Camera Lenses
  
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Camera Lenses


Camera lenses are undoubtedly one of the most important inven­tions in history -- it has truly transformed how people conceive of the world. Now we can "see" all sorts of things that are actually many miles -- and years -- away from us. Photography lets us capture moments in time and preserve them for years to come.


The basic technology that makes all of this possible is fairly simple. A still film camera is made of three basic elements: an optical element, a chemical element (the film) and a mechanical element (the camera body itself). As we'll see, the only trick to photography is calibrating and combining these elements in such a way that they record a crisp, recognizable image.

There are many different ways of bringing everything together. In this article, we'll look at a manual (SLR) camera. This is a camera where the photographer sees exactly the same image that is exposed to the film and can adjust everything by turning dials and clicking buttons. Since it doesn't need any electricity to take a picture, a manual SLR camera provides an excellent illustration of the fundamental processes of photography,and camera lenses.

The optical component of the camera are the camera lenses. At its simplest,the camera lenses are just a curved piece of glass or plastic. Its job is to take the beams of light bouncing off of an object and redirect them so they come together to form a real image -- an image that looks just like the scene in front of the camera.

But how can camera lenses do this? The process is actually very simple. As light travels from one medium to another, it changes speed. Light travels more quickly through air than it does through glass.

When light waves enter a piece of glass at an angle, one part of the wave will reach the glass before another and so will start slowing down first. This is something like pushing a shopping cart from pavement to grass, at an angle. The right wheel hits the grass first and so slows down while the left wheel is still on the pavement. Because the left wheel is briefly moving more quickly than the right wheel, the shopping cart turns to the right as it moves onto the grass.

We've seen that a real image is formed by light moving through a convex lens. The nature of this real image varies depending on how the light travels through. This light path depends on two major factors:

The angle of the light beam's entry * The structure of the lens


The angle of light entry changes when you move the object closer or farther away. You can see this in the diagram below. The light beams from the pencil point enter at a sharper angle when the pencil is closer to the camera and a more obtuse angle when the pencil is farther away. But overall, the glass only bends the light b­eam to a certain total degree, no matter how it enters. Consequently, light beams that enter at a sharper angle will exit at a more obtuse angle, and vice versa. The total "bending angle" at any particular point remains constant.


As you can see, light beams from a closer point converge farther away from the lens than light beams from a point that's farther away. In other words, the real image of a closer object forms farther away from the camera lenes than the real image from a more distant object.

You can observe this phenomenon with a simple experiment. Light a candle in the dark, and hold a magnifying glass between it and the wall. You will see an upside down image of the candle on the wall. If the real image of the candle does not fall directly on the wall, it will appear somewhat blurry. The light beams from a particular point don't quite converge at this point. To focus the image, move the magnifying glass closer or farther away from the candle.


This is what you're doing when you turn the lens of a camera to focus it -- you're moving it closer or farther away from the surface. As you move the lens, you can line up the focused real image of an object so it falls directly on the surface.

You now know that at any one point, a lens bends light beams to a certain total degree, no matter the light beam's angle of entry. This total "bending angle" is determined by the structure of the glass.


Getting the Most From a 50mm Lens


The 50mm lens is unique in that it can produce images that, under the right conditions, seem to have a modest wide angle perspective, and under other conditions have the isolation of a short telephoto lens. Thus it is a very versatile companion for many kinds of photography. When using a zoom lens, you often stand still and twist the zoom ring until you find a composition that works, a lazy approach at best. when using a prime lens you will learn to "zoom with your feet," getting closer to your subject to eliminate distracting background, or backing away to include more context.

I believe this is good training in learning to see effectively with the camera. When using prime camera lenses you become more aware of the viewfinder as a compositional frame, and the various effects that a change of focal length provides. In fact, after using prime lenses for a while, the pictures you make with your zoom lens will improve, as you will have a better understanding of how focal length affects composition.

Who knows, after working with a "classic" 50mm, you might even (horrors!) experiment with manual focus, or learn how to meter the scene manually. Getting back to basics and learning how to gain back control from the camera can be one of the more rewarding aspects of improving your photographic skills.


Which Camera Lenses Should I Buy?


