The correlative of such
elongation of the aperture is an increase in velocity which will always
bring the total exposure to the same figure, whatever be the aperture
employed.
If the aperture be equal to two diameters, the effective time will be
equal to half the time of the total exposure; and if it is equal to
three diameters, the exposure will be good during 2/3 of the total time.
This amounts to saying that the effective time of exposure is equal to n
times the diameter--1, the velocity being supposed always uniform. If
we place the shutter within the objective, it is the diameter of the
diaphragm that it will be necessary to say. The effective time will be
equal then to n diaphragm--1.
From what precedes it results that in no case should the aperture be
inferior to the diaphragm, since the former would otherwise absolutely
suppress the effective time in giving a lower plane corresponding to
an insufficient quantity of light. Moreover, an aperature of this kind
would prove injurious to the quality of the image by successively
uncovering rays which do not form their image identically at the same
point.
Pages:
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174