DOF knob

DOF knob

Hi in the warmth of the summer!

I try to find out how to make DOF knobs for my cameras, an Arca Swiss 6x9, 1982 model, and a Century Graphic 6x9.
I have looked at Hayashi's DOF knob

http://www.largeformatphotography.info/dofknob/

and wonder about the reference point. Hayashi uses a mark on the tension control knob. But when I loosen this knob on my Arca, it moves as I turn the focussing knob. Is there something I have overlooked or misunderstood? Or is this knob just different on more recent Arcas? To work as the "carrier" for the reference point, it would have to snap into a well-defined position when loosened, or?

Another option I have thought of, would be using a turn counting dial mounted onto or instead of he focussing knob. It would imply a counting dial that could be zeroed, preferably by the push of a button, as a second choice by turning back to some sort of indent or mechanically defined stop. Would anybody know of a source for such a knob?
I have browsed the hardware places listed on this site

http://www.largeformatphotography.info/lfforum/topic/498619.html

as well as some others in Europe, but without success. At least, I can not determine from the specs if they have a way of zeroing.
From

http://www.hinkel-elektronik.de/shop/1399.html

I bought their model 15-1-11 by mail-order, believing, that the little black thing that can be seen at the bottom of the picture, was the desired zeroing button; but it is just a lever to adjust friction.

I feel that for 6x9, just a millimeter scale is not precise enough. It would have to be to 0.1 mm.

Good light! - Hening.

I use a modified digital caliper; $16 from harbour frieght. I cut off the ends and have a simple digital axis read out that can be zeroed anywhere. People may find this over kill but, I find it fast and presice. Reads in metric or english. Could be perminatley mounted? I use it like a depth guage from the front end of my Shen-Hao to the optical bench. I used a similar setup on a graflex view II, mounted on the side, but removable.
Regards,

I believe what you are refering to is called a travel dail. They are a bit bulky and expensive. They can be zeroed by moving the face of the dail, which is essentially similiar to a dail indicator.

Thank you for your answer, Dan. - Would you care to post the URL of Harbour Freight? My googling does not bring up anything that has to do with tools. - I have tried with a hand-held caliper, but would prefer something that is fixed on the camera.

Kind regards - Hening.

http://www.harborfreight.com/cpi/ctaf/Category.taf?categoryid=551&pricetype=S&categoryname=MEASURING%20EQUIPMENT

If you want this attatched to the camera you will have to make a bracket to hold it. I cut off the jaws with an abrasive grinding wheel. You will then have to make mounting brackets and a bracket to attatch to the back of the readout to the camera movement. It will be helpfull if you know someone with a milling machine or access to a machine shop. Every camera presents it's own mounting problems. It worked well on a grahic view II mounted as I said. Now I use it as a depth guage and it is just as fast.
It works well and fast for me.
Good luck,