Focal length=power????

[no5094]

When i was checking out the canon 430ex at B&H, they gave a guide number for how far it is good for. Also, they gave it for different focal lengths, and for the shorter focal lengths, the guide number was smaller. Is this just a coincidence, or if when i use it on my 75-300mm at 300mm will it be even larger than that said for 105mm? wasn't sure exactly how it worked, and wanted it cleared up before i buy it. thanks.

[Curtis N]

The flash head zooms from 24mm to 105mm (full frame field of view). When it's zoomed for longer focal lengths, the light is concentrated in a narrower beam and hence will illuminate to a greater distance.

To get additional range with lenses longer than 105mm, you would need a separate attachment such as the better beamer to concentrate the light even more.

[René Damkot]

The flash head zooms (up to 105mm), so the GN gets higher as the 'beam' gets narrower.

[FlashZebra]

There is a fixed amount of light coming from the flash head at any power setting.

You can spread this quantity of light out over a relatively large area needed for wide angle lenses, or concentrate it over the relatively small area required for a telephoto lenses. At the telephoto settings, why use flash potential on areas of the scene that will not be included in the image.

So you can wrangle the light in an efficient manner that meets your needs.

Enjoy! Lon

[no5094]

thanks for the comments.

[CTR]

Does this also apply in bounce mode?

Thanks

[Curtis N]

Does this also apply in bounce mode?No.

When you're boucing flash, you need to think about the flash head zoom from a completely different perspective. For most ceiling bounce shots, the idea is to create a large light source for soft shadows. So the conventional wisdom is to use the widest zoom setting (I believe Canon Speedlites autozoom to the widest setting in bounce position) to light up a large area of the ceiling.

There may be times when you want to zoom the flash to a longer focal length if you're bouncing off a wall or something with a particular artistic goal.