## Converting focus spread in feet to lens change(MM)greenspun.com : LUSENET : Large format photography : One Thread |

Is there a formula for a specified focal length lens which will convert the focus spread of the subject planes, in feet, to the focus spread for the lens, in millimeters?? This would be for a Hasselblad. Thanks.

-- Pete Phelps (petep@bpbinc.com), January 29, 2001

Focus spread?Is this anything like mayonese?

-- Ellis Vener (evphoto@insync.net), January 29, 2001.

Mayonese?

-- Steve Clark (agno3@eesc.com), January 29, 2001.

Those Mayonese really knew how to put on a spread, but the after dinner entertainment wasn't for the faint-hearted or weak-stomached.Pete, do you mean depth-of-field?

That doesn't change whether you measure it in feet, millimetres or Mayonese cubits.

There are 304.8 millimetres to the foot.

-- Pete Andrews (p.l.andrews@bham.ac.uk), January 30, 2001.

OK, I think the penny's finally dropped about what you're asking. You want a formula for calculating the lens extension for any given focus distance, right?

It's: 1/v+1/u=1/F

Where v = subject distance; u = distance from film plane to rear node of lens; F = focal length of lens.

-- Pete Andrews (p.l.andrews@bham.ac.uk), January 30, 2001.

Is this what you're after: the change in lens distance from film plane (how much you need to adjust the lens) to effect a specified change in the plane of focus (in feet) at the object that you're photographing?

-- neil poulsen (neil.fg@att.net), January 30, 2001.

Thanks for the responses. The formula for the focus spread is FS=(1/(1/F-1/VN))-(1/(1/F-1/VF)) from p.l. Andrews, where VN = near distance,VF = far distance, and F = focal length of the lens. This works and allows one to use the Hasselblad range finder to determine the focus spread in millimeters for two different focus planes. This can then be applied to Hansma's formulae for optimum F-stop. Just like a view camera! Keep in mind that all variables must to be in millimeters.

-- Pete Phelps (petep@bpbinc.com), January 30, 2001.

Pete is that how Sinar gets that deal to work on its cameras whereby you do a couple of focuses and it tells you the f stop to shoot at. Kevin

-- Kevin Kolosky (kjkolosky@kjkolosky.com), January 30, 2001.

The Sinar has a scale on the rail which is calibrated to approximately 1 mark to 1.2 millimeters. Thus, a Sinar spread of 10 equals 12 millimeteers. The scale is not in millimeters because Sinar uses the spread to determine the lens tilt angle and their lens tilt is not on axis. All this is in their instruction manual. I have not worked with their F-stop setting capability.

-- Walter M Phelps (petep@bpbinc.com), January 31, 2001.

I'm not sure why one needs to calculate the lens extension shift, for what seems like a simple DOF problem, but glad I could help.

Any 'optimum aperture' is dependent on someone's idea of an acceptable circle of confusion, and this isn't a hard-and-fast, scientifically objective, measurement.

-- Pete Andrews (p.l.andrews@bham.ac.uk), February 01, 2001.

In response to Pete Andrews the treatises presented by Hansma, Peterson, Luong et al derive the optimum F-Stop for a given focus spread. The formula is simply fs=(Focus spread * 375) ^ 1/2 . For a symetrical lens(most normal, and moderate wide angle and tele lenses), where the magnification is less then 1/10, maximum sharpness is obtained. The resulting resolution on the film is measured by R=2/(F-Stop/530), and the maximum print enlargement is R/5. If you want maximum PRINT resolution, this is the way to go.

-- Walter M. Phelps (petep@bpbinc.com), February 01, 2001.

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