On 02/10/2015 15:20, whisky-dave wrote:
On Friday, 2 October 2015 13:22:37 UTC+1, NY wrote:


So what's it got to do with guide numbers?

If you knew anything about photgraphy you'd know, taking pictures
is nit the same.

I think what WD is not saying is this: you need to know the exact
distance from the flash to the subject to use the guide number to
calculate the correct aperture. Rather than measuring from flash
unit to subject, he's measuring from focal plane mark to subject.

Actually you should use both, especaily if the flash isn't mounted in
the usual place. Light comes from the flash gun and the intensity of
teh flash dimishes with teh inverse square law, hits the subject and
travels back to the camera again obeying the inverse square law.

The distance to the camera is irrelevant.



I've rarely used guide numbers because my flash units have either
been completely automatic, metering the reflected light through the
lens, or else have had a sensor on the flash unit which does the
same job, in which case you need to set the aperture manually
according to an ISO versus aperture scale.

Flashes are usualy (apart from ring and dedicated macro which I
couldn't afford (being at school) are built with the subject to flash
distance of around 1 Metre to 5+ metres or so. When I used the GN it
was for distances of about 1-6 inches, which means you've pretty much
go to put them on manual. I also used manual when I was taking
pictures of a baloon bursting.

Most flashes need a diffuser and filters when used that close as you
don't want to stop the lens down to a pinhole, then the GN is useless.

The manual flash units I had used a rotating scale: you preset the
ISO and then found the distance and read off the corresponding
aperture which does the guide number calculation for you (in the
same way that a slide rule performs multiplication or division for
you).

have you ever seen one go below 30cm ?

I have used one below 30cm but it had variable power and few other
useful features, it can even sync to the full speed of my camera even
though its a focal plane shutter.



I tend to read the distance off the lens's focus scale rather than
getting a tape measure out, though at close distances (maybe less
than a couple of feet) accurate measurement may be a bit more
important.

quite a bit more. Didn;t have much succsess foudn the lighting too
harsh and shadows were a problem too.


I'd have to google how exactly you use a GN. From memory it's
something like: every flash unit quotes a series of GNs for
different ISOs.

I only used the GN for 100 asa can;t remmebr altering it for difernt
ASAs

Ask a photographer to explain it to you.


and you divide the GN by the distance to give the aperture (two GN
values are quoted, for feet and metres). That's the principle, even
if I've forgotten the fine details.

That sounds right haven't done those calculating in this milenium.


This is called *sharing* knowledge as opposed to trying to be
superior and saying "if you're not clever enough to know, I'm not
going to tell you" which is childish and unhelpful - I worked for a
guy who thought that this was a way of motivating people, whereas
it does just the opposite for me.

Tomorrow I will teach him how to walk in a straight line, if I have
the time. He is a bit of a waste of time coming up with crap he
doesn;t understand and does even attempt to prove it.