CLOSE-UP FLA5H PHOTOGRAPHY
Sometimes, during wintery weather, when it is unsuitable for any outside
work, there is still an urge to use up a few remaining frames on film before
starting a processing session. So how about some close—up flash?
Flash Guide Numbers for close—ups aren’t the same as your common or
garden GNs and that’s where this Gil and distance table for aperture f/16
comes in handy. Three or four test exposures will establish the Close—up
Gil for your flash gun and then one column of the table will provide
flash distances for any quoted Lens Extension or Reproduction Ratio (R.R.).
I have found that my small Vivitar 215 flash gun has a Close—up Gil of 16
with f16 and 100 ABA film. The easiest way to make the test is to use a
50mm lens and 50mm of extension using rings or bellows so as to give a R.R.
of 1, which is lifesize on film. Set the aperture at f/16 and an exposure
time suitable for flash, generally 1/125 or 1/60 second. Looking across the
“Extension 50mm and LA. 1” line of the table note the flash distances under
Gils 12, 14, 16 and 18. Choose a mid—tone test subject (not too light or too
dark) and position the reflector of the flash gun at 37cm from the subject,
focus subject by moving camera nearer and further, then shoot. Repeat the
test at 44, 50 and 56cm.
After processing the film, one well exposed frame should indicate the Gil
for that flash gun and the film in use. Supposing the answer was 44cm, this
would indicate a Gil of 14. From now on the column under Gil 14 may be used
with confidence at varous Lens Extensions/Reproduction Ratios.
A few further points apply. With a more powerful gun it would be prefe-
rable to test for GNs 16 to 22. I have found that with a light toned subject
OR with a white reflector to bounce the flash onto the shadowy side of a
subject the Gil needs to be increased by one, using a distance mid-way between
the columns. If both these condition apply then the Gil should be increased
by two. The bracketed test may be made at a different extension instead of
50mm but the same procedure applies.
FLASH GUIDE NUMBERS FOR CLOSE-UP
Distances in Centimetres for Aperture f/16
50mm Lens.
Extension GN 10 12 14 16 18 20 22 24
In mm. LR.
5 0.1 56 68 80 91 102 113 126 136
10 0.2 51 63 73 83 93 104 115 125
15 0.3 47 58 68 77 86 96 106 115
20 0.4 44 53 63 71 80 89 97 107
25 0.5 41 50 59 67 75 83 91 100
30 0.6 38 47 55 63 70 78 86 93
35 0.7 36 44 52 59 66 73 81 87
40 0.8 34 41 49 56 62 69 76 83
45 0.9 32 39 46 52 59 65 72 79
50 1 31 37 44 50 56 62 69 75
60 1.2 28 34 40 46 51 57 63 68
70 1.4 26 31 36 42 47 52 57 63
80 1.6 24 29 33 39 43 48 53 58
90 1.8 22 27 30 36 40 45 50 54
100 2 20 25 28 33 37 42 47 50
125 2.5 18 21 25 29 32 36 40 43
150 3 15 19 22 25 2 31 35 37