Bong S. Eliab
School of Arts and Sciences
Ateneo de Davao University
Syllabus | Notes | Papers | Projects
(FYI; you won't be tested on the technical material in this paragraph) Lenses work because light travels more slowly in glass than in air. A beam of light striking a block of glass at an oblique angle is slued around as it enters the glass. This is because the edge of the beam that enters the glass first is slowed down significantly sooner than the far edge of the beam. This bending of light is called refraction. A lens is like a series of prisms stacked up. The prisms in the center of the lens have parallel sides, and closer to the edge of the lens, the prisms have steeper angles. Light passing through near the center of the lens is only bent a little; further from the center, light is bent more. Glass bends different colors by different amounts; this is called chromatic aberration. On a very cheap lens, these different colors do not meet at the same point of focus.
Cameras are only as good as the optics they deliver. Don't buy an expensive camera body, then stick a cheap lens on the front; in fact, do the opposite-invest your money in lenses. Cheap, scratched or smudged filters further degrade your image. If your budget is limited, consider buying a slow (f/4.5) but high-quality name-brand lens. You can often find good bargains on camera equipment through such sources as Shutterbug magazine or the Internet (of course, caveat emptor).
With a 35mm camera the "normal" lens (a lens that shows the subject roughly as the human eye views it) has a focal length of 50mm. Focal length depends on the size of the film. The larger the film, the longer the focal length of the lens. In general, standard lenses for each film format are roughly equal to the film's diagonal. A standard lens is a camera workhorse. It is usually very well corrected and therefore produces sharp, clear images; it is compact and light, and has a large maximum aperture (i.e., it's fast and great for low-light situations). The standard lens is good for full-length portraits of people and for still life work, because of the minimal distortion.
(With a 2 1/4" camera the "normal" lens is an 80mm, and with a view camera the "normal" lens is a 165mm. There are no zoom lenses for large formats. For landscape work, most photographers shooting large-format start with two lenses. For their "normal" lens they choose one of the following: 135mm, 150mm or 210mm. By choosing either the 135mm or 150mm, you get a lens that will also work for close-up photography without needing an extension bellows and rail.. The other commonly purchased lens is a 90mm wide angle. )
Wide-angle lens (usually 20mm to 35mm) have a tremendous inherent depth of field, and thus are great in situations where you need lots of depth of field. They exaggerate the space between close and far objects, lending a feeling of spaciousness. A wide-angle lens will give you a new look at an ordinary subject. Wide-angle lenses are often used for landscapes and by photojournalists.
Wide-angle lenses must be used with care, as they distort images. Tilting a wide-angle up or down creates converging or diverging lines out of lines that are parallel. If this is what you want, do it. But sometimes this can be a problem. Also, people's heads at the edge of a wide-angle frame can be extremely distorted. In general, wide-angle lens are not suitable for flattering portraiture. If you choose to shoot a full-length portrait, get down at about the subject's waist level, keeping the lens parallel to the subject's body plane. Wide-angles are great for shooting architectural interiors (again, watch out for those converging/diverging lines) as they give a wonderful sense of space.
Flash fall-off is a problem with wide-angles. Also, filters can cause vignetting. Also be aware that you can get sky gradations that can be a problem.
Although they are usually heavy and bulky, telephoto lenses are ideal for using in many situations. Telephotos not only get you close to faraway subjects, they (perhaps more importantly) compress foreground and background and have an inherently shallow depth of field. Moderate (85 to 200mm) lenses are great for portraiture. Such lenses flatter most peoples' faces and blur backgrounds, thus eliminating distractions. Use telephoto lenses to isolate and foreshorten elements of a landscape. And of course, large (300mm and up) telephoto lenses are a necessity for sports and wildlife photography.
Be careful of inexpensive zoom telephotos. They're usually very slow and many of them just aren't sharp. When purchasing your first telephoto, start out moderately, for example in the 135mm to 200mm range.
Telephotos must be used with care. Following the rule for hand-holding a lens, you can't easily hand-hold a 300mm, for example, at shutter speeds slower than 1/250. Thus, you must steady that telephoto lens as much as possible to get sharp images. Ideally, use a high-quality tripod and cable release.
Until a few years ago, professional photographers stayed away from zooms because the quality just wasn't very good. That has changed, but as usual, you get what you pay for. However, you can get a fairly good quality zoom that's slow, with a variable aperture at wide open (e.g., at an 85mm setting the zoom will have a f/4.5 maximum aperture and at 35mm it will have a setting of f/3.5.). Disadvantages of zooms are that they are larger and usually slower than fixed-focal-length lenses.
A moderate zoom,with, for example, a 35mm to 70mm range,
can handle perhaps 90 percent of your photographic needs.
Tele-extenders, also known as tele-converters or doublers, effectively double the focal length of your lens. Others extend your focal length by a factor of 1.4. Thus, with a doubler, a 300mm lens can become a 600mm lens, or a 50mm can become a 100mm. They are small and thus extremely portable. However, they have several disadvantages: Stay away from cheap tele-converters; you'll be unhappy with the quality. Tele-converters lose light (1 to 2 f/stops). Lower contrast also can occur. Check the features before you buy. If auto-focus is important to you, make sure you match a converter with that capability to a lens that will match with the tele-converter. Quality tele-extenders can cost $300 or $400. Another disadvantage is that because you're working with such a long focal length and losing light besides, you must use slow shutter speeds. Sometimes this problem is almost impossible to overcome without using fast (and therefore lower-quality) film.
Perspective Control Lenses
These highly specialized lenses allow you to overcome the convergence/divergence problem, especially noticeable in wide-angle lenses. PC lenses are also effective for increasing your depth of field.
If you have color-coded markings for depth of field on your lens, you can maximize your depth of field. First, set your background focal point (usually infinity) on the color-coded mark for the f-stop you want to use. In this position, the focusing index marks the hyperfocal distance. The corresponding f-stop mark on the other side of the focusing index points to the closest distance of the depth of field. Rotate this point to the focusing index and then move the camera, without refocusing or stopping down, until the closest part of your scene is sharp.
(The instructions below are from Steve Traudt of Grand Junction, Colorado via Outdoor Photographer columnist George Lepp)
Ideally you want a lens to be sharp from its closest possible point to infinity. If you're lucky enough to have an older lens with depth of field marks on the lens, this focus point is the hyperfocal distance for the lens at that f-stop.
To use the chart below, set the focus of the lens at the foot scale below the f-stop chosen. Do not refocus the camera even though it will look like the subject area is out of focus. You can expect the zone of sharp focus to extend back half the distance to the camera and from there forward to infinity. For example, the chart says to set the focus for a 28mm lens at f/22 and an indicated five feet. You can then expect the sharpness to extend from 1 1/2 feet to infinity. At f/32 with the same lens you would gain half a foot of depth of field by being sharp from two feet to infinity.
(This chart is for critically sharp pictures with a criteria of a "circle of confusion" of 0.001 inch.) You can have a larger circle of confusion (less sharpness) and use a closer distance than indicated above.
Syllabus | Notes | Papers | Projects
All Rights Reserved 2001
Ateneo de Davao University
10 June 2002