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Thread: Preserving original focal length by capturing the image projected on a ground glass

  1. #1
    Abbazz
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    Preserving original focal length by capturing the image projected on a ground glass

    Modern digital video cameras -- like most point and shoot digital still cameras -- use very small sensors, therefore their lenses have very short focal length and extended depth of field. Users of these video cameras wanting to shoot with a reduced depth of field can use lenses designed for 35mm full frame SLRs, with the help of a specific device which projects the image delivered by the 35mm lens a ground glass disc. The video camera then records the image projected on the ground glass, which retains the original specifications (depth of field, angle of view) of the 35mm lens. It means that a 50mm F/1.2 lens still acts as a normal lens with an angle of view of 40 horizontally and a super shallow depth of field when used wide open. In order for the grain of the ground glass not to appear on the final image, the disc is rotated at high speed during filming.

    There are some commercial devices based on this principle, like the M2 made by redrockmicro, but they are specifically designed for video and very expensive. I was wondering whether it would be possible to build a similar device in order to retain the original focal length of the lenses when used on 4/3rds cameras. It seems to me that it would be feasible to devise a contraption based on a disc made of fine grain ground glass and rotated by a high speed motor. Of course, the maximum shutter speed on the 4/3rds camera would be limited, due to the necessity of blurring the grain of the ground glass.

    In fact, I have found this webpage, which devises the construction of such a contraption for use with a movie camera. It seems quite feasible. Do you think it could be worth the effort to build it if I want to use it with my E-P1? In a world, what kind of image quality to expect from such a device?

    Cheers!

    Abbazz

  2. #2
    Senior Member kds315's Avatar
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    Re: Preserving original focal length by capturing the image projected on a ground gla

    These devices are commercially available, and not too long ago I have seen one being sold on ebay. It even has a vibration device built in to make invisible the effect of the surface of the ground glass. I'll try to locate that for you.

    Cheers, Klaus

  3. #3
    Abbazz
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    Re: Preserving original focal length by capturing the image projected on a ground gla

    Quote Originally Posted by kds315 View Post
    These devices are commercially available, and not too long ago I have seen one being sold on ebay. It even has a vibration device built in to make invisible the effect of the surface of the ground glass. I'll try to locate that for you.
    Thank you, Klaus. I was thinking about making one, as it doesn't seem too difficult. No rocket science involved.

    If these devices are available, maybe someone has already tried to use one with a M4/3rds camera? In fact, I am dreaming of using medium format or large format lenses while retaining their original angle of view. How about using a Schneider Xenon 125mm F/2 or a Cooke Speedic 8" F/2.5 as a normal lens on a digital micro 4/3rds?...

    Cheers!

    Abbazz

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