Is there a formula

[Kevin J. Kolosky]

I own and use both a Sinar P2 and a Zone VI camera. One of the features that I really like on the P2 (actually I like them all) is the focus adjustment feature that is used after setting swings and tilts. For those that don't know how it works, you focus on the farthest object you want in focus. You then set the rear fine focus knob to zero and then use that fine focus knob to focus on the closest object you want in focus. You then set the indicated F stop on the lens, and then move the focus knob 2 stops back for optimum focus.

So I have looked in Stroebel and other places to see if I could find the formula for this. OBVIOUSLY it has to do with the amount of distance between the near and far points one wants in focus coupled with the amount of focus (lens travel) between those two points on the camera. Or to put it more mathematically, the amount of rotation of the focus knobs on the camera is a function of the amount of distance between near and far focus points in the scene. Maybe the answer has something to do with a derivative or an integral or something like that.

I was thinking of taping some scales to the Zone VI to measure the distance in lens travel, and then enter that number in to the formula to calculate know how much to move the camera back in f stops.

Anybody know where I can find this formula?

[jb7]

There's a really good explanation, with tables, by QT Luong, which is linked to on the home page of this site-
There's more information on depth of field in other articles on the home page,, but this article is directly related to focus spread and aperture selection-

http://www.largeformatphotography.info/fstop.html

[Jim Jones]

Lenses for older 35mm cameras often had a DOF scale on the focusing mount. A similar scale can be calculated and attached to a view camera. The calculations involve knowing the degree of image enlargement and the viewer's preference in sharpness. Once the scale is incorporated into the camera, no further math is needed. Harold M. Merklinger has written extensively on focusing the view camera. Google him for more information. Some compensation may be required, depending on subject matter and the degree of enlargement. DOF is more a matter of art than of science and math. As Q.-Tuan Luong says, "In summary, the acceptable circle of confusion on the negative depends on a number of factors, and it is better to determine the one which is best for you, rather than rely on the manufacturer's which is often not critical enough."

[ic-racer]

Yes the aperture to use is related to focus spread as previously described but are you asking about knob rotation and travel on your specific camera? To do that rotate the knob once and measure the travel. Divide that by 12 and you heve the travel for each 'hour' of knob rotation etc. for example on one of my cameras I know I need f16 for 3 hours of knob rotation.

[Mark Sampson]

Having owned and used both Sinars and a Zone VI for many years, I wonder if you could actually design/build such a setup that would actually be workable on the Z-VI. But if you want to try, why not find the relevant Sinar parts and adapt them?

[Bob Salomon]

Look at the Rodenstock depth of field/Scheimpflug pocket calculator. It will calculate what you want on any camera from 35mm to 8x10 and tell if an exposure factor is required and how much of one as well! The ruler is built-in to both sides of the calculator so mm scales do not have to be added to your camera. But an easy way to determine tilt angles would be a help.

[jb7]

My new camera focuses from the rear using a handle rather than a knob. One full rotation equals 2mm travel. I can work out the required aperture using QT's table before I emerge from the darkcloth...
of course, I did need to spend a long time memorizing the numbers...

[Bob Salomon]

2mm?

[jb7]

Inline Trapezoidal spindle...

[Kirk Gittings]

Hmmmmm, I suppose it would be handy but I never found such devices necessary............