that it can display the actual aperture in use. On earlier cameras a mechanical system, that linked the maximum aperture information and the number of stops the lens has been stopped down, was used to display the aperture in use. On later cameras with electronic displays it is done electronically.

The Contax S2/S2b and some Yashica cameras that don't have an aperture display in the viewfinder, don't use this feature.

Automatic Diaphragm Action Pin/Lever. This lever is driven from the camera and causes the lens to stop down to the selected aperture. On later MM cameras when used in shutter priority or program modes, this lever will move just the correct amount to stop the lens down to the required aperture.

AE/MM switch (not shown on diagram). On MM lenses, an extra lug was added which extends just beyond the bayonet mount and engages with the switch on the camera. The switch is a three position switch. The first, default, position is for AE lenses (AE lenses don't have the extra lug so the switch doesn't move). The third position is for MM lenses but there is also a mid position which I intend to be the subject of one of my following posts.