The focal length of lenses used on cameras has the single most effect on the outcome of image making. Not only does the focal length used determine the perspective of the image (Angle of View), it also intrinsically controls the depth of field.

About Lens Lengths

The length between the plane of the film in the camera and the focusing glass element in the lens is called the ‘Focal Length’.

Lenses of shorter focal length are called wide-angle lenses, while longer focal length lenses are called telephoto lenses.

Generally, wide angle lenses are faster lenses, and offer a greater depth of field. Conversely, telephoto lenses use more light (because they in essence ‘magnify the light’) and are generally ‘slower’ lenses. Telephoto lenses also inherently yeild a lower depth of field.

Wide angle lenses

Wide angles lenses are those with smaller focal lengths. Common wide-angle lenses for a full-frame 35mm camera are 35, 28, 24, 21, 18 and 14 mm. These lenses will produce a more or less rectilinear image at the film plane (though some degree of barrel distortion is not uncommon here).

Normal lenses

In still photography, a normal lens is a lens whose focal length is roughly equivalent to the diagonal of the image projected within the camera. This roughly approximates the perspective perceived by the human vision. a 50mm lens is generally considered to be the ‘normal’ lens on a 35mm camera.

Telephoto lenses

Telephoto lenses are ‘long lenses’ whose focal lengths range from 70mm upwards (85mm, 100mm, 200mm, 300mm… ). By compressing space (and thus depth of field) the subjects can appear closer to the camera and appear with a reduced Angle of View.

The Speed of lenses

Wide angle lenses are able to be made with bigger apertures (‘faster’ because you can open to a bigger aperture and there for use a ‘faster’ shutter speed) than telephoto lenses. This is mainly due to the larger amounts of light required to ‘magnify’ the light in longer lenses.

Zoom vs. Telephoto

Zoom lenses refer to an lens whose focal lenght can be adjusted. The confusion between these two terms is usually because many Telephoto lenses are also Zoom lenses (70mm – 200mm lens).

Some Zoom lenses, especially with a wide zoom range, often have a variable aperture. This means that when your Zoom lens is at it’s widest focal lenght it will have a bigger aperture avaialble that when the zoom lens is fully extended. (Remember, longer focal lengths require more light, so they do not have large apertures, so they cannot ‘open up’ as much, and are therefore ‘slower’ lenses.)

Focal Lengths Examined

The following images were made with the same shutter speed and aperture, all while standing in the same place. Notice the reduction in Depth of Field as the focal length increases.

20mm

1/200 sec @ f/5.0

50mm

1/200 sec @ f/5.0

In photography (and cinematography) a normal lens is a lens that generates images that are generally held to have a “natural” perspective compared with lenses with longer or shorter focal lengths.

Consequently 50mm lenses are often the ‘fastest’. Some 50mm lenses can be as fast as f/1.0

70mm

1/200 sec @ f/5.0

100mm

1/200 sec @ f/5.0

200mm

1/200 sec @ f/5.0

White Balance

The human eye adjusts it’s exposure (by dilating or contracting the pupil) subconsciously. It also adjusts very well a wide variety of color casts created by various types of light sources.

There are three main color sources in the minds of photogrphers:
• Daylight (Sunlight, Natural Light)
• Indoor Light (Tungsten)
• Flourecent Light

Each light not only has different qualites but each light soruce also has differnt color temperatures. These different temperatures create different color casts. While our eyes are able to adjust by themselves, photographers need to be consous of the light in a scene and adjust accordingly. This requires changing to the proper light balanced film, or adjusting the *white balance on digital cameras.

White Balance Effects

Incorrectly color balanced photos will have a strong color shift.

• Daylight (Sunlight, Natural Light) = Blue Color Cast
• Indoor Light (Tungsten) = Orange/Warm Color Cast
• Flourecent Light = Green Color Cast

Natural light with indoor (tungsten) white balance: 1/15 @ f5.6; ISO 320. Additionally, note that the image properly exposed the light in the window, forceing the darker foreground inside to be underexposed.

Natural light with a proper daylight white balance: 1/15 @ f4; ISO 800. In this image, note that the foreground inside was properly exposed, forcing the brighter areas of the scene in the window to be overexposed.