zoom lenses

Hi folks. zoom lenses are often referred to "f/4-5.6" etc. can someone explain exactly what this means in terms of the lens' capability?

thanks

This simply means that the effective aperture at the long end is smaller than when the lens is at the short end. In other words, the aperture is not constant across the whole of the zoom range. When zoomed in (long) although the aperture ring may be set to the minimum of f4, the actual aperture is f5.6. There are usually indications on the lens to this effect. A little dot or line usually.

If you set your camera to Av and set the widest aperture with the zoom at the short end of its range, then zoom in (towards the long end), you'll notice the aperture setting on the camera will change.

In practical terms, this is only really an issue when using manual shutter and aperture settings. If you set the exposure correctly for the short end, and then zoom in, your image will be under-exposed.

A zoom lens expressed as a single f number is known as a constant aperture zoom lens and the aperture remains as set for the entire zoom range.

EG: Pentax DA* 16-50mm 1:2.8

so, in essence if we have a zoom lens with a range of 55-250 (say?) then the max aperature at 55 is f4 whilst at 250 it's f5.6 (yes?)

It depends on the lens, but yes, if a lens is described as "50-250mm F4-5.6", it will be F4 max at 50mm, and F5.6 max at 250mm.

Exactly so, yes.

and even then, only if the aperture you selected was wider than the maximum at your eventual zoom setting (assuming an A series or later lens)