Variable Aperture Slide Projector Lens

I have read that a variable aperture slide projector lens can help alleviate

the focusing problems caused by "popped", curved slides to some extent. The

idea is that by stopping the lens down you increase the depth of field thus

making the image more in focus from edge to edge. Does anyone know if this is

true? It sounds good on paper but has anyone had first hand experience with it?

You darken the image, obviously. And your eye can't compensate with a slower shutter speed.

I haven't seen a projection lens with a diaphragm, but a little shopping will turn up quality lenses of differing speeds. For example, I have one Schneider Vario-Prolux 70-120mm that is f/2.8; and another that is f/3.5.

 

Another idea might be to insert a fixed stop (Waterhouse stop) into the lens. It could be thin metal or maybe cut from a thin fibreboard card. Don't make it too small, or you will have poor screen brightness. F/4 or 4.5 might be practical limit, depending on screen size and reflectivity.

Well it probably works, but I personally would prefer the bright image and you can alter the focus if you want to see each section of the screen in pin sharp focus. I do remember being impressed with an old Leica projector from the 1960s with a 300w tungsten bulb and an f3.4 lens - very sharp, but rather dim too. It should be said that the better the slide gate warming, uneven focus becomes less of an issue (although it is always there to some extent unless you use glass slides).

Glass slides solved the problem for me. "Leica Dia" and "Pro Loc" had a very good system, but I don't know if they are still available.

Leica made a projector lens specifically for cardboard mounted slides, with a less than flat field, to somewhat accommodate the curvature of "popped" slides. They also made a similar lens for glass mounted slides which has a flat field.

Kodak also made curved-field lenses, indeed they were standard in the 1980's. These days most slides are mounted much flatter than they were then, and flat-field lenses are more appropriate.

 

With a Carousel projector, you get less popping if you use a tray, instead of the stack loader, as the projector "pre-heats" the slides.

 

Or, use a projector with autofocus, and live with it hunting, and fighting you when you try manual focus. (I don't like autofocus projectors, but you might.)

Just from my own experience, I find that slides 'pop' to differing extents, depending on the type of mounts, temperature at the gate, how long the slide is projected, the slide's humidity and the type/make of film, etc. Hence, I can't see that a variable aperture projector lens will be much practical use compared to, say, reducing the temperature at the gate by using a cooler bulb (which will potentially make focussing trickier) and so forth. Warm, 'dry' slides are part of the key to reducing popping as well as a good well-cooled projector. I find in practice that many adverse factors can be minimised, but few eliminated completely - most slides will shift focus during projection at some point; some by a large margin, often beyond the capability of many autofocus systems to handle. Also, keeping the projector's axis 'precisely' at right-angles to the screen can also help in mitigating out of focus effects of a popped slide (as well as the dreaded keystoning). AC

One of the reasons that the Leica projectors were so sharp was that the illuminating system concentrated most of the light at the center of the projection lens. This effectively gave it a "stopped down" characteristic with very little light loss.

In the past, a similar lens was made by ISCO for the Reflecta trade mark, but i think that is discontinued.

Also the special Leica Super Colorplan 90 mm f/ 2,5 flat field was discontinued. See the Braun projection lens range: maybe is still avalilable the 85 mm f/ 2,8 B-MC.

Ciao.

Navitar did make a 4" variable aperture projection lens. (I think it was f 2.8 to 16.) However it was primarily used to adjust light levels to match different projectors in a multi projector presentations. It does increase DOF, but in order to make much of a difference you need to make the image pretty dark.

 

If sharp projection is very important, the best thing to do is to mount the slides in a glass mount (Gepe is probably the most available mount). However, if you do a lot of projections, I strongly suggest duplicating the original slides and projecting the dupes.

Read through out this old thread :http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=000nFn

"Or, use a projector with autofocus, and live with it hunting, and fighting you when you try manual focus. (I don't like autofocus projectors, but you might.)"

 

Aha, this does not happen with the Leica "pro" projectors. It is one of their good points. The AF focusses the slide once when the slide appears in the gate and then switches off. You then touch up or alter the focus manually without fighting the AF.