Focal Length

• For equivalent perspectives, working distance is determined by the 35 mm equivalent focal length of the lens. The longer the focal length of the lens, the longer the working distance.
  
  A 50 mm focal length is considered a normal perspective. A 100 mm focal length gives a 2x magnification of a normal perspective, and a 25 mm focal length gives an 0.5x magnification. This means that to achieve the same perspective as a 50 mm focal length, a photographer would have to be twice as far away from the subject with a 100 mm focal length, or twice as close with a 25 mm focal length.
  
  The photographer adjusts working distance by adjusting the focal length of the lens. For example, if the working distance is too short, the photographer chooses a longer focal length and backs up, maintaining the same perspective.
  
  

Crop Factor

• Digital photographers have to take into account the crop factor of their particular camera when calculating 35 mm equivalent focal lengths.
  
  A link to sensor sizes and crop factors is listed in Resources. Olympus four thirds DSLRs, for example, have a crop factor of 2x. That means that a 50 mm lens mounted on an Olympus DSLR has a 35 mm equivalent focal length of 100 mm.
  
  Calculating 35 mm focal lengths is especially tricky with the small sensors found in point and shoot digital cameras. Fortunately most such cameras will list the range of equivalent focal lengths on the lens.
  
  

Macro Photography

• Macro photography is close-up photography. Sufficient working distance is critical in macro photography, both to avoid intimidating subjects and to ensure correct lighting.
  
  Many macro subjects, such as insects, will not allow photographers to get too close. For this reason, a longer focal length lens may be necessary.
  
  Even when subjects allow macro photographers to get close, lighting may be an issue. The camera lens may block ambient light and the on-camera flash. The solution is to use a longer focal length lens that allows greater working distance.
  
  Inexpensive extension tubes convert any long focal length lens, even those not meant for macro photography, into macro lenses. The lenses will not focus to infinity with the extension tubes, but the tubes can be removed when infinity focus is desired.
  
  

Portrait Photography

• Many photographers consider 50 mm lenses to be too short for portrait photography, as 50 mm lenses necessitate the photographer being so close to the subject that the subject may feel uncomfortable or unnatural.
  
  A more common focal length range for portraits is 70 mm to 135 mm, which allows the photographer more working distance than a 50 mm lens.
  
  

Shutter Speed

• Most photography is hand held. A rule of thumb for minimal shutter speed in hand held photography is 1/focal length. At slower shutter speeds photos may blur. For example, shutter speed should not drop below 1/50 second for a 50 mm focal length or 1/150 second for a 150 mm focal length, when the camera is hand held.
  
  Shorter focal length lenses give photographers more options for shutter speed, and correspondingly more options for aperture and ISO. Shorter focal length lenses are preferred to longer lenses wherever possible, unless some special effect of a longer lens, such as limited depth of field, is desired.