« on: August 27, 2012, 01:00:48 AM »
There are many techniques out there - and it is good to understand them and apply them to the situation.
HDR and Photoshoping multiple images are ways to do it in some cases. You can get some very stunning, dramatic effects. But sometimes multiple exposures dont work - especially when sky elements are moving - fast moving clouds, direct sun, so on, dont always play nicely with HDR (I have the NIK product); This is also true in action / sports shots.
I also have a growing set of 4x6 Lee for many critical shots where there is a delineation that is soft or hard. It makes for a natural transition and its one full exposure. Sometimes I like to apply that effect.
For the original question, it has not been fully explained...
You use Graduated Filters to affect the exposure an specific areas of the image. Most often, its a bright sky and darker landscape, but it could be any number of situations where sections must be exposed down.
You use Solid ND Filters to extend the exposure time across the entire image. This can be useful for very wide open shooting in bright light where you cant get a fast enough shutter speed; or more conventionally for timelapsing water/waterfalls, bluring out people in crowded areas, or to add a certain texture to the image with a range of stationary and moving elements (a softened field in front of a rock solid mountain for example). I have a custom Lee-format filter that has Solar Film to shoot the solar eclipses and transits.
I used to use round when I was playing 77mm. Now I have a number of lenses - so going square Lee (or Cokin) allows one filter to be used on a variety of lenses. And of course, with Graduated Filters you need to position them.
One note to add, beyond Grads, Solids, Polarizers, and Protection filters, most everything else can definitely be done in Post.