I have a Canon 700D and I am new to photography (around 1 month). I got two kit lenses with my camera - the 18-55mm and 55-250mm. I usually use the 18-55mm lens as most of photography I have yet done has been indoors. I also heard somewhere that this lens is good for portraits and the other is a telephoto lens which is good for comparatively distant shots.
Yesterday I was clicking friends' pictures on our roof, but the background-blur effect was not quite clear. Just to check, I changed the lens to the 55-250mm and it was really good at creating blurred backgrounds with sharply focused objects (my friends). Should I use this lens for shooting portraits then?
I know some theoretic basics about Depth of Field and that aperture should be wider to create shallower DOF, but what I am not able to understand is that for same settings(Automatic modes - portrait mode), shouldn't the 18-55mm lens also create the same effect? It looks like that lens does not increase its aperture as nicely as the other lens does.
Answer
The 'blurred' background in case of the 55-250mm lens is due to the longer focal length, as compared to that with the 18-55mm.
A closely-related correlation between the focal length and the field-of-view also plays a role here, as depicted in the fantastic example below:
The photo^ on the right is taken with a typical stock lens like the 18-55mm, taken from up close (as depicted by the camera near the man in the middle picture), and has a (relatively) sharp distant trees in the background. The photo on the left is taken with a lens with a large focal length (e.g. 55-200mm, telephoto). One can clearly* see the same tree and buildings in the picture, which are on the left of the man's right shoulder in the picture on the right.
And there you have it, the 'blurred' background (in a narrow field-of-view) with a lens having a larger focal length.
Caleb's answer nicely clears the misconception of 'same settings' between different lenses on a camera.
*: pun intended
^: picture credit - from a PPT slide : Fredo Durand, CSAIL, MIT
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