Anomalous aperture behavior in some tube lenses

[Lou Jost]

Regular readers of this forum know not to stop down the tube lens that has an objective or reversed lens mounted on it. Rik has several posts explaining how the aperture doesn't behave as expected; it does nothing when stopped down slightly (not even changing the exposure) and it vignettes the image when stopped way down. I've seen this behavior in all of the 35mm camera lenses that I use as tube lenses.

But not all tube lens apertures behave this way. The aperture on my 210 mm Sironar-S view camera lens behaves just like a normal aperture when an objective or reversed lens is mounted on it. Stopping down improves the CA and, beyond f/8, darkens the image, and it never vignettes, even at f/32.

I recall somewhere that Charles Krebs also encountered a lens whose aperture behaved this way when used as a tube lens.

I have no idea why this would happen. It is a very desirable behavior in a tube lens. Does anyone know why some lenses would behave like this?

[rjlittlefield]

It has to do with the position of the aperture. If the tube lens aperture is far enough forward, and the objective is mounted near enough to it, then the tube lens aperture can become the limiting aperture across the entire field. When this occurs, then it acts like the iris in lenses used the ordinary way. It's basically the same as placing an iris just behind the objective, and using that to be the limiting aperture to stop the whole system down.

--Rik

[Lou Jost]

Thanks Rik!