So is it a good idea to leave on a UV filter all the time for protection? As Garbz said, it would be tragic to break/scratch the front element of your lens.
I don't agree with Garbz here. Leaving the UV filter on when there is no excess UV light to be filtered puts an extra barrier – with 2 refracting sides – in front of the lens. That can only do one thing: degrade the image.
For lens
protection I use a lens cap, which was designed to do precisely that, and a lens hood (
never shoot without a hood!).
You don't use a screwdriver to hammer in nails either, do you?
Also, how can you tell when there will be ghosting due to your UV filter, and when are you 'supposed' to use your UV, i.e. what would be an example of a situation where there is lots of unwanted UV light?
Use a UV filter (
only) when there is an excess of UV light. There is an excess UV light at altitude (over 3,000 feet), in the desert and savannah, at the beach, at sea, and in snowscapes. If you're not in any of these circumstances there is no excess UV light to be filtered. So use a UV filter only in
those situations. Not in others.
Is it true that you should leave on a polariser for most outdoor shots, just to increase the colour saturation?
No.
Firstly, you must
want that colour saturation (in the sky more than anywhere else) in that particular image. Secondly, the lens axis angle in relation to the sun determines whether or not the light
can be polarized, and by how much. Thirdly, polarizing slows down your available shutter speeds by up to 2,5 stops. You will probably need a tripod.
Important: polarizers and wide angle lenses don't match! You'll get blotchy skies!
And I'd love a good ND grad, but they're so expensive
An UNgraduated ND filter is more useful. It enables you to slow down your shutter speeds, or open up your aperture (for more selective DoF), by several stops.
Instead of using an ND grad you'll get better results with HDRI.