What size lens would be best for the following?

Hi, i'm new to photography. I have a nikon d60 and i currently have a 18-55mm lens. I would like to shoot photos of mainly architecture like bridges and some scenery and portraits. I'm going to buy a polarizing filter tomorrow and i might buy another lens or two. Any suggestions on a suitable size lens size for each of the applications?

Added (1). By size i mean focal length. I was thinking of some kind of wide angle

Added (2). Ah, thanks… I was thinking a nikon 55-200mm. I have seen filter screw on type fish eyes. Any experience with those?

The 18mm end of your zoom lens is wide angle. You could buy a prime (non zoom) at a wide angle which will probably give better results than the zoom lens, but the benefits will be small.

For architecture, a fisheye lens is always fun but they are quite expensive and quite niche. I don't think you would want to overuse the effect. I took some fisheye photos of the Clifton Suspension Bridge, see below.

For portraits, depending on if you are thinking of full-body portraits or just head and shoulders, anything between 50mm and 90mm will do nicely. You might might find a short telephoto (eg 90mm) lens useful for architecture too, since it compresses perspective and you can achieve things like my second link.

Have fun!

Given a crop factor of 1.5x, your 18 mm is actually a 27mm in 35mm format.(basically the image sensor is smaller than the standard 35mm film).

The lens for architecture/nature are opposite for the ones you nee for portraits, though it doesn't mean you shouldn't use them at all.

For architecture and scenery, you would need a wide angle lens. Your 18-55mm may be just enough.
For indoor portraiture, you would need a fast lens (wider the aperture, the faster can be the shutter speed).So it is recommended to go for fixed focus lens (prime lens) which have wide apertures of 1.4 or 1.8. There will be distortions if you use a lens will small focal length and take a person's picture from close. It is therefore better to go for 85-105mm (in 35mm format) for using in portraiture. So look out for a prime lens that has a fixed focus length of anywhere between 60-70mm for your camera. Also keep in mind that your camera may not have auto focus motor in the body and the lens must have. So the 50mm f/1.8 lens from Nikon will work only in manual focus for you.

I have a 55-200mm too… But its widest aperture is 3.5, and that too only when its at 55mm length. So I find it difficult to shoot with it, say, in fashion shows.

For architecture and landscape you should be looking to extend your range at the wide-angle end. I'm not familiar with Nikon, but you ought to be looking for a lens which sits beyond the w/a end of your current lens; I use a 12-24mm for landscape and architecture, on a 35mm slr. Your nearest equivalent, on a crop sensor, would be a 10-20mm zoom, or nearest Nikon equivalent. For portraiture you should be looking for a 50mm wide-aperture prime lens.