Camera body or Lens problem?

I have Nikon D5100 with 50mm prime and 18-105 VR lens. Whenever I take a picture, the subject (my girlfriend who has long legs, slim) looks short or sometimes she looks normal. Is it because of the focus point. Are there any reference point where the picture will look lean and slim. Or should I just switch to full frame camera.

Please advice.

Id recommend switching to a full frame camera. My friend had the same problem so she switched to a full frame camera and the problem seemed to be gone. Hope this has helped you a bit.

It's almost certainly not the focal point. It's more likely to be the focal length. If you shoot with the 18-105 at a wide setting, it might look strange. The 50 should be OK. Try shooting with the 18-105 set to around 85mm and experiment with how far back you stand when shooting. You'll eventually find the ideal distance.

If you use a short focal length (18mm to about 26mm) and thus have to have the camera close to the subject the human body will look distorted. Stand back and use a longer lens. For full length about 30mm is fine, but for head and shoulders you need about 50mm, and your 50mm prime would be ideal. You can use longer focal lengths (70mm, 90mm, etc) if you wish and have the room to do so.

Look back through your portraits and look at the metadata to see the focal lengths of the plesasing and displeasing photos.

Switching to full frame is not necessary, and will not affect the 'drawing' of the photos, but if you did so you should multiply the numbers I have given by 1.5, thus a 75mm or 85mm is good for head and shoulders, 50mm for full length.

Always have the camera at least 8 feet (2.4 metres) from the subject.

From your description I would say the problem is due to one or more of the following factors, with varying degrees of influence:

1. Angle of view.
2. Lens focal length.
3. Distance between subject and camera.
4. Pose.
5. Clothing.
6. Lighting.
7. Setting and props.

We can advise with more certainty if you post some sample pictures in an online gallery and provide a link here. If you do, it would be most helpful to include some that you consider successful and others you regard as failures.

It's a lens problem… Or more accurately, a photographer problem. You are shooting too close with a wide angle lens.

Shoot at focal lengths of 50mm and greater.

That has absolutely nothing to do with full frame vs crop frame cameras. It has to do with the angle of the shot.