Should i swap lenses would appreciate someone's input?

I currently have a D3100 with kitlens Nikon 18-55mm f/3.5-5.6G AF-S VR DX NIKKOR
and a tamron 70 mm - 300 mm - F/4.0-5.6 which I bought last week.

I like to take pictures with a short DOF to really draw attention to the subject getting it as sharp as possible with soft blur around of things like flowers objects and people.

This is with my tamron lens: http://sphotos-d.ak.fbcdn.net/hphotos-ak…

I'm usually out taking pictures of scenery in woods and such.
I'm thinking a prime would be more suited for the pictures i want to take.

Should i Swap my tamron 70-300mm for a prime lens:
Nikon AF-S 35mm f1.8 G DX Lens: £125
Nikon AF-S 50mm f1.8 G Lens: £160
Nikon AF-S 40mm Micro f/2.8 DX G Lens £200

What you want for 'selective focus' shots is a narrow depth of field, only two things effect DOF, how close you are to the subject, the closer you are the narrower the DOF, and aperture, the larger the aperture (smaller the f number) the narrower the DOF.

How close you are has by far the most effect. With a 50mm lens anything around 10ft + from the lens will have a similar DOF no matter what aperture you use. Get the tree within 3ft of the lens and the DOF narrows dramatically and large apertures exaggerate the effect.

As one of the first compromises in the design of zoom lenses is aperture, wide aperture primes are best for these selective focus shots. As auto focus is less useful with a narrow DOF shot as it can focus where you don't want it to just as easily as where you do, older manual focus lenses, which are available cheaper, will work better, you might also find that you can afford one without having to sell your 'long reach' lens which is useful in it's own right.

The widest apertures are usually available in the good o'le 'nifty fifty', one should be in every photographers bag for just this effect, it's also a good portrait lens, but for landscape it's not so good.

Focal length just alters how close you are to the subject to frame your shot, frame the same shot with any focal length lens (by altering your distance from the subject) and DOF will be exactly the same when using the same aperture.

Guess what? 50mm prime will have the same DOF as a zoom set to 50mm at the same aperture and distance to subject.

This is true for other equal focal lengths also.

Tips…

Use wide apertures
Use long focal lengths
Keep subject closer to camera
Keep depth behind subject