Working out Minimum Focusing Distance : LUSENET : Large format photography : One Thread

How does on work out the minimum focusing distance given the maximum extenstion of the camera, the focal lenght of the lens and the flange distance?

As an example for a 450mm lens with flange distance of 428.4mm and a 8x10 camera with a maximum extension of 570mm, what would be the minimum focusing distance and magnification?

Thank you.

-- Natasha Sekha (, October 01, 1997


The extension past infinity focus, e, is the max extension minus the flange-focal distance. Here e = 570 - 428 mm = 142mm.

The image distance from the lens, i, is about the focal length, f plus the extension e. So here i = 450 + 142 mm = 592 mm.

The object distance from the lens, o, is then given by the lens equation: 1/f = 1/i + 1/o.

For this example, 1/450 = 1/592 + 1/o. This gives o = 1876 mm, or about 1.9 meters from the lens (not from the film).

There will be slight deviations (possibly larger for lenses of telephoto design).

-- Ben Weiner (, October 20, 1997.

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