Focus problems RTS III 167MT

[didzis]

I have two used Contax manual focus bodies –
RTS III
167MT.

I bought both of them used, second-hand. Both cameras have standard split-image, micro-raster screens installed. FV-1 on RTSIII and FU-4 on 167MT.

I have discovered what I believe to be a focusing problem with both of these cameras. I believe that the actual plan of focus is BEHIND the one indicated by the split-image. I discovered it, when shooting portraits – when focusing on the eyes of the subject, the plain of focus worked out somewhere near the ears of the subject. Atter this I performed careful tests (having a camera on a tripod, with careful focusing). I tested both cameras with all of my c/y lens 28 2.8; 50 1,4; 85 2,8 and 135 2,8. All the tests were carried at the widest aperture at the minimum focusing distance. Both cameras displayed focusing error of approximately 1 cm behind the plane of focus as indicated by the split-image. The error identical/similar was with all lenses, so I believe it is not the lens problem. The strange thing though, is that both cameras should display so similar an error.

The reasons for this could be the following

- poor focusing technique by me
- something wrong with the seating of the focusing screen
- camera focusing needs adjustment.

Perhaps such focus shift is a known issue with these cameras? If so, can this be fixed?
Or perhaps it is the seating of the focusing screen - are there any tricks in seating it correctly?

Any comments would be appreciated.

 

[wang]

There is nothing wrong with the camera bodies, the problem is the use of split image in focusing.

Unlike rangefinders, SLRs do not have focal plane to adjust. Everything is standard and it does not make mistake in focusing. On the other hand, rangefinders are labour intensive in focal plane adjustment.

I regularly make mistakes when I am using split images for focusing. It is difficult to focus in situations like portraits at 1m using 85mm when you are using transverse frame. It is easier with vertical frame. This is the reason why the matte screen is often used for macro photography when the use of split images is difficult to get a good result.

The experiment you did clearly points out that the errors made by split images are significant.

I have three of the 85mm, focusing with the 1.2 is the most accurate. My AX has split image, but I use the matte part of the screen for focusing.

This is how I do it but I am sure other people have other ways. I fixed the focusing barrel at 1m because I like the perspective at this distance. I looked at both eyes of my subject and keep them at the matte part of the screen. I focus by moving the camera to and fro until I find both eyes are in focus. Before I press I have to frame. The use of complete matte screen will make focusing and framing into one step. This way of focusing will work for oblique views as well. For oblique views, split image focusing at one point is difficult to get a good result.

 

[didzis]

Thanks , Wang, for your answer!

What you are saying is something similar to issues discussed in this thread about the ease of focusing of 85 1.4
http://www.contaxinfo.com/cgi-bin/discus/board-auth.cgi?file=/14022/137066.html
Look at the third comment from the top by Edmond Yiu. He says that one cannot trust split-image or microprism at close and medium distances. Some other users on that thread also report that they get the best results using matte screen. On the other hand some users said that split works fine for them (not specifying though whether they use it at close distances).

I did a quick visual test (without shooting any film) to check whether focusing on the matte screen would produce any difference with split-image or microprism indication. The trouble is that I could not find any discrepancies between these two readings. But again, that was a very quick test - I will have a more thourough test later.

If it works out that matte screen is better for focusing at close to medium distance, I think I will change the focusing screens on both cameras. The trouble with having the split-image is that I tend not to trust my eyesight and fall back onto the split-image reading (which may be faulty) - in this sense it is an unnecessary distraction.

BTW I also understand that split/microprism works better with wide angle lens.

 

[wang]

Matte screen, to me, is the most accurate way of focusing. It is very technique dependent and, sometimes, tiredsome. It allows two point focusing and ,to me, is vital for oblique views of portraits.
Split image focusing is quicker but it is less accurate. It works well with wide angles because accuracy of focusing is less important. Often, wide angle subjects have long lines and is great for split images.