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# Thread: Focusing on curved film plane

1. Heres a site with some good pix of an Agfa Chief where you can see the curvature of the film plane.

http://www.foundphotography.com/Phot...f_120_mod.html

What I am working on is basically a wood box with the a lens on the front and a door that opens in the back. Think of it as a wee sized pano camera. 6x9,.

2. This is how I measure field curvature of lenses.

First mount the lens on a view camera and focus a point light source at the centre of the ground glass. Measure the distance between the film standard and the lens standard in some convenient way. I use a digital vernier caliper.

Then turn the camera so that the image of the light source now falls at the edge of the ground glass. Focus again. Measure between the standards again using the same fiducial points as before.

The difference between the second and first measurement is how much the image "surface" bends away from a plane. If you know the focal length of the lens then this "bend" gives you enough data to calculate the radius of image curvature by good old Euclidian geometry.

Alanrockwood's warning about astigmatism is very pertinent. A stigmatic lens (any simple lens, for example) will give two focus distances at the edge of the ground glass. In one setting the point source will focus as a short vertical (tangential) line. In the other setting the focus will be a short horizontal (sagittal) line. Take both measurements, make an average, and call the result the "image distance".

3. Originally Posted by Maris
This is how I measure field curvature of lenses.

First mount the lens on a view camera and focus a point light source at the centre of the ground glass. Measure the distance between the film standard and the lens standard in some convenient way. I use a digital vernier caliper.

Then turn the camera so that the image of the light source now falls at the edge of the ground glass. Focus again. Measure between the standards again using the same fiducial points as before.

The difference between the second and first measurement is how much the image "surface" bends away from a plane. If you know the focal length of the lens then this "bend" gives you enough data to calculate the radius of image curvature by good old Euclidian geometry.

BINGO

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