Choosing the Right Lens

Prime Lenses: Don't zoom, they are fixed in one position. Advantage: generally cheaper and produce more sharpness/crispness/clarity. Disadvantage: you have to switch lenses often or move around! And you may have to buy several prime lenses to cover your bases, instead of one zoom lens.

Zoom Lenses:
Advantage: you don't have to move as much to get the shot you want, and don't have to switch lenses often. Disadvantage: more expensive, heavier, less sharpness.


What's the Difference between Fast and Slow Lenses?

"Fast" Lenses = you can shoot in lower light (fast lenses go to a low F-stop number, like F1.8 or F2.8)

"Slow" lenses = not great in low-light (slow lenses don't generally go to an F-stop number lower than F4 or F5.6)


Lenses are measured in length, by Millimeters (mm):

-Wide Angle (16mm, 24mm, 35mm...): show more of the scene
-Long (100mm, 200mm, 300mm...): Shoot things farther away (like a telephoto lens)
-Mid-range "normal" lenses are somewhere in between (40mm to 80mm? although it depends on the size of your sensor because lenses on full frame cameras will appear wider...see below).


Longer Lenses Condense the Background. See example image below (image credit: Lynda.com):

Image on left: 100mm lens.  Image on right: 24mm lens. (The photographer shot with a zoom lens and walked closer to the subject when taking the the wide angle shot on the right to be able to crop it the same.)

See how the background looks more condensed and closer in the image on the left? This can also be more flattering when taking portraits, as it doesn't spread the features as much (image credit here.)

Image on left: wide angle lens (24mm?). Image on right: longer lens (100mm?).

How does your sensor affect your lens? 

Cameras with a full frame sensor will make lenses seem wider. (These are generally more expensive or professional cameras so may not affect you). If you upgrade your DSLR to a full frame sensor camera, you may notice your lenses suddenly seem wider. (Some lenses may not be compatible when you upgrade).

In addition to an offered aperture (like F2.8) and length (like 50mm), lenses also contributes to the overall Color, Contrast and Sharpness of your image. Whereas these elements can be added later in Photoshop, it can often be to your advantage if it's done in the camera and offer a unique creative style.

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Summary:

My favorite lens for portraits is the 50mm f1.8. You can find it at B&H here. You generally want to use a 50mm or longer lens when shooting a portrait (I'd say 50-100mm. Some say 80mm is the best.)

It's not very wide, so you will want a zoom lens as well. The more expensive ones that go to f2.8 are the nicest and assure that blurry background. Something like one of these.

I also have a fixed 28mm f2.8 lens I love. They say prime lenses (those that don't zoom" are a bit sharper. This is a bit too wide for a portrait, but great for group shots.)