I recently received the following e-mail:

Q: “I recently purchased a Pentacon lens marked as: Pentacon 4/300 Made in G.D.R.
F stops are marked 4 thru 22, however it will not stop down  past 11. (note: there is a red dot over the 11)
Do you have any suggestions on correcting this problem?”

A: “Hello

This is a pre-set lens.  It has a facility to enable you to rotate it quickly from fully open down to the aperture that you have chosen, without having to look at the aperture ring.  It appears to be pre-set at f/11 right now, so when you rotate the ring it goes from fully open down to f/11 and then stops.  This is correct.

Hold the lens in your left hand, with your forefinger on the numbered ring (the ring that has 11 on it), to stop it moving.
With your right hand grab the knurled ring that is just in front of it, which is currently locked to the aperture ring.  This is the ring with the red dot on it, and pull it towards the front of the lens.  You can now rotate it to the left and drop it back into f/16 or f/22.  That then becomes the pre-set aperture.

If you want to set a wider aperture, first rotate the ring with the white numbers to the higher aperture (in fact, you rotate both rings together), then hold the numbered ring as described above, push the pre-set ring forward, and rotate it to the right to the wider aperture that you have selected, and again let it engage at your chosen aperture.  There are click-stops for each aperture number and at the half-way point between each number.

All of this is quicker to do than to describe.

The aperture on the 500mm Pentacon lens is set in exactly the same way. 

Earlier versions of these Pentacon lenses were labelled  “Meyer-Optik Orestegor”.  They are operated in the same way.

Best wishes
Mr Pentacon Six”
 

Alternative method

There is another way to achieve the same result:
 

1)  Pull the spring-loaded ring forward and rotate it to the right (viewed from above) so that the red dot on the ring aligns with the red aperture index mark on the lens just behind the aperture ring.
2)  With one hand pull the spring-loaded ring forward and hold it in the forward position, without turning it.
3)  With the other hand, rotate the aperture ring until the required aperture value is opposite the index mark and the red dot that has been aligned with it in (1).
4)  Release the spring-loaded ring, and it will lock in place at the selected aperture setting.

Now you can raise the camera to your eye - or put your eye to the camera viewfinder, if the lens is mounted on a tripod - and swing the aperture ring to the left (viewed from above) to obtain maximum aperture for focussing and composition.
Then swing the aperture ring to the right.  It will stop at the correct pre-set aperture, so you do not need to take your eye from the camera viewfinder.
As soon as it is there, you can fire the shutter, confident that the lens is at the aperture that you had previously chosen.