"dennis@home" wrote in message
web.com...

You claim your so called DoF mark on the camera is relevant to guide
numbers so tell us why?

Ah, I thought I'd explained, Dennis.

WD was using the focal plane mark (what he calls the DoF mark) as a
measuring point for measuring the flash to subject distance, as an
alternative to measuring from the flash head to the subject.

The distance is needed for calculating the correct aperture - see
Applications section of https://en.wikipedia.org/wiki/Guide_number

f-number = GN/distance

Given that he was using very short distances, I'd have thought that flash
head to subject would be the more correct thing to measu beyond a couple
of feet, the error of an inch or two between the two distances would be
negligible.

The Wikipedia article says "The position of the camera is not relevant"
which I take to mean that you could have the flash unit in a separate place
to the camera (joined by extension flash lead to hot shoe) and the exposure
would be the same no matter how far away the camera was - all that matters
is where the flash unit is in relation to the subject.

I suppose this makes sense: if you illuminate a subject by a given set of
lights, then you don't have to change the exposure if you move the camera
closer or further away