Need help -- Focussing mechanism -- Screw type.

[Shen45]

Does anyone have a link or diagram showing how to construct a focusing system using a length of threaded rod.

Steve

[Brian Stein]

No specific plans but:

look at chamonix / phillips cameras
http://home.online.no/~gjon/tsmith.htm
not for a camera but principle similar http://www.instructables.com/id/Easy-to-Build-Desk-Top-3-Axis-CNC-Milling-Machine/step2/The-X-Stage-Rails-and-Motor/
also http://www.diyphotography.net/the-diy-macro-rail

hope this helps

[erie patsellis]

Depening on the length desired, there are numerous lead screws available on Ebay as well, more precise than a threaded rod.

[Dan Fromm]

If you can find pictures of Rilex or Brand press/view cameras, I think you'll find them enlightening.

[domaz]

New-Vue cameras use that mechanism as well.

[GPS]

Often times I was thinking (and had even cameras built so) that if the front standard is mechanically precise and doesn't wiggle on the camera rails it doesn't need any screw focusing mechanism at all - manual focusing is then precise enough and lighter. Just a thought...

[Jack Dahlgren]

One or two rails running in the focusing direction.
On the rails a carriage which holds the front standard.
The carriage should have sufficient length and rigidity to resist turning - an H or square shape with rollers or slides at the corners will help prevent rotation.
Through the center of the carriage have a captive nut or thread the body of the carriage.
The threaded rod will go through the nut and will need some mechanism to resist the pulling and pushing. This can be done with a thrust bearing at the end, or by putting a shoulder on it. That bearing should be attached to the camera body.

This approach keeps the knob in the same place in relation to the camera. An alternative is to have the thrust bearing in the carriage and thread the mount on the camera body, but then the screw moves in and out as it focuses.

Alternatives would be a scissor type of focus attachment, but that is going to be a bit sloppier.

Threaded rod is plenty precise as long as it fits closely with what it is screwed into. A loose fit will cause lash.

[Shen45]

Thanks everyone for the info. I have plans afoot to build a 5x4 with at least 550mm bellows and the ability to take a 90 mm lens on a very shallow board. The only "mental" hurdle was the mechanics of the focusing arrangement.

I have located a very suitable bellows material [I hope :) ] so in the next week or so I will build a full sized trial bellows as a proof of material.

I suppose my first task should be to build a working screw focus assembly. Or just a very quick rough and ready "prototype" camera. The only problem I see with that is if the prototype works well I know I will never bother to finish it off to a higher standard.

Steve