Painted point in the middle of enlarger lens

[Pancolart]

This question might sound stupid but i have few lenses most probably for enlarging and they have painted a point in the middle of one glass element inside. Any idea why?

[Sevo]

I know blocked lenses from dark field applications in microphotography, where the direct beam from light source to film has to be suppressed. Similar techniques may have been used for the technically related point light sources in enlarging.

[Scheimpflug]

I've heard of "center filters" being used to darken the center of wide-angle lenses to compensate for the light falloff at the edges, to balance the exposure across the frame. Perhaps this is an internal version of that?

[Pancolart]

Thank you both for reply. I only cannot imagine how this center block wouldn't have bad impact on the image produced. I'll make tests.

[Sevo]

There is no rule that says that apertures must be disc-shaped - donut shape apertures occur in mirror lenses as well.

[visualopsins]

Catadioptric (mirror) Lenses have center blocked. (Oh!, now I see Sevo has already said that ).

Another interesting (to me) lens is the Zone plate aperture shape. I plan to experiment with these.

[greg]

Zone Plate Comment

I do hope you have success with them on a camera. I, unfortunately, listened to my eye doctor/surgeon and had zone plate lens replacements installed during cataract surgery (Restor lens). They claimed I would never need glasses again, etc since the were almost infinitely variable. Well, HALF true, I don't need glasses to function normally, the downside is there is no sharp focus at any distance. They are like a soft focus lens. I have to wear jewelers or reading glasses to actually see a sharp image and the DOF is like an f1.0 lens.

[Pancolart]

Hey Greg that is too bad to hear. You should have had a Vario-Pancolar installed .

[Pancolart]

Vario-Tevidon 18-90mm also has such dot. It has reflective surface towards the front.

[Sevo]

On old TV and cine lenses, in-lens mirrors are not that uncommon as provisions for mirror-out finders - on close inspection you'll see that the dot (indeed the mirror surface) is at 45° angle and semi-transparent, while the lens barrel may have a cover at that side that unscrews to expose a finder socket (but they often did not use a different optical group on budget/CCTV lenses, so the mirror often is there while the finder socket is missing).