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I've got a Nikon D3100 with an 18-55mm zoom lens. I'm keen to experiment with some other lenses and a friend of mine recently reccommended I purchase a fixed 50mm f/1.8 lens as he said it's good for portrait photography and capturing really sharp images. He also said that as it isn't a zoom lens it will sharpen my composition skills.

My question is - I currently have an 18-55mm zoom lens, so by purchasing a 50mm fixed lens, won't I be purchasing a spec of lens already covered by my 18-55mm lens? What are the main differences between these two lenses?

Finally - can anybody vouch for the Nikon fixed 50mm f/1.8 lens as being a good lens to go for?

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This is basically a duplicate of Between Nikon 18-55mm II and 50mm f/1.8, which suits my needs better?, although I must say you have phrased the question more coherently (and the answers to that one are a bit over-specific). –  mattdm Oct 15 '11 at 12:55
    
And a related but not duplicate question (about a prime lens of a different focal length): If I have a 18-55mm lens, is there a point in buying a 35mm prime lens? –  mattdm Oct 15 '11 at 12:58

4 Answers 4

up vote 18 down vote accepted

At 50mm on your 18-55, the max aperture is f/5.6. On the 50mm f/1.8, the max aperture is - obviously - f/1.8. It is perhaps not immediately obvious, but f/1.8 lets in 10-12 times more light than f/5.6. That is the difference between shooting at 1/10 second shutter speed (which is absolutely a no-go for moving subjects) and shooting at 1/100 (which is a usable shutter speed for moving subjects). Big difference indoors at night, for example. It lets you shoot without flash, or with the flash used as mere fill flash instead of it being the main light-source.

Note that Nikon has two variants of the 50/1.8, one with a built-in autofocus motor and an older one without. Do get the new one.

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Advantages:

  1. You'll get f/1.8 @50mm which is much useful in low light conditions.
  2. Your images will be slightly sharper than usual. Specially if you shoot around f/2.8.
  3. You'll get shallower DOF which will get significantly better background blur and subject isolation.
  4. You will get the taste of walking zoom method :)

Disadvantages:

  1. It will not AF in your Nikon D3100 body.
  2. You will lose zoom flexibility and will surely miss wide angle.
  3. You'll need to switch between your lenses often.
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3  
3. Decreased depth of field, not increased! –  Staale S Oct 15 '11 at 8:51
    
I have the Nikkor 50mm f1.8 AF-S SWM (Silent-Wave-Motor) which has autofocus for my D5000 which is, just like D3100, without in-body focus motor. –  Andrei Rinea Oct 15 '11 at 9:41
    
@Staale S: Thanks and corrected! –  fahad.hasan Oct 15 '11 at 14:35
    
@Andrei Rinea: I have not heard about that model before. Theres a 50mm f/1.8 G for Nikon DX which will AF with D3100 but not the typical 50mm f/1.8. Any link to the product you're talking about? –  fahad.hasan Oct 15 '11 at 14:36
    
@ShutterBug I believe that you and Andrei are thinking of the same lens -- the 50mm f/1.8G AF-S. It's not DX only, though: its image circle covers a full 36x24mm sensor. –  Evan Krall Oct 15 '11 at 20:20

To add to Shutterbug's answer, it might be worth looking at the new Nikon 40mm f2.8 Macro. On a crop sensor like your D3100's it is still just long enough for portrait work, and the f2.8 aperture will still blur the background nicely, but you also gain a true macro lens into the bargain, and you don't lose autofocus like you would with the 50.

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I do have the 40mm Micro Nikkor 2.8. It's quite nice for non-macro work such as portraits! –  Andrei Rinea Oct 15 '11 at 9:40

The advantage is that kit lenses are usually crap, and prime lenses rock.

Now seriously, I can tell you about my experience with the Samsung GX-10 kit 18-55mm f3.5-5.6 lenses (and at least its Pentax K10D counterpart was reviewed as better than equivalent Nikon and Canon kit lenses, I do not know if they were much more different than its twin camera) vs Pentax KA 50mm f1.7.

The 50mm f1.7 produces images that are sharper, with more contrast, more vivid colours, almost zero geometric and chromatic aberrations, less vignetting, and an incredibly narrow depth of field if needed.

And your friend is quite right, having a fixed length lenses pushes your creativity further because you do not have to care about choosing a focal length (and can focus on other important things) and you will need to move around the scene to get the best framing, possibly noticing things that with a zoom lenses you would have missed.

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