Studio Flash head units

Sorry I typed it wrong 200/100 = 2x = 1 stop

To double the illumination of a subject, you need to quadruple the power of the light(s). This is due to the fact that when you double the distance between light and subject, the light will illuminate four times the area. It actually is the same progression as the f stop on a lens. You can easily verify this with a lux meter.

Doubling the ISO, the flash will reach only 1.41X further, not twice as far.

This is due to an "obscure" physic law: it is called the "inverse square law".

I don't think your talking about the same thing here. The lights and distance aren't changed... just the ISO on the camera from 100 to 200 ISO which gives an extra 1 Stop.

I've just confirmed this on the manual dial on an old Sunpak flash. Distance and ISO are on the outer dial and f stops on the inner. Move the ISO one stop and the distance 10 ft in this case moves to the next smaller f number = 1 stop

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That sounds logical ... Also, other distance related variables come into play, such as zoom head position and any modifiers, like shoots, that will narrow the light spread ( and effectively increase the brightness for their reduced angle of coverage). I'm just glad I never had to do all this flash stuff on film in the past! Now with digital we really are spoilt by being able to check test shots immediately.

It was actually a lot easer in the past. You just had a Guide No which you divided by the approx. distance and that was the aperture that you used.

Flashbulbs were similar and very powerful considering the size....
http://www.flashbulbs.com/flash_info.htm

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Well we can still do the maths in the same way if we want. I was really referring to the greater ease with digital chimping of checking the results of using umbrellas, snoots and things, allowing fast fine tuning, like adjusting directional lighting or power of bounced our reflected flash. Surely that all must have harder to get right on film ... I guess pros could have used a Polaroid.

Polaroid was the answer ........ Just reminded me I still have a 5x4 Polaroid back....
I wonder if it's worth selling ...... takes smaller film packs than 5x4.

It gets pretty easy to predict.
Often, my very first picture is as good as it gets. Nice and relaxed, bang. Then they start thinking...
I'm working on a staff shot right now where they chose the first one
They were paying by the hour tho', so it was handy to keep them busy showing them variations

Possibly some useful info link

Best regards

Don't know if anyone is still interested but as far as I can see nobody has mentioned Bessel Studio Lighting at Swadlincote, Derbyshire. Their prices are more than competitive, and they are a British Company too. I have a couple of old 500w units and they seem ok to me, although I'm not exactly an expert. Incidentally, I have no connection with Bessel other than as a satisfied user.