"Returns the bellows factor, the ratio of effective aperature to actual aperture."
(1+ (magnification :focal-length focal-length :object-distance object-distance)))
+(defun extension-tube (focal-length &key original-object-distance original-image-distance original-magnification
+ new-object-distance new-image-distance new-magnification extension-length (units :feet))
+ "Computes the parameters for using extension tubes.
+Requires: 1. original-object-distance, original-image-distance, or original-magnification
+ 2. new-object-distance, new-image-distance, new-magnification, or extension-length
+Returns: original-object-distance, original-image-distance, original-magnification, original-bellows-factor
+ new-object-distance, new-image-distance, new-magnification, extension-length."
+
+ (multiple-value-bind (focal-length o-od o-id o-m o-bf)
+ (close-up :focal-length focal-length :object-distance original-object-distance
+ :image-distance original-image-distance :magnification original-magnification :units units)
+
+ (cond
+ (extension-length
+ (multiple-value-bind (focal-length n-od n-id n-m n-bf)
+ (close-up :focal-length focal-length :image-distance (+ o-id extension-length) :units units)
+ (values o-od o-id o-m o-bf n-od n-id n-m n-bf extension-length)))
+ ((not extension-length)
+ (multiple-value-bind (focal-length n-od n-id n-m n-bf)
+ (close-up :focal-length focal-length :object-distance new-object-distance
+ :image-distance new-image-distance :magnification new-magnification :units units)
+ (values o-od o-id o-m o-bf n-od n-id n-m n-bf (- n-id o-id)))))))
+
+
+