Topic: Aerial image photography for scanning slides?

[EarlJam]

Prior to CGI for motion picture special effects, aerial image optical printers were state of the art. In brief, you project an image into a point in space and then focus the copying camera on that same point. This eliminates artifacts that would be caused by photographing the image off an intervening screen.

I recall building a rudimentary system with my dad in the late 60s, using a Kodak Carousel  on one side and (I think) a Praktica SLR on the other, primarily as an exercise to better understand the process. If memory serves, we built a screen out of vellum to have a visual reference for where the image was. Once the projector and camera were focused, we removed the screen and made the exposure.

With the release of new Ektachrome, it strikes me that something on this order, utilizing a digital camera, might be a means of scanning mounted slides with generally on-hand equipment. Once I've finished clearing space in the garage, I might try a proof of concept setup and check the accuracy of my recollection.

[Bryan]

I assume the room would have to be completely dark and maybe have a black backdrop behind the projector.  Otherwise wouldn’t you possibly pick up reflections?

[BernardL]

If you would try with the setup as pictured: pure aerial (no ground glass) image at finite distance,  you would find out that you suffer from severe vignetting, i.e. keyhole vision. Try to imagine the path of light for a point in the image (red dashes) near the edge.
One way around is to insert a ground glass.
The other is to have the intermediate image at infinity, and the two lenses (projection+taking) as close as possible; even so, in most cases, you will have some vignetting.

[Pete_R]

If you would try with the setup as pictured: pure aerial (no ground glass) image at finite distance,  you would find out that you suffer from severe vignetting, i.e. keyhole vision. Try to imagine the path of light for a point in the image (red dashes) near the edge.
One way around is to insert a ground glass.
The other is to have the intermediate image at infinity, and the two lenses (projection+taking) as close as possible; even so, in most cases, you will have some vignetting.

+1

As you have shown it, you'll only get the very centre of the image.

Presumably the point is to be able to load the slides into the projector so that you can zip through them pretty quickly rather than loading them individually into a slide copying setup. Another way of achieving that would be to remove the projector lens and, using a macro lens on the camera, focus directly on the slide in the projector gate (maybe some modification to the projector would be required). Alternatively, remove the camera lens and mount a macro lens on the projector and project directly onto the camera sensor.

Edit:
I've never looked at optical printers before but looking at this diagram, I'm not sure they use an aerial image. Seems they use a lens between the original and copy so are projecting the image directly onto the copy emulsion.

[EarlJam]

Thanks, all. I'm running on 50+ year old memories as it is, and it's been more than 40 years since I last encountered an optical printer. Aerial image was quite common in the late stages of film effects, with Oxberry being the most widely used AI printer. Here's a clip from a cinematography manual (link follows) with some additional details.

https://books.google.com/books?id=jaXWAQAAQBAJ&pg=PA110&lpg=PA110&dq=ilm+aerial+image+optical+printer&source=bl&ots=PJPWtUxfHk&sig=Sb59Yq2oTVSgy7zKnVhjte9ZT3I&hl=en&sa=X&ved=2ahUKEwi93IT4yurdAhULO60KHcxcBtw4ChDoATAGegQIAxAB#v=onepage&q&f=false

As Peter notes, it's possible to use a macro lens on the camera and shoot the slide directly, using the projector as a light source and transport. I used such a system in my video production days, basically the still image version of a telecine. There are a couple of designs online that use this combination along with a controller to advance the tray and trigger the camera, thus automating the process. For the moment, though, I'm intrigued with the physics of photographing a point in space, into which an image is projected.

[Pete_R]

OK. Get that. But the difference is the aerial image is the same size as the original, not enlarged as in your first diagram. As projector lenses are designed to enlarge the image on to a screen you would need a different lens to make this work.

[Francois]

A long time ago I was reading stuff on digitizing super 8 movied and I was reading something from a guy who built his own telecine machine. He had used a projector which he had modified by replacing the lamp with a lower powered one, replaced the condenser unit by a ground glass and took one of the condensers and glued it to the front of the projection lens. He said this allowed him to directly film the filmstrip as it was going through the gate.