I was tinkering with my 550ex yesterday after noticing for a while that the
flash confirmation light was not lighting in situations where I thought I
had enough flash power for my exposure.
I noticed that the flash distance readout was consistently greater than the
distance I got when doing the GN/F=D calculation myself.
For example with 550ex at 100 ISO
GN 180 (feet) divided by f11 = 16 feet readout says 20ft
f8 = 22.5 feet "
30ft
f5.6= 32 feet "
40ft
f4 = 45 feet "
60ft
Since the readout on the 550ex goes 2,5,7,10,15,20,30,40,60 in each case the
calculated carry distance is a couple of feet over breakpoint on the
readout.
Does the 550ex always round up to the next breakpoint?
As a further test I set up a grey card to fill the viewfinder at 10ft
distance and set the camera to f18 ( 180 / 10ft = f 18 at ISO 100 ) indoors.
With no flash compensation set on the flash or body I fired a test shot and
a burst of FE lock at the card. The flash confirmation lamp failed to light
and the fe lock indicated underexposure. I repeated the test increasing
aperture by 1/3 stops until I reached f14 at that point I got flash
confirmation and fe lock indicated middle tone exposure. This would
indicate a GN closer to 140. I know that 180 is an optimistic projection
but I expected better than 140.
I would be curious to hear what other 550ex user get in the way of distance
indications at various f settings, and what GN you believe you are getting
from your unit.
Thanks,
Joe.
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