I have a
and am planning to buy this
does that make any sense? I'm interested in taking pictures that has lot of DoF.
Something like these...
Short answer: Yes.
Long answer: You can still get the shallow DoF look with any lens by doing any combination of the following: focusing close/decreasing working distance, ensuring the background is far away, and increasing the focal length.
You can even take it to the extreme and get very close, more than your intended composition, and stitch:
The 50mm 1.8 is an excellent choice for small DoF type pictures, as well as many other types of pictures. The large aperture makes it a good lens for low light situations, and the focal length is ideally suited for portraits. For the price, it is an unbeatable lens that should be in any camera bag. A strong buy recommendation from me.
if you're using the 18-200 dx, it means you don't use a full frame body. therefore, remember the 1.5 factor. so, the equivalent lens of what you want is the 35mm 1.8. this will give you a 'real' 50mm focal length. also notice that the 50mm doesn't autofocus (no inner engine) and therefore won't autofocus on d40/x/60, only on d80/90/200 etc. this might be even more relevant when shooting with small F number, since focus misses are more likely to occur.
There are two key specifications of a lens that determine the depth of field, the focal length and the aperture. (There are other factors such as distance from the subject and sensor size, but those are not directly tied to the lens).
In general, to get a really shallow depth of field you want a wide aperture (low f value) and a long focal length, then have your subject fairly close. Obviously there are some limits to that, and framing your shot correctly is more important that limiting the DOF.
The 50mm 1.8 is going to have a much narrower depth of field that the 18-200, because you have at least 2 full f-stops difference in aperture size, which will allow you a lot more flexibility.
Also, when you are researching a lens with shallow depth of field in mind, look at the number of "Diaphragm Blades". This is not directly linked, but bokeh is very important with a shallow dof, and the higher the number of blades, the smoother your background can be.