Filter positions - filter "zoom"

[simons-photography]

Since buying my first GND filters I've been wondering,

I've seen advice saying to put the filter as close to the lens as possible but surely this way you use less of the filter, of course with a zoom lens thing will change continuosly, has the concept of ajusting the filter distance ever been explored ?

as I have a size A polarizer and can't spend £ 60 on a size P one as I have all other filters in size P I'd be inclined to find a way of putting the P filter holder on the end of the A sized holder.

is anyone with me on this or have I gotcha all confused as to what I'm on about

[Wheatfield]

You want the filter as close as possible to the lens.
The farther away it is, the more chance you have of seeing the surface of the filter in the depth of field.

[simons-photography]

ah yes there is that too, but say I'm using 18mm on my lens, if I sudenly zoom to 55 then I'll be using 1/3 the filter I was before, that start to make filter use more of a hit and miss thing I exprect that at longer focal lengths I'd be wiser to use the hard edge GND filter instead of a soft one