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Old January 10th, 2006, 11:29 AM   #1
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auto focus system for 35mm adaptor systems

This question is for all those who have used either commercial or home made 35mm adaptors for actual film/video production.

I'm wondering how easy/hard you've found pulling focus on 35mm lenses under low light conditions and especially with tracking, dolly or jib shots?

We all know that 35mm lenses offer huge depth of field capability which is great for that film look - but for those who have dared shoot wide open with 35mm lenses and had to pull focus during complex camera moves I'm wondering if there might not be a better way.

I've had the thought and idea of developing an external electronic auto focus system for use with 35mm lenses. It would measure the distance between the lens and subject and convert the distance into a position on a motor which would adjust your lens focus for you.

It could work like the "push to focus" features found on most cameras (my VX, XL1 and XL2 had this) where you only push the button when you want focus.. or alternatively hold it down so the camera continues to focus while moving your position or the subject moves.

I'm wondering if there would really be any practical use for this or not, or what do you guys (and gals) think?
Dennis Hingsberg is offline   Reply With Quote
Old January 10th, 2006, 11:53 AM   #2
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Sounds interesting. What would be cool is to go a step further and implement a sensor device that could measure distance from the lens and to say an actor wearing the sensor, so if they were walking towards the cam, the focus would pull automatically.

But im getting way too complicated now;)
Wayne Kinney is offline   Reply With Quote
Old January 10th, 2006, 12:51 PM   #3
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Why not just gut the "passive" auto-focus system from a cheap SLR and use that?

With passive autofocus, a sensor looks at the GG and moves the focus on the lens until it finds the setting with the most contrast (in focus) You've already got a GG in there so why not take advantage of it? :)

Here is a link describing the passive system better than I can...

I've talked to someone who tried this with their 35mm adapter and said it worked like a charm.
Jesse Rosten is offline   Reply With Quote
Old January 10th, 2006, 01:01 PM   #4
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I prefer "Active" methods for auto-focus because it actually measures distance which can be easily translated into focus ring position on a 35mm lens.

"Passive" methods usually "hunt" for their focus which can be annoying during shooting if it gets "lost" so to speak. The other problem is you need to have enough contrast in the frame for it to work which may not always be the case.

To me the "Active" method seems more precise and something I can easily do at with some electronics. Also if using an "active" method it means you could potentially aim at your subject. It would also work really well in low light, low contrast situations.

Maybe I need to experiment with both methods.
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Old January 10th, 2006, 09:57 PM   #5
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Okay this is probably way off but still here it goes.

If I wanted autofocus, then to me the easiest way to go about it, given three of my lenses are autofocus anyway, would be to twin mount on the same tripod head, the autofocus camera, which I already have, harmonised to the AGUS or whatever.

I would make little light cogbelt pulleys or grooves to fit around the lenses, or maybe cogbelts to fit the gear pitch of the follow focus systems already here, then use identical lenses in the AGUS or whatever, zero set them before eech setup, then fire away, using the SLR camera autofocus to drive its own lenses and the AGUS lenses.

The upside of this is you also get a set of high res still images, very near to POV of your video camera.

There'd be all manner of parallax problems but there, it is doable.

A hack from an existing SLR camera design using the mirrow and viewfinder system to power the AGUS lenses directly would also be doable and likely aid sharp focus by eye, which is one of our issues with these things

-- but not for the simple likes of me who wields hacksaws and scraper blades and finds the high science of soldering fine wires and comprehending the mysterious flow of electrons one of the darkest arts known to man (correction human) kind.
Bob Hart is offline   Reply With Quote
Old January 11th, 2006, 08:08 AM   #6
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I probably wasn't very clear in my original post.

The goal would be to develop a system that would automatically keep focus on a subject while tracking or dollying, or while the subject is moving (real time).

In other words it would be an electronic/automatic rack focus system that uses the measured distance to the subject to determine the position of the focus ring on the 35mm lens barrel all in real-time.
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