Hi, I'm Joe. I teach photography at the School of Visual Arts in New York. I'm going to talk to you a little bit about choosing a good portrait lens.
The normal lens for your camera, whatever is the normal focal length, the focal length that shows things in a size perspective that's approximate to human vision is actually equal to the diagonal measure of the film plane of the size of your sensor in the camera. And usually a good portrait lens is somewhere between 1-1/2 to 2 times whatever is normal for your camera.
For a full frame camera, that normal dimension, that diagonal dimension, is about 43 millimeters and most people sell you a 50 millimeter lens as normal.
So if you've got a full frame camera, really anything from about 75 millimeters to 105 or 110 millimeters is a good focal length for a portrait lens.
Portrait lenses are different than wide angle lenses. Wide angle lenses, they tend to distort things a bit and make things curvier and you don't want to use that for somebody's portrait or you end up with a really, really big nose and a smaller head and lots of extra sort of weight and fatness that people don't really thank you for.
Longer lenses give you the opposite of distortion. They give you something called compression. And most people look good with a little bit of compression on their features.
Most digital cameras aren't full frame cameras and unless you're spending a lot of money on a camera, your DSLR is probably using something called an APS sensor. An APS sensor conforms to the size dimensions of something called APS film which doesn't really exist too much anymore. But APS film is smaller than the full frame sensor and the diagonal of that sensor is about 29 millimeters.
A good portrait lens for your APS sensored camera is going to be somewhere between 42 and 56 millimeters. Somewhere around there.
A lot of the zoom lenses that come with the camera actually rack from about 18 millimeters to 55 millimeters so if you just use that zoom lens all the way at the 55 millimeter end, you've got a pretty decent portrait lens right there.
If you're using a zoom lens with a camera for a full frame camera, you just need a zoom lens that goes, you know, beyond 75 millimeters and you can take it anywhere between 75 and 105 and 110 and that will be a good place for taking portraits.
With your APS camera, it's very often the case that the kit lens that comes with the camera which ranges somewhere between 18 and 55 millimeters, if you just rack that lens all the way out to 55 millimeters, you're in a good place to shoot portraits and you should just leave it there.
If you find that you're not close enough or you're too close, instead adjusting the zoom on your camera, walk back and forth so that you leave the focal length at the right place to give you that portrait lens compression without changing the focal length and take your chance on bringing it back to the wide end of the spectrum, which is going to make your subject distort and give them the big clown nose, and otherwise do all sorts of unflattering things.
And those are a few tips for choosing a good portrait lens.