Pentax 40 MZ-3i - Manual (Page 30)

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16. Technical Data 15. Exposure corrections
The automatic exposure systems are adjusted to a subject reflection factor of
25%, this being the average reflection factor for subjects shot with flash.
Dark backgrounds that absorb a lot of light, or bright backgrounds that reflect a
great deal of light (e.g. backlit scenes), can result in overexposure or under-
exposure of the subject, as the case may be.
Exposure corrections in auto mode
To compensate the above described effect, exposure can be corrected by opening
or stopping down the camera’s aperture. If the background is mainly bright, the
flashgun’s sensor will cut off the flash too soon with the result that the
subject will be underexposed. With a dark background the flash is cut off too late so
that the subject looks too light.
Bright background:
open the aperture 1/2 to 1 f-stop
(e.g. from f/5.6 to f/4)
Dark background:
close the aperture 1/2 to 1 f-stop
(e.g. from f/8 to f/11)
Exposure corrections in TTL mode
Many cameras have an adjusting facility for exposure correction which can also
be used in TTL flash mode.
Note: Please observe the corresponding explanations in the Operating
Instructions for the camera or SCA adapter.
Here, exposure correction by changing the aperture on the lens is not possible.
This is because the camera’s automatic exposure system will regard the changed
f-stop as a normal working aperture.
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Table 1: Flash durations at the individual partial light output levels
Partial light output Flash duration LC-display Guide number
P=Flash Power in seconds at ISO 100
50 mm zoom
1 1/200 1 40
1 - 1/3 1_
1/2 + 1/3 2¯¯
1/2 1/600 2 28
1/2 - 1/3 2_
1/4 + 1/3 4¯¯
1/4 1/1500 4 20
1/4 - 1/3 4_
1/8 + 1/3 8¯¯
1/8 1/3000 8 14
1/8 - 1/3 8_
1/16 + 1/3 16¯¯
1/16 1/5000 16 10
1/16 - 1/3 16_
1/32 + 1/3 32¯¯
1/32 1/8000 32 7
1/32 - 1/3 32_
1/64 + 1/3 64¯¯
1/64 1/13000 64 5
1/64 - 1/3 64_
1/128 + 1/3 128¯¯
1/128 1/20000 128 3,5
1/128 - 1/3 128_
1/256 + 1/3 256¯¯
1/256 1/26000 256 2,5