Pros and cons of lenses?

Manual focusing lenses and auto focusing lenses. Which one are better? Does manual focusing lenses have any advantages over auto focusing lenses? I'm using a Nikon D3100. Will there be a trouble if I choose to use a manual focusing lenses?

Your biggest problem with using manual focus lenses on your D3100 is the meter won't couple to the lens. So I would not recommend using manual focus lenses unless you are able to upgrade to a camera which can meter with manual focus lenses, like the D200/300 or D7000/D7100. With your current camera, it would be more trouble than it's worth having to either calculate exposure on your own or use an external meter. Otherwise, some manual focus lenses work quite well. Better is subjective, there's no such thing. What works for you may not for everyone. So you'll have to figure out which is better for you and why. Manual focus was at one time the only game in town and with technological advancement comes convenience. Six of one, half a dozen of another. Advantages are of course, cost. Most manual focus lenses are substantially less expensive than their autofocus counterparts because many optical formulas have changed very little between each iteration, and many are still being made new today, so you can save money by choosing function over convenience. One of my favorite lenses is an AI Nikkor 55mm f/1.2, which I use on a D7000 and it is very fast and very sharp. It is my go-to portrait lens. It's suited to that purpose for me so I can say it's better in a lot of ways than the AF-S 50mm f/1.4G I used to have because I don't necessarily need autofocus. But truthfully, it's a flawed lens that I have learned to master. Doesn't mean it's technically a better lens. But I like it.

Once upon a time manual focus was the only option. The introduction of the Minolta Maxxum 7000 with its auto focus system in 1985 sparked the auto focus revolution.

The older manual focus 35mm film cameras had focusing aids in the viewfinder which, unfortunately, your D3100 doesn't have.

However, according to this site: http://imaging.nikon.com/...lity02.htm AF D or G lenses can be manually focused (with electronic rangefinder) and function in Auto and Scene Modes as well as P, S and A and can use 3D metering.

You may find manual focusing advantageous in low light or in macro photography. Auto focus will be better for action and sports.

If your local camera store has any Nikon AF D or G lenses maybe you can try them out and see if manual focusing is for you.