I mean the focal length.
Using no zoom is a small number like 20mm and using telephoto zoom is a large number like 100mm.

In practise when using the filter use the zoom lens at least some of the way too.

The filter works by removing light from different directions depending on how you turn it.

So if you have a wide point of view you inevitably have light from different directions.

The result if used on a wide pov is, and try this for yourself so you can see, that the sky will be very bright one side of your pic. and dark on the opposite side. Message edited on: 11/30/2005 02:24

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html