Zoom head on flash? Why?

[Vetteography]

I have an old Sunpak flash that works well enough on my new EOS (and is compatible as it only pulls 5 volts).

It has a "zoom" head on it where the lens will click out away from the bulb 3 notches.

Why? What is the purpose? When would you use this feature and how do you decide how far to "zoom" it? I see this advertised as a feature on some flashes but haven't sen an explanation of what it does.

Help a newbie!

[DaveG]

The beam of light that comes out of the flash is in the shape of a cone or a piece of pie. If you were using a wide angle lens you'd want this cone to be very wide so that the whole of your picture would be covered by the flash's light. On the other hand if you were using a longer lens you'd want to concentrate the light in a smaller area so that less flash light was wasted and that you could get more range out of your flash.

The flash accomplishes this (usually) by moving a lens inside the flash head called a fresnel, pronouced "fre nell" not "fresss nell" for some reason. The fresnel can focus light into a beam and is exactly the same thing (well, bigger) that's used in lighhouses to magnify their light.

If you are able to look at the flash's instructions it should indicate the wide angle, normal and telephoto settings. On the Canon flashes like the 550 & the 580 the camera sends instructions to the flash as it senses the zoom focal length being changed and the flash head zooms along with the lens.

[Vetteography]

Thanks for the lesson, it is much appreciated. Makes perfect sense now that you have explained it!

I don't have the instructions on the flask, it is about 15 years old and whatever documentation I had is long gone. I looked on the net but no luck so far.

Nice thang about digital is I can experiment with it and see for myself, eh?

[pierrot]

The more you pull the head out, the narrower the cone: so it's the tele position.
The more the head is retracted, the wider the cone: suited for wide-angle lens.
In between: for normal coverage of a standard length lens, eg. 28 to 50mm on a dRebel/dRebel XT/20d etc.

Just try various combinations (head position/focal length): it doesn't cost a cent, thanks to digital ;)

[Jon]

Probably the positions are 28-35, 50, and 85 mm or so. With the DRXT, you'd need to apply a correction factor. Use the widest setting at 18 on your kit, mid stop at about 30 mm, and the longest when you're at 55 and you should be about right.