One of the benefits of owning a DSLR is having the freedom to change lenses according to your needs. We’ll look at the basics of what a lens is all about from focal length, to aperture, to wide vs telephoto, to demystifying all those strange symbols. Today, let’s look at focal length.

Focal Length

When light rays hit a lens, they converge at a single focal point. In it’s simplest sense, the distance from the middle of the lens to where the lines converge is the focal length of the lens denoted in mm. Lenses range from 10mm at the wide end up to 300mm for a consumer telephoto lens.

There are two types of lenses:

Prime - These lenses have a fixed focal length. As such, they usually have a larger aperture than zoom lenses. Some notable lenses include 30mm f/1.4, 50mm f/1.8 or f/1.4, 85mm f/1.8 or f/1.4.

Zoom - A zoom lens has the ability to vary its focal length and as a result, can give you different angles of view.

Lens manufacturers produce many different ranges of focal length but they usually fall under three categories:

Wide Angle - Lenses wider than 50mm (eg. 10-22mm, 14-24mm)
Normal - 50 mm
Telephoto - Lenses longer than 50mm (eg. 70-200mm)

You may have seen the words “35mm equivalent” when talking about focal length. This is simply a comparison to 35mm film cameras, not to be confused with focal length of a lens. (The film used in 35mm cameras were, surprise surprise, 35mm wide!)

So why do we need to bother with knowing the 35mm (full frame) equivalent? It’s used more as a standard so we can make comparisons. Most DSLR’s have a crop sensor (except for full frame DSLR’s like the Nikon D3, Canon 1Ds and 5D) meaning compared to a 35mm camera, we only see a crop of the actual field of view. Different camera companies use different crop sizes. The crop value is referred to as the Focal Length Multiplier(FLM). My Nikon D300 has a crop factor of 1.5. This means that compared to a full frame camera like the D3, I see 50% less with my D300 than I would with a D3.

So, if I am using a 50mm lens on my D300, that would be equivalent to using a 75mm lens on a D3 because 50mm x 1.5 = 75mm.

I found this great drawing here that shows you the differences between a full frame and crop sensor. This one uses a crop factor or FLM of 1.6, which is what Canon uses.

The RED box represents a full frame sensor and the BLUE box a crop sensor.

Now, don’t get confused with all the numbers. A 50mm lens is still a 50mm lens regardless of which camera body it’s used on. The focal length doesn’t change but the field of view varies according to the sensor size.

Here’s an example:
To get the same field of view as a 50mm on a D300, I would need a 75mm lens on a D3. (50mm x 1.5 = 75mm)

Or, to get the same field of view as a 15mm on a D3, I would need 10mm on a D300. (15mm / 1.5 = 10mm)

So what does this all mean to you? Well, if you have a crop sensor body, which most of us do, you will have more “reach” or telephoto than if you were using a full frame body. Most photographers like to use an 85mm lens (on a 35mm camera) for portraits. If you have a crop sensor body, you can get a 50mm lens and it will be close to 85mm on a 35mm film camera. Therefore, you will like using your 50mm lens as a portrait lens.

Next up, lenses and aperture!

Laura is a Vancouver wedding and portrait photographer. Visit laurahana.com.