Today’s Question: I obviously understand the zoom feature of lenses. But I don’t understand what is meant by the zoom setting for a flash. How do you “zoom” the light of a flash?
Tim’s Quick Answer: The zoom setting on a flash enables you to narrow (or widen) the coverage area of the light from the flash to better match the field of view of the lens based on the focal length.
More Detail: To make the best use of the light emitted by a flash unit, you want it to cover the full area of the subject or scene you are photographing. If the light from the flash covers too small an area, you will obviously not have even lighting within the scene you are photographing. If the flash covers too large an area, you are wasting the light that falls outside the scene you are photographing.
Many flashes have a zoom setting, so you can adjust the coverage area of the flash based on the field of view of the lens focal length you are using. In fact, some flashes include an automatic zoom setting, where the flash coverage area is adjusted automatically based on the focal length of the lens currently in use.
Note that you may need to compensate for the cropping factor of your image sensor if you are using a camera with a sensor smaller than full-frame. In other words, with a camera that does not have a full-frame sensor you may need to set the flash to the effective focal length based on sensor size, rather than the actual focal length of the lens itself. For example, on a camera with an APS-C sensor a 50mm lens would have the field of view of an 80mm lens, and you may need to set the flash accordingly.