What does the mm of a digital lens signify?

Question

I'm new to the digital SLR world and I'm trying to find a good lens for portrait photography what would be the different in a 35 vs. 85?

Answer 1

Basically, the smaller the number, the wider the lens will go, therefore allowing you to photograph a bigger area, like a landscape. The higher number indicates how much the lens will zoom in, allowing a close up of distant objects.  

Answer 2

As they do in MythBusters....

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WARNING: SCIENCE CONTENT

The focal length of a lens is simply a calculation.

1/f = 1/S1 + 1/S2  where:

f = focal length

S1 = distance between the object and the lens

S2 = distance between the lens and the projected image

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Now back to the non-science stuff.  There are lots of focal length guides.  Most all of them are based on the 35mm format.  If you are using a DSLR, you need to divide the numbers by whatever the crop factor for you camera is to get the find the most ideal lens.  Some common ones are 1.5x for Nikon, 1.6x for Canon, and 2x for Olympus.  Here are some examples (in 35mm format) of focal lengths, what type of lens they are considered, and typical use.

< 21mm:  Extreme Wide Angle: Architecture

21-35mm: Wide Angle: Landscapes

35-70mm: Normal:  Street and documentary

70-135mm: Short Telephoto: Portraits

135-300mm: Telephoto: Sports, Large Birds and Widlife

> 300mm: Super Telephoto: Sports, Birds, Wildlife, Surveillance

Some of my favorite portrait lenses (I shoot Canon) are:

EF 85mm f/1.2L

EF 24-105mm f/4L IS

EF 135mm f/2L

EF 70-200mm f/2.8L IS

and now for some that are a little less expensive :)

EF 50mm f/1.4

EF 85mm f/1.8

EF 28-135mm f/3.5-5.6 IS

A 35mm focal length lens is too short on Canon and Nikon for standard portraits (but would be decent on an Olympus).  85mm is good on all the Nikons and Canons, but a little long on the Olympus.

Answer 3

The mm, or millimeters, signifies the focal length, or zoom, of the lens.  35mm is a good closer in lens, whereas the 85mm is better if you want to be a bit further from your subject.

And if you're primarily concerned with portrait photography, make sure that its f/stop is low, like f/1.8 or f/2.0.

Answer 4

As was already said, the mm is the focal length. Basically what that tells you is what you'll see through the lens.

The human eye sees at roughly 50mm. Anything with a focal length less than 50mm will see more than you see through your eyes. For example, a 25mm lens will see twice as wide as your eye. Anything with a focal length more than 50mm will see less than your eye. For example a 100mm lens will see half as much (and everything will be twice as large).

For a portrait lens you usually want something around 50mm with a large aperture. An f-stop of 1.8 or 1.4 will give you nice background blur on your portrait.