EXPOSURE 71 
It should be recognized that the figures given are for use 
with particular exposure systems and will yield good well- 
exposed negatives. The character of the same may not agree 
with individual opinions, and in this case higher or lower 
numbers should be used until with a given developer the 
desired character of negative is obtained. All plates have 
more or less latitude and, therefore, considerable variation in 
exposure may be given and good negatives be still obtained. 
EFFECT OF THE STOP APERTURE.—The relative exposures 
are as the squares of the aperture of the stops or diaphragms. 
For instance, if the exposure with stop f:8 is 5 seconds, with 
f: 16 it is not twice as much, but 4 times; because, according 
to the above rule, 8° — 64 and 16° — 256. Therefore, the 
exposures are as 64:256, or as 1:4. This applies to all 
lenses irrespective of their focal lengths. 
VARIATION OF F VALuES.—When copying and enlarging, 
the lens is not working at its equivalent focus, the actual 
working focus being more or less increased; therefore the 
values of the stops are altered. Mr. Alfred Watkins gives 
the following table which shows the necessary variation of 
the exposure due to the alteration of the f value of the stop: 
Lantern slide making Thin Neg. Medium Dense 
Neg. Neg. 
Copying Black and Photograph Coloured 
White Object 
15 times focus from lens 4 y, 1 
i H 1M 
ee ati. ¥ Vay VA 
SUA f: CRihhtae yy 1 2 
eK, I au 
2% “* i tetas 4 ly 3 
Bi oa Aru 1 Zz 4 
(Copying same size) 
