+
+(defun magnification (focal-length distance)
+ "Returns the image magnification: the ratio of image size to objecct size."
+ (float (/ focal-length (- distance focal-length))))
+
+(defun bellows-factor (focal-length distance)
+ "Returns the bellows factor, the ratio of effective aperature to actual aperture."
+ (1+ (magnification focal-length distance)))
+