barrel lens, need basic help

[Geraldine Powell]

I just got my first barrel lens. It is a Goerz apo red dot artar 8 1/4 ins. I am planning to use it in a speed graphic but I don't understand how it works. The diaphragm does not close completely. Is that normal? The aperture ring seems to work properly, but above that is another ring that turns, revealing a thin slot above the aperture numbers. What is that for? It does not seem to do anything.
I tried looking up the answer but it seems too basic a question for anyone to have had a discussion about it.
Geraldine

[Jim Jones]

The slot above the aperture numbers is probably for a waterhouse stop with an opening in a shape different than a perfect circle. This was sometimes useful in making halftone negatives. It may even facilitate some pictoral negatives with strange bokeh. Otherwise, keep the slot covered. As for the diaphragm not completely closing, they never do except in a few rare shutters where the diaphragm also acts as a shutter. Barrel lenses are usually used with an external shutter.

[Geraldine Powell]

Thank-you! That was simple. I was afraid there was something wrong with the lens.

[brianjnelson]

The slot is for filters. This is a process camera lens and the slot puts the filters between the lenses for color separations.

[Steve Hamley]

The diaphragm is probably fine. I checked 4 barrel lenses and none closed completely. But it does need to close until the aperture ring reaches the smallest marked aperture! If it doesn't, something is not right.

Cheers, Steve