Hey John,
I recently purchase a 17mm Tilt Shift Lens. I know that Nikon is releasing a range of their own soon, if not already. Canon produces 17, 24, 45 and 90mm, and came in response to people struggling with vertical objects such as posts, lines of windows, anything really, in wide angle shots. Horizons arc, buildings lean back, funny things happen at the edges. I'd say 24mm and wider is where you start to see this unwanted effect. You can of course do some correction in PS, but that takes time and effort for what can be avoided by using a tilt shift.
Dp review has done an extensive review of my len's cousin the 24 TSE mkII and it explains the pros and cons of using a tilt shift lens.
http://www.dpreview.com/lensreviews/can ... 5_tse_c10/
One of the cons is price; they are expensive! I bought mine from Hong Kong "Digital Rev" due to the strength of the Aussie dollar and saved about 500 at the time, but still i was pushing the mid 2000's. So it's not a cheap option for many, but you get what you pay for with build quality of an "L" series lens; amazing sharpness and colour rendition. I have doubtless confidence that Nikon will be just as expensive, but will probably be better than Canon's in the long run.
The major con with the 17mm TSE is that the front element (the front glass of a lens) protrudes a fair way out of the front of the lens. Infact, Canon didn't even bother putting a thread for circular filters because the front element stands so proud.
Having a 17mm lens comes with difficulties. Vignetting! Those unwanted objects towards the edge of the frame, such as a filter holder or a lens hood can bleed into the image when at the widest angle. In response to this i went for the largest sized filter holder and filters. So that they would clear the field of view, or be wider than the 17mm itself. Cokin is the company that makes the filter holders in 4 sizes, A, P, Z-PRO and X-PRO being the largest. I have the X-PRO holder & a universal adapter (because i have no thread to screw a filter ring into) that will sit on any lens up to 120ø. They make filters to hold these sizes, but quite frankly i'm not a fan with their filters. Here's the webpage if you scroll down it will show you the sizes:
http://www.cokin.fr/ico15-A.html The best you can get is Singh Ray filters but once again come with a price tag. You can only buy them from the company direct:
http://www.singh-ray.com/index.html. On their website they specify only up to the Z-PRO size but i asked them to make ones to the X-PRO size, which cost an extra 30 dollars or so each ND filter.
So the filters themselves used in the recent shots were Graduated ND (neutral density) filters and are great for balancing out the sky / landscape. The human eye can see about 12 levels of black, that is the dynamic range. The best cameras in the world can only see about 6-7. This leaves a big gap and is the reason why storm shots can be exposed usually for the cloud only, leaving a black silhouette landscape. esp in low light conditions such as sunset/ sunrises. So, graduated - meaning that they go from dark to clear from one end of the filter to the other, means that you balance out the difference. In these shots i worked out that there was about 2-6 stops of dynamic range on the top image that i had make up. Having the darkest part on the top of the images where the sky was lighter than the landscape meant that i got a balanced image exposure wise. The bonus is that the darker you go, the more light you shut down, the longer the exposure will be - hence more movement in the water captured. There is a fine balance of course, i like a little bit of movement without getting a white washy type of image - i like to capture the water spilling over with undulations. personal preference really.
When i get a chance i will show what my set up looks like so you can get a picture of what i'm talking about.
Here's a vid on you tube about ND filters:
http://www.youtube.com/watch?v=aPcEezGP_Xc
Also, go down to Vanbar at Sth Melb or Carlton and ask them to show you the filter set up.
Hope this helps cause it is worth every bit of investment.