What does this lever do??

[redshift]

I have several old lenses that have a rotating lever in a slot next to the aperture scale. It's not obvious they have any affect when turned. Some have described them as Waterhouse stops. It doesn't appear they are removable and the lens has its own iris so that isn't likely. It seems to rotate about the same axis as the f stop ring. I thought it might be a friction lock for the f stop scale but it doesn't seem to have any affect. What is it for??

[Louis Pacilla]

It is a waterhouse stop slot.

In the graphic arts and process camera work there are a bunch of different shaped stops that are used for either or both. There may have been times when both a square stop in unison w/ the round iris stops may have be used? But I'm not 100% sure of that though.

[Sevo]

It is a waterhouse stop slot.

In the graphic arts and process camera work there are a bunch of different shaped stops that are used for either or both. There may have been times when both a square stop in unison w/ the round iris stops may have be used? But I'm not 100% sure of that though.

I don't think so. For colour raster work, square or rhombic apertures were used, and they were rotated to different angles for each plate to minimize moire.

[Louis Pacilla]

I don't think so. For colour raster work, square or rhombic apertures were used, and they were rotated to different angles for each plate to minimize moire.

Thanks for straightening me out Sevo . Exactly the reason I added "But I'm not 100% sure of that though". I guess I was 100% wrong.

So the answer to the OP's question. The slot was used by the process & graphic arts crowd using a variety of different shaped waterhouse stops for various reasons. Sevo gave an excellent example of how and why the square stop was used.

[Steven Tribe]

It is connected to a concealed semi circular hidden ring which can close the waterhouse stop completely - to prevent dust entry to the barrel.
Quite common on the F9 and upwards process lenses.
But could have the extra function described by Sevo as well.

[redshift]

Thanks, now I have a better understanding of the process. I've looked closer at the lenses and now see that the lever I was looking at opens and closes a window into which a Waterhouse stop was inserted. I wasn't seeing that the slot needed to be closed when a stop wasn't in place. Makes a lot of sense now!

[Bob Salomon]

It has a big effect when turned. They open a filter slot. If that slot is open during exposure you can have a problem. Tape them closed so they can't accidently open.

Many images were not continuous tone. They were made up of different shaped and size dots. Larger ones for newspaper reproduction and smaller ones for high end magazine work. The stops inserted into the lens through the slot controlled the size and the shape of the dots.