e 
  
At £/5.6, the curve matches the cosine fourth power curve. At f/8 and 
smaller apertures, the distribution curves are almost identical to 
those of camera Bl. The effect of the A.V. filter is similar to tbat 
for camera Bl. The transmittance characteristics of some À.V. fil- 
ters deteriorated because of the long period of use. This caused a 
non-rotational symmetrical exposure distribution over the picture 
format as found in Figure 10. | 
Figure ll shows the results for camera C. The lens attached to this 
camera shovs significant vignetting so that the exposure decline at 
the periphery and corners is very noticeable. The relative exposure 
distribution curves match the cosine fifth power curve and no improve- 
ment is observed by stopping down the lens aperture. Although the 
effect of the A.V. filter itself is significant, the exposure distri- 
bution curve obtained by fitting the A.V. filter matches only the 
cosine third power curve, and is still insufficient for a satis- 
factory density distribution over the picture format. 
2.3.2 Effective Exposure 
(b 
Measured exposure times for various cameras are shown in Table 1. Th 
measurements have been made at the full aperture (f/4 or £/5.6) and 
f/8, and at the slowest and the fastest shutter speed settings, e.g. 
the longest and shortest exposure times. Each value shown in the 
table is the average of six measurements. The standard deviation of 
the measurements is 0.5 to 22 for the slow shutter speeds, and 2 to 3% 
for the fast shutter speeds. The percentage error from the marked 
shutter speed is shown in parentheses after each measured value. 
Since the shutter is of the between-the-lens type, the effective expo- 
sure time becomes longer by stopping down the lens aperture. Accord- 
ing to the table, the efficiency of the shutter is between about 70% 
and 80%. The cause of the very slow measured speed compared with the 
marked speed for samples No.3 and No.4 is, in our opinion, that the 
driving spring of the shutter blades cannot give sufficient tension to 
obtain marked exposure times due to the deterieration over the long 
period of use. Other sample shutters are of the rotating disc type 
and the measured speeds are more or less faster than the marked speeds. 
3. Instrument for the Measurement of Glare Spot 
3.1 Purpose of the Measurement 
In the early period, a blue-colored circular glare spot at the center 
of aerial color photographs was often observed. Although this phenom-. 
enon is less conspicuous with recent aerial survey cameras than it is 
with old cameras, it is still somewhat troublesome: the spot reduces 
the image contrast and deteriorates the color rendition. We have 
developed an instrument to investigate the cause of the appearance of 
(7)