The camcorder guys figured out long ago how you can get the shallow DOF of a larger sensor system (typically full frame) on a camcorder equipped with a much smaller sensor (typically 1/2.8") by doing basically the following thing.
Mount the larger format lens so that the image gets projected on a piece of ground glass (i.e. where you would normally have film).
Film the ground glass from the other side, possibly using macro mode of the camcorder.
There are many improvements, like shaking the ground glass to reduce grain and I understand there is a whole science about choosing the right kind of ground glass etc.
The obvious advantage of this is that you get the shallow DOF. The disadvantage is that the setup looses light, you get no AF and there is some quality degradation.
There also used to be a similar idea in stills photography in the 1990s with the Nikon E series - it was a quasi full-frame camera which means it used a piece of optics called a telecompressor to use the whole FOV of 35mm lenses on a 2/3" sensor. I dunno what it did to DOF though.
I was wondering if you could use a similar setup today in stills photography. In particular what I have in mind is the paper-thin DOF you get with tele lenses on large-format systems. It is a personal thing, but I like old-fashioned portraits shot this way, as well as the ability to use a full range of camera movements.
I was wondering if you could have a setup along the following lines: A large format camera + a DSLR with a good macro lens used to capture the ground glass. The DSLR could even be APS-C, it doesn't really matter as long as you can take sharp pictures of the ground glass.
Maybe there is a way of doing this without the ground glass, i.e. with optical lenses alone?
Has anybody tried something like this (maybe with a middle format lens if large format is too cumbersome)?
The "telecompressor" you mentioned is a focal reducer, a device that concentrates the image in order to project it onto a smaller sensor.
This approach reduces the backfocus distance (the distance from the back of the lens to the sensor). So it only works on mirrorless cameras using lenses designed for DSLRs (which have sufficient backfocus distance to make room for the adaptor).
They do exist though, Metabones were first to market with this type of adapter for mirrorless cameras (though they've existed for telescopes for years). The focal reducer maintains the depth of field you get with the larger system, i.e. you can get the "full frame" DOF and bokeh using an APS-C sensor.
It would be possible to design a focal reducer, or ground glass based approach like the one you describe for larger formats. No-one to my knowledge has bothered trying to do this, however. The reason for this is that contrary to popular opinion medium and large format systems don't really offer shallower depth of field compared to 35mm.
Technically a larger format gives shallower depth of field for the same f-stop, but that's exactly where the problem lies, you can't get lenses with the same f-stop for larger formats.
Medium format lenses are typically f/2.8 or slower, large format lenses tend to be for like f/5.6 or f/8. When you work out the depth of field equivalences they end up about equal, or more commonly worse than their 35mm format equivalents.
Sure there are a few exceptions, such as the large format 150mm f/2.8 Schneider Xenotar (which you can't afford), or the 178mm f/2.5 Kodak Aero Ektar (which is radioactive) but on the whole there would be very little to be gained by the time you've taken the losses of the adapter / ground glass into account. Lenses on the speedbooster show quite minimal loss of sharpness, however that's only a 0.71x, for large format you'd need a 0.28x!)
Large format cameras are basically already half-built DOF adapters. Just throw a macro lens on your smaller digital camera to photograph the focusing screen. With large format cameras, you will also have more room to add modifications, such as motors to move the ground glass to avoid capturing its texture.
Plenty of DIY DOF adapters can be found online. You may need to move or substitute components because of the larger size. For instance, the Fresnel Lens may make the plano-convex lens unnecessary. Here is the general schematic to follow.