When Enlarging, Reducing, or Copying we frequently want to know the positions for lens, negative and enlarging board. The formula is a simple one.

Multiply the focus of the lens by the times of enlargement or reduction plus one. For instance, if we wanted to enlarge a 1/4-plate to 8 1/2 X 6 1/2, this would be twice and not four times, as many people imagine. The 8 1/2 X 6 1/2 is double the length of the 4 1/4 X 3 1/4, and it is this that decides the degree of enlargement and not the difference in area. Supposing our lens was 7 inches in focus, then in this case 7 in. X (2 + 1) = 21 inches, the distance from the lens to the enlarging easel. To find the distance from the lens to the small plate we divide the distance from the screen by the times of enlargement. In the case named it would be 21 ÷ 2 = 10 1/2 inches. If we were working reductions in a camera the method would be exactly the same but in this case the 21 inches would be the distance from one copy to the lens and the 10 1/2 inches the necessary camera extension from the lens to the plate.

The following table will save calculation.

 Reproduced to: - Same size. Double. 3 times. 4 times. 5 times Distance from lens to: - Copy. Enlar. Copy. Enlar. Copy. Enlar. Copy. Enlar. Copy. Enlar. With lens of ins. ins ins. ins. ins. ins. ins. ins. ins. ins. 4 in.focus 8 8 6 12 5t 16 5 20 4 7/8 24 5 10 10 7 1/2 15 6 5/8 20 6 1/4 25 6 30 6 12 12 9 18 8 24 7 1/2 30 7* 36 7 14 14 10 1/2 21 9t 28 8| 35 8 3/8 42 8 16 16 12 24 10 1/4 32 10 40 9 3/4 48 9 18 18 13 1/2 27 12 36 11 1/4 45 10 3/4 54 10 20 20 15 30 13 1/4 40 12 1/2 50 12 60
 Reproduced to : - 6 times. 7 times. 8 times. 9 times. 10 times. Distance from lens to. - Copy. Enlar. Copy. Enlar. Copy. Enlar. Copy. Enlar. Copy. Enlar. With lens of ins. ins. ins. ins. ins. ins. ins. ins. ins. ins. 4 in.focus 4 5/8 28 4f 32 4 1/2 36 4 1/2 40 4 3/8 44 5 51 35 5 3/4 40 51 45 5 5/8 50 5* 55 6 „ 7 42 6 7/8 48 6 3/4 54 6 5/8 60 6 1/2 66 7 8 1/8 49 8 56 7 7/8 63 7 3/4 70 7 3/4 77 8 9l 56 9 1/8 64 9 72 8 7/8 80 8 3/4 88 9 10 1/2 63 10 1/4 72 10 1/8 81 10 90 9 7/8 99 10 11 5/8 70 | 11 1/2 80 11 1/4 90 11 1/8 100 11 110

LANTERN REFERENCE TABLE. Focus of Lens.

 Distance between lens and screen. 4 in. 5 in. 6 in. 7 in. 8 in. 9 in. 10 in. 11 in. 12 in. 13 in. 14 in. Diameter of Disc. ft. in. ft. in. ft. in. ft. in. it. in. ft. in. ft. in. ft. in. ft. in. ft. in. ft. in. 10 feet 7 6 6 0 5 0 4 3 3 9 3 4 3 0 2 9 2 6 2 4 2 2 11 „ 8 3 6 7 5 6 4 9 4 2 3 8 3 4 3 0 2 9 2 6 2 4 12 „ 9 0 7 2 6 0 5 2 4 6 4 0 3 7 3 3 3 0 2 9 2 7 13 „ 9 9 7 10 6 6 5 7 4 11 4 4 3 11 3 7 3 3 3 0 2 9 14 " 10 6 8 5 7 0 6 0 5 3 4 8 4 2 3 10 3 7 3 3 3 0 15 „ 11 3 9 0 7 6 6 5 5 8 5 0 4 6 4 1 3 9 3 6 3 3 20 „ 15 0 12 0 10 0 8 7 7 6 6 8 6 0 5 6 5 0 4 7 4 3 25 " 18 9 15 0 12 6 10 9 9 4 8 4 7 6 6 10 6 3 5 9 5 4 30 „ 22 6 18 0 15 0 12 10 11 3 10 0 9 0 8 2 7 6 6 11 6 5 35 " 26 3 21 0 17 6 17 0 13 1 11 8 10 6 9 6 8 9 8 1 7 6 40 „ 30 0 24 0 20 0 17 2 15 0 13 A 12 0 10 10 10 0 9 2 8 6 45 „ 33 9 27 0 22 6 19 3 16 10 15 0 13 6 12 3 11 3 10 4 9 8 50 „ 37 6 30 0 25 0 21 5 18 9 16 8 15 0 13 8 12 6 11 6 10 9