As discussed earlier, there are many different types of lenses available to a photographer to create unique and different images. Also, in conjunction with the other attributes of a camera (aperature & shutter), lenses work to create a sharp, focused image from a variety of distances. A very common camera is the Single Lens Reflex or the SLR. This camera combines the physics principles behind lenses and mirrors to take focused pictures that appear the same as they will record on the film. You can put different lenses on the SLR camera to get different focal lengths and thus differnt focuses. Essentially the light entering the camera goes through the compound lenses and bouces of a morror through a pentaprism to the viewer's eye while they are choosing the object to take a photo of. The shutter button lifts the viewing mirror while opening the shutter, to allow the light to hit the film and form a negative.
Prime and Zoom Lenses
Lenses come in either prime or zoom. Prime lenses have a fixed focal length. In order to fit more or less into the viewfinder, the photographer must physically move forward or backward. Focal length in a zoom lens can be changed by turning the zoom ring on the lens. A zoom will have a variable focal length such as 28mm-105mm. While zoom lenses are more convenient for storage and versatility, prime lenses often have a sharper lens and work better in low-light situations. In addition to focal length, the lens speed and its focusing distance should be considered. The lens speed is determined by the f-stop setting. A faster lens will be heavier and more expensive. The focusing distance is the distance from the lens to the subject needed to achieve a focused photograph.
Lenses have truly revolutionized certain industries. The telephoto lens is often used in espionage and spying as you can be far away from a subject and observe great detail. The macro lens is incredibly useful for CSI forensics as thay can take permanent pictures on crime scenes of tiny pieces of evidence that they can later compare. The macro lens is applied to medical reseach as it is used in numerous optical instruments for medical purposes. Macro and telephoto lenses are commonly used in nature photography to get close-ups of animals that a person can't approach or to examine tiny specimens in detail. Telephoto lenses with high speed shutters make action moments in sports photography easier to capture.
Issues with Camera Lenses
Given camera lenses are somewhat thick (at least thicker than a Thin Lens) they do have a few challenges that affect the created image (real image) which appears on film. These challenges come in the form of aberrations which are created by the way light reacts with the convex lens and bends.
Different colour rays of light bend at different angles and reach a focal point somewhat different than another colour ray of light. This creates a soft fuzzy image for some of the colours that are not reaching their focal point at the film plane (see diagram 1). Photographers try to implement lenses that have special features that adjust this aberration by aligning the focal point of two of the colour rays (red and blue) or three of the colour rays (red, green, blue). These Achromatic (two rays) or Apochromatic (three rays) lenses have vastly improved the camera’s ability to provide sharp and clear images in colour film photography (see diagram 2).
All lenses with spherical surfaces have imperfections as to how light passes through them and reaches a common focal point. As a result of these miniscule deviations from a perfect lens the light rays that enter towards the edge of a convex lens (further away from the optic axis) don’t reach the same focal point as rays that pass through closer to the center of the lens (along the optic axis). This is shown in the diagram below (See diagram 1) and is somewhat exaggerated to prove the point. Camera lenses are designed with the minimum of spherical aberration but some imperfections do exist and affect the resulting image.