Spinning/vibrating DoF adapters

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enricosavazzi
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Spinning/vibrating DoF adapters

Post by enricosavazzi »

It seems these things are used to artificially decrease the DoF, i.e. produce images equivalent to those made with very fast lenses. They seem to be used mostly for movies, but probably could be used in photography as well.

I found the following by accident:
http://www.modo35.com/products.html

and this short explanation:
http://ezinearticles.com/?35mm-DOF-Adap ... id=1270610

As far as I understand, a lens projects an image on a ground glass screen, and a second lens picks up this image and relays it to the sensor. So this is a compound optical system, but the intermediate image is projected on a screen instead of being aerial. I am not sure I understand how this affects the DoF of the first lens, though. Something is missing in the explanation.

An interesting idea is to have the ground glass spin or vibrate (in direction(s) perpendicular to the lens optical axis) to effectively erase any crud sitting on the screen (but large particles are going to produce streaks, so it is not foolproof). It might have applications in photomacrographic equipment, but I don't have an immediate use for it.
--ES

SONYNUT
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Post by SONYNUT »

these were used to be able to use regular slr lenses on video cameras that have to much dof...

this is why video dslr's have become popular..no more need for these toys...

no-one really liked them but it was a way around the system for the indie filmmakers


as for using them in photography...there is no point...just use a lens with low f #
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rjlittlefield
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Re: Spinning/vibrating DoF adapters

Post by rjlittlefield »

enricosavazzi wrote:As far as I understand, a lens projects an image on a ground glass screen, and a second lens picks up this image and relays it to the sensor. So this is a compound optical system, but the intermediate image is projected on a screen instead of being aerial.
Correct.
I am not sure I understand how this affects the DoF of the first lens, though.
It doesn't, and that's the point.

The intermediate screen essentially becomes the "sensor" for the first lens, and as we all know by now, using a large sensor permits using a long lens that also has a large diameter entrance pupil, producing shallow DOF. The shallow-DOF planar image on the intermediate screen is then copied onto the much smaller sensor of the video camera, using a short lens as a relay with fractional magnification.

If the same field of view were imaged directly onto the small sensor of the video camera, then the effective entrance pupil would be that of the short lens, with its necessarily smaller diameter producing greater DOF.

It's interesting to note that the compound system manages to combine the shallow DOF of the large lens with the lesser resolution of the small lens. From our usual standpoint as macro photographers, it is giving the worst of both worlds. But of course shallow DOF can be quite valuable as an artistic tool, and this is why the devices exist.

It is also interesting to ponder why the intermediate screen is required at all. At first blush, it seems like one should be able to stick in some sort of field lens that would allow the shallow-DOF intermediate image to be redirected through the short lens to the small sensor. I wrestled with this problem for a long time before I finally figured out the sticking point. It's the aperture --- again! If you remove the intermediate diffusing screen, then the aperture of the short lens ends up blocking all light rays except the ones corresponding to a small entrance pupil, and those rays produce an image with large DOF. With the diffusing screen in place, the aperture of the short lens ends up blocking a large fraction of all the rays, but in equal proportion, so that the resulting image is relatively dim but retains the shallow DOF implied by the entrance pupil of the large lens.

--Rik

SONYNUT
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Post by SONYNUT »

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Just shoot it......

enricosavazzi
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Post by enricosavazzi »

Of course, I had forgotten to check the Encyclopaedia Galactica (a.k.a. Wikipedia). :D
--ES

SONYNUT
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Post by SONYNUT »

lots of people used them for a time in my business.(movie/tv).but once dslr came out with video..they all ended up on ebay now..lol
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