50 PHOTOGRAPHIC FACTS AND FORMULAS 
Then multiply the focus of the lens by the times of enlarge- 
ment plus 1, and the result will be the distance between lens 
and sensitive surface. This distance divided by the times of 
enlargement will give the distance between the lens and nega- 
tive. Example: to find the distances for enlarging a 4x5 
negative to 16 x 20, the times of enlargement — 20 — 5 = 4. 
Using a 6% in. focus lens, 6% & (4+ 1) = 32Y and 32% 
—4—8% in. The greater distance is called the major con- 
jugate focus and the lesser the minor conjugate. In reducing 
or copying, these distances are reversed, that is, the major 
conjugate or greater distance is between the subject and the 
lens and the minor between the lens and the sensitive surface. 
How To Care For High GRADE LENSES.—The lens should 
be capped when it is not in use. Avoid sudden and extreme 
temperature changes. The lens should not be exposed to 
acid fumes. Avoid placing or leaving lens where dampness 
or moisture may collect on it. Do not permit lens to fall, or 
subject it to a sudden jar. Occasional cleaning is not only 
advisable but necessary when dust, finger marks or moisture 
show on the lens surfaces. For this a camel’s hair brush and 
a clean, soft, linen cloth are desirable. Proceed as follows: 
Remove dust with camel’s hair brush. Breathe on lens 
surface, and with cloth wipe lightly with a circular move- 
ment. If finger or dust marks cannot be removed, rub the 
surface gently witha tuft of cotton moistened with clean warm 
water. Dry the surface with a piece of clean linen cloth. 
Under no circumstances use abrasives or any kind of polish- 
ing or cleaning material. Never use acids, alcohol, alcoholine 
or other solvents on the lens surfaces or on the mount. Only 
in extreme cases should lens elements be removed from their 
metal mounts to clean inner surfaces. When replacing same, 
see that they fit evenly and firmly in the mount. 
