I Need a lens'swhich blurs the background of an portrait, I Have Nikon D90, 18-105 lens?

I Need a lens'swhich blurs the background of an portrait, I Have Nikon D90, 18-105 lens?

Added (1). Thanks Jens

Smear vaseline around the edge of the lens.

Your lens already should do that relatively good, when using it at 105mm.

A typical portrait lens would be a 50mm prime

The 50mm f/1.4D AF, 50mm f/1.8D AF, 50mm f/1.4G AF-S and 50mm f/1.8G AF-S will work perfectly on your camera.

The older D lenses are cheaper and come with an aperture ring, this enables you to combine them with extension tubes to give them macro capabilities as well. The newer G AF-S lenses have faster and perhaps slightly more accurate autofocus.

Edit:
Oh, and stay away from your lens with vaseline. Especially if you don't have a UV filter on it.

You should be able to do that with your lens if you put the subject very far from the background and the camera very near the subject. The larger the aperture size (lower f/number) the easier to blur background. If you realize you need a new lens to do it more easily, get a non-zooming lens with the lowest f/number you can afford. The 50mm f/1.8 is the best for its value in this aspect.

There are three ways to decrease the Depth of Field (what you call blur background), all of which work in conjunction with each other.

1.move the subject closer to you.
2.increase the focal length of the lens.
3.increase the aperture of the lens (make it larger).

Also the further the background from the subject, the less noticeable it will be as even if the background is blurry, it might still be annoying.

Here are some examples:

If you set your lens to 105mm, and the aperture to f/5.6, then;

with the subject 10ft away, the DoF (total range of the in-focus area) 7inches.
with the subject 20ft away, the DoF will be 2.4 feet
with the subject 30ft away, the DoF will be 5.5feet.

But if you change the lens to 18mm and use an aperture of f/3.5, then:

with the subject 10ft away, the DoF will be 23ft
with the subject 20ft away, the DoF will be at infinity.
with the subject 30ft away, the DoF will be at infinity.

These are theoretical DoFs using a DoF calculator, and are just examples of two extremes of your lens. You can see that 18mm is almost worthless for portraiture use. At 105mm, f/5.6, and the subject even 20ft away, the 2.4ft DoF may be your best combination for nice portraits with a blurry background.

Reducing the aperture to say f/8 will all but kill any shallow DoF you are trying to achieve. So experiment with different distances, focal lengths, and apertures, keeping in mind the three things I mentioned above.

If you find this combination does not provide the sufficient effect you are looking for, look into a 50mm f.1.8 or a 105mm f/2.8 macro (which also works well in this application).

Indeed using a long lens is the way to do it, so anything above about 70mm is good. Even a 55mm can work. Use at full aperture e.g. F4.5, f5.6 or whatever lowest number is available, not f11 or f16. Keep the subject close to the camera, with good separation from the background that you want blurry.

I'm also using 18-105 lens with my Nikon D90 - that's the lens since I bought D90 and it is really doing good for me.