Photography Lesson: Focal Length

The focal length refers to the distance from the lens to the point where the object becomes focused.  The unit of measure is in millimeters.  Focal length is also a measure of how strongly the lens focuses or diverges (defocuses) light.

  • A shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.  This results in a wider angle of view.  Inherently, short focal lenses have a larger depth-of-field.  This means that near and far objects are in focus.
  • A longer focal length or lower optical power on the other hand magnifies distant objects, has a narrower angle of view, and smaller depth-of-field. This means that you get a clear image of your subject while the background remains blurred (or out-of-focus).

Short Focal Length

Long focal length

Remember those lenses you see: 18mm, 35mm, 55mm, 200mm and so forth?  That tells you the focal length of the lens. For example: an 18mm lens means that the object comes into focus 18 mm from behind the lens; a 55 mm lens means that the object comes into focus 55 mm behind the lens.  Also notice that the longer the focal length, the longer the lens tube is (compound lenses excluded, of course).

Pentax 40mm and 70mm lens

The human eye has an equivalent focal length of 50mm. This is why a 50mm lens is called a standard lens–suitable for a variety of subjects from portraits to landscapes.

Lenses with focal lengths less than 50mm are called wide angle lenses because they fit more of the view into the photo.  For example, if you used a 25mm lens, the diagonal field of view is twice (2x) that of a standard (50mm) lens.  This is useful when you need to take a shot of a wide view (like a building or a scenery) or a large group and you can’t step back any further.  The common focal length for wide angle lenses is 28mm. This is ideal for landscape and architectural shots.  Focal lenghts less than 20mm is known as ultra wide angle lenses. 

In the photo below, you’ll see the effect of focal length in terms of the area captured.  The subjects are the same.  With a shorter focal length (left photo), you see more of the area surrounding the subjects, but the subject appear farther.  With a longer focal length (right photo), you see less of the area surrounding the subjects, and they appear nearer.

Focal length: 24.4mm

Focal length: 42.5mm

While wide angle lenses capture more of the view, they do distort the edges of the photo.  This can be used to exaggerate subjects for special effect.  There is a special type of lenses called fisheye lenses that really distort the image—it’s like looking through a water droplet.

Sample Fisheye Photo (Courtesy of http://joiedevivre79.wordpress.com/tag/lomo/)

Lenses with focal lengths greater than 50mm are commonly known as telephoto lenses.  While the focal length increases, the angle of view decreases (less of the background is captured).  This is ideal for getting close-up shots of distant objects.  The shallower depth-of-field brings to focus the subject and blurs the background–this is ideal for taking portrait shots.

Lenses with focal lengths from 85mm to 135mm, also known as short telephoto lenses, are ideal for taking portraits. For sports and wildlife shots, it’s better to use lenses with focal lengths of 200mm to 300mm.  However, professional sports and wildlife photographers use lenses with focal lengths of 600mm or more.

To give you an idea of the field of views available for different focal lengths, look at the chart below.  Above each shot, you’ll see the focal length (in mm), and the field of view (in degrees).  Notice that as the focal length increases, the object gets nearer and view becomes tighter (more confined).

Field-of-view available per focal length

Imagine in the days of old when you needed to change lenses for macro photography, normal photography and telephotography.  To solve this problem, lens maker made compound lenses.  These lenses incorporated lenses of different focal lengths into a single tube!  Imaging not needing to change lenses for taking macro shots, normal shots and telephoto shots.  Since lenses are quite expensive, it is wise to invest in a lens that covers a wide range of focal lengths such as a 28-200mm lens. Look for one that incorporates a macro lens as well.

Next Lesson:  The Camera

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s