Scale and Base-scale and font sizes

[Wookey]

I used to have 
 scale 1 2000
 base-scale 1 1000

in my layout. That worked OK, but the symbols were very small and the text
was microscopic.

So I added 
 code metapost
 fonts_setup(10,16,20,24,28)

And that made the text a sensible size.

I just changed base scale to read
base-scale 1 2000

which makes the symbols a much more sensible size, but makes the text too big.

My understanding of these two values is that scale sets the scale of the
output (i.e how big the final PDF sheet is in map mode, or how many sheets
in atlas mode) , whilst base-scale 'has the same effect as if the map
printed in base-scale would be photreduced to the scale' (quote from thbook)

But if it was photoreduced then the fonts and symbol size shouldn't change.
So I am confused about how this should work. When would I ever want scale
and base-scale to be different? Why does changing base-scale effect the size
of symbols and text?

On a related point I tried doing an atlas version with the defaults (i.e.
1:200) and get loads of 'number too big errors' from metafont.

I can see why such a scale is a problem in map mode (page larger than PDF
can manage), but in atlas mode the page size is limited to A4 (or whatever)
- so why can I not print an atlas (with a lot of sheets) showing the system in
high detail? Is it a fundamental problem of the way the PDFs are generated
or can it be fixed?

Wookey
-- 
Aleph One Ltd, Bottisham, CAMBRIDGE, CB5 9BA, UK  Tel +44 (0) 1223 811679
work: http://www.aleph1.co.uk/     play: http://www.chaos.org.uk/~wookey/