Lens comparison

[Mzaki]

Which of the belo mentioned lenses is better for portrait pgotography asuming I am using the same focal lens (50mm) on EOS 550D:

EF 50mm 1.8 II

EFS 18-200 mm

[inkista]

Yes, the 50/1.8 II is a more typical portrait lens. It's all about the maximum aperture.

[Mzaki]

According to the charts published on dpreview.com the EF 50 is sharper then the EFS 18-200 at any given f stop. Would that count?

[inkista]

Sure, but as I said above, it's more about the maximum aperture (that link up above will explain why).

The 18-200 lens is an f/3.5-5.6 lens. The 50mm is an f/1.8 one.

The 18-200, when it's set to 50mm can only open up to f/4.5.

The 50 can open up to f/1.8. That's a difference of 2 2/3 stops. Which is a difference of 2^2.6 which is about 6x the amount of light.

So, compared to the 18-200, the 50, in the same situations, could use an iso that's one-sixth lower, or a shutter speed that six times higher, if all the other settings are the same.

In addition, the depth of field at f/1.8 is shallower than at f/4.5, so throwing the background out of focus is much easier.

[jml79]

You are also not really comparing apples to apples. The 50mm is a prime lens and doesn't suffer any of the compromises of a zoom lens, the 18-200 has a huge zoom range and not only would it suffer some, it would suffer a lot of the disadvantages of a zoom lens. There is a reason you can't find any really huge zoom ranges in the higher end lenses or in a really fast lens. There are just to many quality compromises.

Your 18-200 is a lens meant for convenience not quality (not that it's bad). It is great for family vacation and learning and playing but if you can take your time and control the situation the 50mm will give a better chance at taking a better picture (careful wording here, great lenses can still take awful pictures if you do everything worng).

Of course it is also faster and will let you take pictures in less light and with less DOF like inkista said.