Zeiss Flektogon 2.8 m42 Need Help!

[sandymandy]

Hi,

recently i got a Zeiss Flektogon 2.8 "Zebra" version, m42 Mount /w adapter on my eos 1100D.

http://farm7.staticflickr.com/6188/6071754657_6531f96493_z.jpg

Im really happy with this lense, i got it for cheap and on my APS-C the field of view is quite standard.
I just got one problem and if i set the aperture to 2.8 and twist the focus ring out to the maximum so i can get really close to subjects. The aperture ring starts to shift a little. I wonder why its like that?
I first checked if its perhaps just loose but its not and when i set it to minimum focussing distance there is no way of moving the aperture ring back to 2.8.
I also tried holding the aperture ring in place and then just focussing to minimum distance but that doesnt work. The Focus ring will just block.

Ok i just checked again and it seems the aperture ring moves a little bit when im focussing. Any idea why?


*edit*

I found this online:

Let's start with the f-stop ring set at 2.8 and the focus ring set at infinity. Now I begin to turn the focus ring clockwise, getting closer to the shortest distance of 0.18m. As the focus ring passes the 0.3m mark, it begins to turn the f-stop ring counterclockwise. Ever so slightly. By the time the focus ring is at 0.18m, the f-stop ring has turned to position f/4.
What's more strange is that the aperture blades are still in the fully o

and the reply:

The lens is really designed to be used with a hand-held exposure meter; probably it was built in the days before through-the-lens metering was available (certainly the case for Exakta). As you move into to close focus, you need to open up the aperture some more to keep the same effective f-stop, so for example an f/2.8 aperture setting on a close focus might only give as much illumination as f/4 would at infinity. If you had a through the lens meter, it would see the reduced illumination, but a hand-held meter simply does not know the distance to the subject or the focal length of the lens, so it can't compenstate. The Flektogon had a built-in mechanical compensation as I remember it. As you focus to shorter distances, it opens the aperture up. Of course, if you start off at f/2.8, it has no where to go, so instead it rotates the aperture ring to at least tell you what its effective aperture really is.

Experiences? thanks already.

 

[sandymandy]

double post