Handbook of High Speed Photography - IET Labs, Inc.

4.4 GUIDE NUMBER CHART.
A guide number expresses the relationship between the lamp-tosubject
distance and the lens aperture :
_ Guide Number
Effective lens aperture (f/number) - . '-r~. ,. . ,
r
S + lamp-to-subject dist. in ft.,
where S is the distance of the apparent source behind the reflector.
Just as its name implies, the guide number is a help in establishing
the correct aperture. It is just a guide and never should be accepted
as gospel...particularly in high-speed photography where film is prone
JUU
6R 1540 HIGH INTENSITY
GR 1538
(with auxiliary capocitor)
GR 1540 MEDIUM INTENSITY
GR 1538 HIGH INTENSITY*
100
50
GR 1540 LOW INTENSITY
GR1538 MEDIUM INTENSITY*
GR 1538 LOW INTENSITY*
10
5
GR 1538
24k-l50krpm RANGE
Appliot also to GR 1331
and GRI539
i
10 50 100 500 1000
FILM SPEEO-ASA
5000 10,000
Figure 4-3. Guide number vs film speed for various GR stroboscopes
and settings.* The outputs of the Types 1531 and 1539 are the same as
the outputs for corresponding 1538 settings. Note that "high intensity"
occurs on low-speed range (i.e., low-flashing rate). Data are for singleflash
operation. Film should be processed in accordance with manu
facturer's recommendations.
Effective lens aperture (f/number) = guide number/(1.5 + distance from
lamp to subject in feet). When camera is placed close to the subject,
this computed f/number setting should be multiplied by the K factor
found from Figure 4-5 to find the recommended lens setting.
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