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136 MOTION PICTURE PROJECTION
In event that lenses we supply do not make the picture close enough to the desired size, on account of an error in measuring the distance, report at once the exact width of the picture they produce and we can then allow for the error and determine what the distance actually is and the focal length required.
If you want Gundlach lenses to make a picture the same size as it is made by some other lens send the lens to us to be measured because you cannot depend upon the focal length engraved on the mount.
Computing the Focal Length
The focal length required is ascertained by a computation based upon the size of the opening in the aperture plate, the size of the picture wanted and the distance it is to be projected.
The distance is somewhat uncertain owing to errors made in measuring it which we have known to amount to as much as fifteen feet but in case a mistake has been made by which lenses of the wrong focus have been secured it is easily rectified. We should then be informed the exact width of the picture made by the lenses the customer has received and as we have a record of their exact focus, we can calculate from these two factors what the correct distance is and determine the proper focal length of the lenses to send in exchange. The distance of projection can be obtained by referring to the architect's plans of the theatre if these are available.
Measure the distance accurately, and you can depend upon us to supply lenses of the correct focal length.