Pradovit slide viewing question

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Ive got a Padovit 150 with colorplan P2 lens and have found that only a certain part of the slide is in focus at any one time and have to shift focus constantly to get other areas in sharp focus. Is this because the slide is not totally flat in the mount, or is there something wrong with the projector?

-- Karl Yik (karl.yik@dk.com), April 16, 2002

Answers

Is this because the slide is not totally flat

Yup! When you project the slide it bulges due to the heat of the light source - After a few seconds you can see it plop and then you get the focus problem... There are special projector lenses that resolve this issue but then the glass mounted slides are not sharp since they are flat...

best regards,

-- Boris BRECELJ (bbrecelj@mac.com), April 16, 2002.


Karl

This question comes up regularly. Look into the "Projector" section of this forum. The answer is that it should be in glass to get perfection, but also that the the Pradovit 150 is not that good a projector in my opinion.

-- Robin Smith (smith_robin@hotmail.com), April 16, 2002.


I think what you have going on is an incompatibility issue between the particular kind of slide and film emulsion, verses the use of an appropriate curved field or flat field projection lens. I don't know what kind of Leica lens (curved or flat fiield) that your P150 has, but I have heard that in recent years, the major film makers changed their technology with their slide films, so that when mounted and projected, they were a lot more flat than was available with earlier slide films. For this reason, when I project my newer slides, I need a flat field lens.

-- Steve Brantley (sbrantley@nccommerce.com), April 16, 2002.

Wow you get some real science fiction on the internet. Film emulsions have changed a lot but the film stock still curves and it still pops from heat. There is only one way to get corner-to-corner sharpness with a projected transparency: anti-newton glass mounts+flat-field lens+flat screen+projector postitioned so film plane is parallel to screen. A curved-field lens may or may not have the same curvature as the film. And if you project dupes, the curvature is in the opposite direction and a CF lens only exacerbates the problem.

-- Jay (infinitydt@aol.com), April 16, 2002.

I'll have to agree with Jay here. I see no difference between modern and older films when it comes to slide popping. The only advance I see is in the use of the modern 300W. projector bulbs with dichroic reflectors, that deliver less heat to the gate than the 500W. bulb on my old Carousel 850. Even so, the slides still pop anyway.

One idea for use with cardboard mounted slides would be to build a curved screen. But the curvature would be opposite to that used in theaters: the edges would have to curve away from the audience and not in to the audience. That would be just too weird to suit me.

-- Bob Fleischman (RFXMAIL@prodigy.net), April 16, 2002.



So what is the difference between a slide mounted in Plastic as opposed to a slide mounted in cardboard?

-- karl yik (karl.yik@dk.com), April 17, 2002.

The Leica CF lenses were originally designed around the typical curvature of Kodachromes in carboard mounts, or so I believe. In the sense that fewer people are using Kodachrome nowadays it perhaps means that CF lenses are even less useful than they once were (if we assume that K and E6 films curve differently (they might...)). In my opinion they were no good anyway as they gave less sharp edge to edge images than the flat field lenses. Of course, a simple curved screen does not really help as the slide assumes a spherical section so the screen would have to be similarly shaped which is even more daunting.

-- Robin Smith (smith_robin@hotmail.com), April 17, 2002.

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