7.3 NORMAL LENSES
 
 
 
 
 
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A "normal lens" for a 35mm camera usually refers to a lens with a 50mm focal length. On a digital camera, an equivalent lens will have a much smaller focal length because image sensors are much smaller than 35 mm film. When you zoom your lens and look at the image on the LCD monitor, the scene looks about the same as it does to the unaided eye. Looking at the LCD monitor with the lens zoomed all the way out makes everything appear closer than it actually is. With it zoomed out to a wide-angle, everything looks farther away.

burial_hill_tree.jpg (26073 bytes) A normal lens gives you the feeling that you're looking at the scene with your own eyes.

A normal-focal-length (50mm) lens isn’t necessarily the one photographers normally to use. Many photographers prefer the wider angle of view and greater depth of field provided by a slightly shorter focal length.

Changing Apertures

A lens’ maximum aperture is determined by dividing the actual diameter of the aperture opening into the focal length of the lens. That’s why the aperture might change from f/2.6 when zoomed out to f/4 when zoomed all the way in on a subject.

 

little_harbor.jpg (30560 bytes) It’s hard to look at a photo and tell what focal-length lens was used to take it. However, objects in an image taken with a normal lens look normal in their spatial relationships.

 

See for Yourself

A lens is called normal because it captures a scene just as the human eye does. This seems to violate common sense, because the eye’s angle of view is much wider than any normal lens. However, you can demonstrate for yourself why a specific focal length is normal for your camera. If you are a passenger in a car, try zooming the lens as you watch the traffic ahead on the LCD monitor. The longer focal lengths make distant cars appear right on top of you; in reaction you might even try to put on your brakes and then discover the cars are nowhere near as close as you thought. With shorter focal lengths, cars look far ahead, even when relatively close. A normal focal-length makes the cars appear in the same distance relationship as you perceive them ordinarily.

Another demonstration is to take two photographs of greatly different size and tape them to a wall. Look at them one at a time on the LCD monitor with the lens zoomed to a normal focal-length a little above it’s widest angle of 28mm. Move close enough so each fills the LCD monitor. You’ll discover you are at the correct distance for viewing the prints. With a longer focal-length you would feel too far away, and with a shorter one too close.


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