Lens Construction
Lenses, and lens quality, are of major concern to a photographer. Lenses were originally ground by hand. Niépce and Daguerre shared the same lensmaker in Paris. Today, lenses are machine ground and computer-designed.
The simplest way to form an image is by using a pinhole. This will form an image, but to get a sharper image, you need to use a smaller pinhole, which (greatly) increases exposure time. Pinholes are also subject to diffraction, or a bending of light, around the edges.
So, we introduce the glass lens. This allows rays of light to be focused, which gives a sharper image with a much sharter exposure time.
Compound Lenses
Contemporary lenses are invariably of the compund type. That is, they are made with multiple lens elements, combining to correct image aberrations. This is necessary because light is not single, unified source, but has different color and wavelengths.
The number of these elements, along with their inherent material quality (the quality of the glass) serve to create the quality of image. More elements mean that seperate lenses are doing their own job, rather than combining multiple tasks into one piece of glass. Better glass simply means the glass is more opticallly pure, and does not introduce its own aberrations.
There are several different shapes of lenses, or elements. Without going into the physics used for each, the main shapes are: plano convex, plano concave, converging meniscus, and diverging meniscus. Most simply put, these are combined to bend (refract) and shape the image onto the planar (flat) film surface.
Focal Length
The biggest difference in lenses is their focal length. The focal length is measured from the center of the lens to the film, or the focus point, where the lens focuses light. A shorter focal length bends light at a sharper angle. Simply looking at the difference in the shape of the outer element of a 28mm lens compared to a 50mm lens will show the 28mm being more rounded:
Short lens (left) vs. longer lens.
Focal length determines both the magnification of and image, as well as the angle of view. Angle of view is the width of the image. A ‘normal’ lens is called normal because it best approximates human vision—both magnification and angle of view—for a particualar film format. A short lens (shorter focal length than normal) will have an extended angle of view, but lower magnification, while a long lens will have the opposite, a compressed angle of view with a higher magnification.
