camera Lenses in photography

 Introduction to Focal Lengths

This introduction will give a brief overview of the most common focal length categories and their characteristics.  Later posts will cover each focal length separately and in more detail.

The focal length of a lens refers to the actual length of the lens from a point inside its front lens element to the point where it comes to focus -- in our case, that is on the camera sensor -- when the lens is focused at infinity.  Modern optical systems and zooms can make this calculation more exacting, but this definition is sufficient to understanding the elementary difference between lenses and how to use them creatively.  Focal length is typically expressed in millimeters for modern DSLR lenses. 

What we call a "normal" lens is one where the focal length is approximately equal to the diagonal of the sensor plane.  On a full-frame (35mm) sensor that would be a 50mm lens.  On the smaller APS sized sensor that would be a 35mm lens.  All other focal length categories are given relative to what is "normal".  For instance, a wide angle lens is one that is wider that a normal lens, while a telephoto lens is longer than a normal lens.  

Wide angle lenses take in a larger visual area that makes them most suitable for wide landscapes, while telephotos act more like telescopes for close-up viewing of distant subjects.  These are the obvious, practical reasons for selecting one focal length over another.  More importantly, however, are the creative characteristics each focal length has.  

Wide angle lenses:

A wide angle lens is one with a focal range from around 14mm to 35mm on a full frame sensor, or 10mm to 28mm on an APS sensor.  The smaller the focal length, the more area the lens covers.  A 14mm lens (10mm on APS) would cover a viewing angle of 114° (81° with APS).  On the other, longer end of the wide angle spectrum, a 35mm lens would have an angle of view of 63°  (44° with APS).

Normal lens:

The normal lens of 50-60mm on a full frame camera (35mm on an APS sensor)  is most closely related to the natural perspective the human eye has of the environment.  It is best used when you want to recreate a scene that feels natural, as if the viewer is actually part of it.  In a creative sense, a normal lens places the viewer in the scene without drawing attention to the fact that the camera was even present.  It downplays the role of the photographer in the scene

Portrait Lens:

A "portrait" lens is so-named because it causes the most natural perspective on the human face.  In addition, it has a slight telephoto effect that throws the background gently out of focus.  This serves to put more attention on the main subject.

Telephoto Lens:

Telephoto lenses range from 135mm on up.  The upper practical limit of the telephoto range is 200-400mm (135-300mm in APS), but even longer  lenses can be had at 600-800mm.  In addition to magnifying distant objects, telephoto lenses have the drawback of also magnifying camera movement.  A shutter speed that might be sufficient to stop camera motion with a 50mm lens needs to by three times greater to stop the same motion with a 200mm lens.  In other words, every time you double the focal length, you will need to increase the shutter speed the same amount.  For instance, 1/250 second with a 100mm lens is equivalent to 1/500 second with a 200mm lens.

Fortunately, most modern telephoto lenses are equipped with a vibration reduction technology that can effectively freeze the action for you and allow lower shutter speeds to be used hand-held with long lenses.  Still, it is best to use very long focal length lenses on a tripod to minimize motion blur by increasing steadiness.  With extremely long telephotos it is almost impossible to steady them without a tripod or monopod.

Conclusion:

The real reason for choosing one focal length over another is not necessarily the practical one of simply including more or less of a scene, or putting you closer to your subject.  A more important point to consider when choosing a focal length is how it can contribute creatively to your image.  When you want to bring the viewer right into the action, a wide angle lens used up close would be the best choice. The compression effect of a telephoto lens will relate a foreground to background in a very different way than the expanse created by a very wide angle lens. You might want to switch to a normal or portrait lens in situations where you want to illustrate the subject in a more natural way.