Digital Photography SLR Lens

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Digital Photography SLR Lenses

At the top end of digital cameras is the "Digital SLR Camera" or DSLR.

SLR stands for "Single Lens Reflex". In a digital SLR camera, the image displayed to the photographer through the viewfinder is the actual image that will be photographed. An assembly of mirrors and prisms reflects the image coming through the lens, and allows the photographer to see EXACTLY what (not whatever) the camera sees.

When the shutter is released (i.e. the button is clicked), the mirror (which had reflected the image to the photographers eye throught the viewfinder) is physically moved away and light falls on the light sensitive CCD element. This is a level of complexity that, ordinarily, may cause concern for service and maintenance. But in reality the mechanisms are extremely simple (at least in concept) and should never break down under normal use.

Digital SLR cameras allow enormous lattitude in adjusting exposure settings when taking pictures. Most DSLR cameras also have auto modes, some of which are useful, while others are not. Snapshot modes or point-n-click, for example, are to be avoided like bad cliches. Autofocus is a powerful and useful tool, coupled with a good lens (preferrably vibration resistant or VR) this can make for spectacularly sharp photographs. Note that most autofocus lenses will also allow the photographer to use manual focus when needed.

Most SLR cameras have interchangable lenses, allowing the photographer even more freedom in selecting a suitable lens type for any specific photo shoot.

Rather than having to invest in multiple cameras with different fixed attached lenses, DSLR cameras allow the photographer to use a single camera body and purchase additional lenses only as needed.
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DSLR lenses are attached to the camera by a "bayonet". It is essentially a screw-type lock, a release button allows quick and easy removal and replacement of the lens. Compatible lenses will offer all the automatic features (such as focus and aperture control) by connecting to the digital signal points and rotors that are part of the cameras bayonet.

Cheaper DSLR cameras may have a plastic bayonet, while more expensive models offer metal bayonets. Plastic bayonets WILL BREAK easily, especially when used with heavy lenses. Unless you intend to only use small or light lenses, avoid purchasing a digital camera with a plastic bayonet.

The term compatible can be misleading when it comes to lenses. Always scrutinize the true compatibility of a lens with your specific camera model. Some third party lenses, while cheaper and often with the same features, may not work with ALL the features that a lens made by the camera manufacturer can offer.

The ability to quickly switch lenses allows the photographer to keep many lenses handy. Depending on the photo shoot requirements, the photographer can easily change the lens and continue shooting. There are several kinds of lenses available for SLR cameras.

A Normal lens is generally a fixed magnification, usually 35mm or 50mm, although any lens that is not a macro or fisheye lens is often considered as "normal".

The Macro lens is used for extreme closeups, usually of small objects. The focal point on a macro lens allows the subject or object to be placed extremely close, providing a high quality photograph with a detail level that a normal lens cannot provide. Macro lenses come in various zoom levels, allowing for creative photography of small subjects (ants, scorpions, etc) or small objects (seeds, hotdogs, etc).
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A Fisheye lens is also known as a Wide Angle lens. With an extremely low magnification (usually under 30mm), it allows a larger area to be captured in a photograph. However, this causes rounded edges in the image, almost like looking out from a fishbowl - hence the term fisheye.

A Telephoto lens is a fixed magnification lens, usually a very large magnification (400mm or more). A telephoto lens can be considered a mini-telescope, allowing photographs to be take of distant subjects. While there are smaller magnification telephoto lenses available, they are usually not worth the cost when compared to the benefits of a similar zoom lens.

A Zoom lens offers variable magnification, either linear (that is, gradual zoom) or fixed (only specific magnification levels). Almost all lenses used on DSLR cameras today are zoom lenses, they are versatile and cheap, allowing a single lens to either take close ups (lower magnification) or distant shots (higher magnification).

Most lenses today are auto-focus. While some professional photographers grouse and grumble about the evils of auto-focus, the fact remains that it usually works pretty well! Almost all auto-focus lenses can be focused manually if needed, for example if shooting a fast moving subject/object at a fixed distance, that the autofocus system may not be able to lock on to.

The quality of the glass used (a camera lens is, in essence, a collection of glass lenses housed in a tube) dictates the quality of the photographs that a lens can deliver, as well as its price. Higher quality glass makes higher quality lenses, for higher prices.

Glass is also heavy. The more glass in a lens (i.e. the higher the magnification), the bulkier and heavier that lens will be. Many high zoom lenses are so heavy that they have to be used on a tripod, or the photographer could get serious arm and shoulder injuries holding the behemoth up.
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All auto-focus lenses suffer from a major problem, vibration. Besides the gyrating arm of the photographer under a heavy lens (which is why camera tripods were invented), the autofocus mechanism in a digital camera is, quite simply put, an electric motor. A shaft in the bayonet connects the motor to an actuator on the lens, the motor spins and moves the lens focusing mechanism. The motion of the motor and lenses produces torque, however minute, and torque causes motion. The heavier the lens, the more power the motor has to use, producing more torque. The constant adjustment by an auto-focus motor may result in blurry photographs.

Some professional photographers simply turn off the auto-focus and focus the lens manually. In some cases when the subject is at or beyond the farthest focusing point of a lens, one simply has to turn the focus ring to its maximum distance setting (infinity). However, this does not always work. Many high end lenses have VR or Vibration Resistance. The simple ones either dampen the focus action or produce counter torque, while not true VR, it may still work.

The high end VR lenses have a separate motor built in the lens. When used with a compatible camera (and note, not all VR lenses work with all DSLR cameras) - the camera will send the focusing signal to the motor in the lens via the bayonet wiring connection - and not use the cameras internal focusing motor. The lens then controls the focus action and ensures that there is no vibration due to torque or lens motion. VR lenses are often heavier due to the additional machinery built into them.

This discussion of DSLR lenses must include one important caveat. All lenses have an aperture rating of a minimum "f-stop". The wider the lens (i.e. larger surface area), the more light it can gather, the lower the f-stop. All lenses have a diaphram that can deliberately narrow and restrict the amount of light entering, this is the aperture control that will allow you to increase the f-number when needed.

The lower the minimum f-stop rating of a lens - the better its quality. Any lens rated f/3.5 or higher - is NOT the highest quality. Heartbreaking for most, to be sure, since 99% of the lenses sold today are above this threshold. But the old maxim holds true - you get what you pay for. Note that many discount zoom lenses have a variable minimum f-stop number - you have to read the description carefully. The lowest f-number will be on the lowest zoom, as you zoom out, the lower quality of the glass results in an increased minimum f-stop rating. So any rating expressed, for example, as "f/2.8-f/5.6" is actually a practical minimum f-stop of f/5.6 - NOT f/2.8!

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