50mm prime camera lenses come in a breathtaking range of speeds, focusing options, and prices. Canon, for example, offers the EF 50mm f/1.8 II for about $75 street, while the EF 50mm f/1.0L is almost $3,000! Other makers continue to offer both autofocus and manual focus versions in various speeds. There are even 50mm macro lenses that can be substituted for a standard lens, but these lenses usually have smaller maximum apertures that mitigates their advantages over a zoom.


If you can afford it, the best combination of speed and performance is usually found among the manufacturers' 50mm f/1.4 offerings. These camera lenses can be used in very low light, are usually quite well made, and can cost several hundred dollars or more. My Canon 50mm f/1.4 USM is one of the sharpest lenses I own.

However, if you are just starting out or are on a budget, my recommendation is too look into your camera maker's entry level 50mm lens. These camera lenses, with maximum apertures of f/1.7 or f/1.8, are 1/2 stop slower than the f/1.4 lenses, but still very fast compared to a zoom. You can obtain 95% of the benefits of the f/1.4 lenses at only 1/3 to 1/4 the price (usually under $100). Be aware, however, that some of these lenses are more cheaply constructed, possibly with plastic lens mounts. Canon made two versions of the a 50mm f/1.8; the first version with metal lens mount (picture at the top of this page) is more desirable and can often be found used.

Issues When Shooting With Fast Camera Lenses


Accurate focus is essential when shooting with a fast lens at wide apertures. The wider the aperture the shallower the depth of field (or zone of apparent sharpness) in the photograph. You will need to learn to use the AF system of your camera to make sure the area you want is sharp (for example, your subject's eyes) even if it is not in the center of the frame. Usually this is done by first locking in an auto focus setting, then recomposing. However, with fast moving subjects you may not have the time to do this. If your camera has multiple focusing points, you can try selecting one and keeping it over your area of critical focus. Or you may want to experiment with use of manual focus (which I do quite a bit). In any case, expect that a percentage of images will be rejected due to soft focus. Don't worry, this happens even to professionals; with practice your averages will improve.


Another issue is camera automation. If your camera has a built-in flash and "easy" shooting modes (such as "portrait," "action" and "landscape") it may beep at you in some dim lighting conditions and pop up the flash or even refuse to take a picture. Your best bet is to put the camera into a more flexible auto exposure mode, such as aperture priority (Av or A). Generally in this mode the camera will assume you know what you are doing and permit a shutter speed that might cause camera shake. (I have made sharp images with a 50mm lens as slow as 1/15th second by bracing myself against a wall or door frame.) Program (P) mode is okay too, provided the camera will permit a slower shutter speed without flash.

As of the time of this writing, most digital SLRs that accept interchangeable camera lenses have sensors that are smaller than a traditional 35mm film frame. Images made with these cameras are cropped from the center of what would be a full frame image if shot on film. This produces a "focal length multiplier", typically 1.5x (Nikon) or 1.6x (Canon). For example, a 50mm lens on a Canon Digital Rebel renders an image equivalent to an 80mm short telephoto lens on a film body.

While a 50mm lens on these cameras is still quite useful, particularly for candid portraits, the focal length that best equates to a "normal" lens on these cameras is the 35mm, becoming an effective 52-56mm lens on a typical digital body. Most vendors produce a reasonably fast 35mm f/2.0 autofocus lens that can be used quite well as a normal lens. Canon, Nikon, Leica and a few others even make high speed 35mm f/1.4 optics, at correspondingly high prices. A few vendors have adopted the so-called "Four Thirds" format (Olympus, Panasonic) which is smaller yet, producing a 2x multiplier.


Summary

If you are considering a new camera purchase or already have a camera and would like to make pictures of your family that don't look like snapshots, consider buying a classic 50mm "normal" lens instead of (or in addition to) that "consumer" . This 50mm lens will allow you to make pictures in natural light without flash that share at least some of the qualities of the very finest photographs ever made. As with any new equipment, you will have a learning curve as you figure out how to use the focal length compositionally and achieve good focus. But you may find that pictures of your friends and family begin to exhibit a much more natural and evocative quality.All camera lenses are not created equal!To find the best camera lenses for your particular model will require a little research.Camera lenses come in a wide price range and quality.

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