Is a 50mm f/1.8 or a 55-200mm f/4.5-5.6 a better lens for portraits, and why?

Question

I just recently bought "Canon EF 50mm f/1.8 II" for a Canon rebel T3. This was about $100. I just came across "Canon EF 55-200mm f/4.5-5.6 II USM" which is only about $40 more right now.

My wife wanted the 50mm because a friend of hers recommended it for portrait shots and yes the pictures are better than the standard lens the camera came with.

What I am trying to figure out is whether the 55-200mm would give me the same great pictures with zoom. That's the part missing in the 50mm.

The part I am completely confused about is "f/1.8" vs "f/4.5-5.6"

The reason this lens was bought was to take portrait pictures.

Answer

The numbers 1.8, 4.5 and 5.6 refer to the aperture of the lenses. The smaller the number, the bigger it's light gathering power, and the smaller the minimum depth of field you can achieve with it. The 50mm is then a lot faster (i.e. gathers more light) than the zoom, by almost an order of magnitude.

The pros of the 50mm are then:

• better ability to take pictures hand-held in low light
• better ability to get a shallow depth of field.

One of the reasons the pictures taken with the 50 look so good is the fact that with this lens you can throw the background out of focus. The background-defocussing ability of a lens is proportional to the diameter of it's entrance pupil, thus:

• for the 50mm/1.8, the entrance pupil is 28mm across
• for the 55-200 @ 55mm, it's 55mm/4.5 = 12 mm
• for the 55-200 @ 200mm, it's 200mm/5.6 = 36mm

This means that the 55-200 will also allow you to defocus the background. You will, however, need to go to the long end of the lens, which in turn means a long working distance: you have to stand far from your model. With the 50mm you would stay closer.

Although I don't know this particular zoom, it's also common for this kind of zooms not to provide a very good sharpness when used at their long end and full aperture. In summary I would expect the 50mm to provide nicer images and the 55-200 to be more comfortable to use.

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