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Mike Lopez
4-Jul-2005, 11:15
Can anyone out there recommend a good guide to lens magnifications for a given focal length and subject distance? For example, yesterday I used a 240mm lens on my 4x5, and I was focused on a plant about 4' away. What magnification was I working at? If I had the bellows draw to switch to a 300mm lens without moving my tripod, what then? Any recommendations would be appreciated.

Michael S. Briggs
4-Jul-2005, 11:53
The equations for magnification, subject or object distance and image distance are given in the Lens Tutorial,
http://www.photo.net/learn/optics/lensTutorial.
For most purposes, for most LF lenses other than telephotos, it is accurate enough to measure the distances to the shutter or aperture.

Mike Lopez
4-Jul-2005, 13:54
Thanks, Michael. I'll have a look.

Leonard Evens
4-Jul-2005, 17:12
You should certainly follow Michael's advice. But briefly, here is how you calculate the magnification if you have the distance.

First convert the distance to mm. To do this, find it in inches and multiply by 25.4.

Next divide the distance in mm by the focal length in mm. Subtract one from that and then take the reciprocal. That is the magnification.

In your example, 4 feet is 48 inches or 48 x 25.4 = 1219.2 mm. Divide that by 240 (the focal length) to get 5.08. Subtract 1 to get 4.08. Finally, take the reciprocal of that to get 1/4.08 which is about 0.25.

For a 300 mm lens it would be the reciprocal of 1219.2/300 - 1 = 3.064 or about 0.33.

Mike Cockerham
4-Jul-2005, 20:10
Since you are using a lf you can measure the subject size and image size on gg to get magnification.

Mike

David A. Goldfarb
4-Jul-2005, 20:18
I usually either measure, as Mike suggests, or estimate the size of the object in the world compared to the width of the format, and then I have a table on the back of my camera to determine the exposure factor quickly.

Alan Davenport
5-Jul-2005, 09:01
A handy tool is Phillip Salzgeber's QuickDisc. (http://obelix-new.ideefix.net:8124/disc/) You'll find a PDF file there, which you print and then cut out the disc and a scale that is calibrated with exposure increase factors (both time and in f/stops.) Just measure the image of the disc and read the magnification and exposure increase directly from the scale. I carry one at all times. Best thing is, it's free!