On Fri, Jun 08, 2001 at 05:30:04PM +0800, [EMAIL PROTECTED] wrote:
Hi
I have a non-original TTL flash (30 guide number) for my Z1p. If I use it together
with the built-in flash (14 guide number), is it equal to 44 guide number?
No, guide numbers (GN) don't add together like that.
Two flashes of GN 10 together, will give you a GN of 14.
A flash of half the GN only gives out 1/4 as much light.
This is because the GN is the flash-subject distance multiplied by the aperture
f-number. This is no more than a device for mathematical computation (just like
momentum in Newtonian mechanics has no physical meaning.) So doubling the GN means
that you can use the same aperture for a subject twice as far away, but since the
subject is twice as far away, you need four times as much light. Perhaps a more
natural, from a photographer's point of view, way to look at it is that with twice the
GN, you need to increase the f-number by a factor of two for the subject at the same
distance; that is you have to close down the aperture by two stops, hence letting 1/4
of the light in.
I don't think using the RTF flash will not give you any noticable extra power over the
GN30 flash. I guess it might be useful for fill purposes if you use the GN30 flash
off camera, or bounced.
It was a good question though. I wouldn't have asked myself this question, but now
that I've thought about it, it's quite an interesting answer!
Yours,
Frank
(who has roots in HK.)
--
Francis Tang, Postgraduate Research Student, LFCS, Edinburgh.
Visiting: AG14, FB Mathematik, TU Darmstadt, Deutschland.
Tel: +49 174/3545241 (D2 Voda) ZNr: S215/215
Email: [EMAIL PROTECTED] WWW: http://www.dcs.ed.ac.uk/home/fhlt/
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