Zoom on flashes?

JayClark79 · Oct 16, 2009

What exactly does the zoom on a flash do?? My flash goes from 24mm to 80mm but im not sure what its doing to the light?

c.cloudwalker · Oct 16, 2009

If I'm correct about your question, the same as a zoom lens but for the light coming from your flash. You are either narrowing or widening the area of light coverage.

Derrel · Oct 16, 2009

The zoom on the flash does three things. First, it narrows or widens the beam spread. Second, it changes the effective range of the flash unit; at telephoto zoom ranges, both old (Vivitar 285HV for example) and modern flashes SB 800, EX 580 EX-II will be able to shoot flash exposures with accurate and repeatable results at LONGER distances than when their beam spreads are set wide; and third, zooming the flash to a wider or narrower beam changes the effective power, as measured by Guide Number. When bouncing flash off a high ceiling, it sometimes pays to zoom the flash to a normal to telephoto setting.

As I tried to explain to a 20-something forum member here a few days ago, the effective power of a speedlight is measured by Guide Number,and not by watt-seconds. Guide Number changes depending on the zoom setting. To use the GN to determine an exposure, divide the GN by the distance to the subject, in feet. With a 580 EX-II set to its 35mm zoom setting, the GN is 118; taking 118 and dividing it by 10 feet, means that at 10 feet, the correct exposure is f/11.8. With the flash set to its 14mm widest angle setting, the GN is 49, so 49 divided by 10 feet means an exposure of f/4.9 is needed.

Let's look at the Canon 580 X-II and real-world full-power manual Guide Numbers, measured in Feet, at ISO 100, and the various zoom settings. These Guide Numbers are for use in average-sized indoor rooms. In large, totally open areas or in small,confined spaces, GNs tend to go down,or up, respectively.

580 EX-II Guide Numbers at ISO 100, in Feet: Zoom of 14mm=49; zoom of 24mm=92; zoom of 28mm=98; zoom of 35mm=118; zoom of 50mm=138; zoom of 70=164; zoom of 80=174; zoom of 105mm=190.

Overread · Oct 16, 2009

slightly offtopic but it fits in here - but a quick question from a macro shooter. Were one to use a telephoto macro lens at close distances - say 150mm macro. Would one get a softer distribution of light (since its more spread out) if one were to set the flash head to be zoomed to a wider focal length - since the subject is very close to the flash itself?

Big Mike · Oct 19, 2009

slightly offtopic but it fits in here - but a quick question from a macro shooter. Were one to use a telephoto macro lens at close distances - say 150mm macro. Would one get a softer distribution of light (since its more spread out) if one were to set the flash head to be zoomed to a wider focal length - since the subject is very close to the flash itself?

I don't think that the light would really be any softer when 'spread out'. There might be some subtle differences in the quality of the light, but that would likely be most noticeable at the edges where it falls off.
The softness of light is a direct result of the size of the source and the proximity to the subject....and at either zoom setting, the flash still uses the whole head/lens.