DSLR camera lenses collect light and project it onto the CCD sensor, where the image is recorded. The amount of light falling on the sensor has to be controlled, brightly illuminated subjects will require less light to fall on the digital photo sensor. Conversely, poorly lighted subjects will require more light to be passed in order to take an acceptable photograph.

Lens aperture is measured in units called "f-stops" and is written in the format of "f/[number]". A larger f-stop number indicates a smaller aperture; while a lower f-stop number means a larger aperture. Most lenses range between f/1.4 (largest aperture) and f/22 (smallest aperture).

Any lens with a minimum f-number of f/3.5 or higher is NOT the highest quality SLR lens. Several discount zoom lenses have a variable minimum f-stop number - you have to read the description carefully. The lowest f-number will be on its lowest zoom setting; as you zoom out, it 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-number of f/5.6 - NOT f/2.8!



The amount of light that like can pass through a lens is limited by the diameter of the glass lens. Higher end lenses will be larger, thereby allowing more light to be collected. Since the physical lens has a fixed diameter as constructed, a moving diaphram within the lens dynamically changes the f-number as needed. Note that every lens has a minimum and maximum setting that it is capable of delvering, usually the largest aperture is advertised since it directly affects the quality of most digital photography (lower f-number).

While the maximum aperture is not a serious limitation, it does have an impact on certain types of photography. Brightly lit subjects, for example under the sun or under studio lighting, results in excessive light falling on the digital CCD photo sensor. A lower aperture size (higher f-number) is desired to preserve image quality and prevent over-exposure of pictures. Excess light can also saturate subtle colors, or blur fine lines and details. Generally a f/22 f-number is the smallest aperture that most lenses offer, and should be considered as a required maximum f-stop.


In addition to the amount of light that is collected by the lens, the SLR camera utilizes a mechanical shutter that opens for a predetermined period which also controls the amount of light being passed. This is known as Shutter Speed and is measured in seconds. Generally the exposure of a photograph is only a fraction of a second, for example 1/250th of a second, and shutter speeds are noted in this fractional manner.

A faster shutter speed usually indicates a well built SLR mechanism (i.e. higher cost). Good quality DSLR cameras should be able to achieve upto 1/4000 shutter speed. Fast moving subjects require a faster shutter speed to "capture" the image without any motion blur. Since a faster shutter speed means that the CCD photo sensor is exposed for a shorter time, a good quality lens with a large aperture (lower f-number) is required to get a properly exposed and sharply focused digital photo. Note that a high shutter speed also reduces the clarity of things beyond the subject (backgrounds), this is known as Depth of Field.


Conversely, there are some photograph subjects that require a sharp and clear depth of field, for example, scenery pictures. Using a fast shutter speed in that case will result in only the nearest object being in focus, while the rest of the scenery will be blurry or with improper exposure.

Using a smaller aperture (higher f-number) with a slower shutter speed in this case will result in less light falling on the digital CCD sensor for a LONGER time, allowing the entire image area to be captured in vivid detail and in sharp focus. In some cases, photographers place filters in front of the lens to deliberately reduce the amount of light even further, effectively increasing the higher f-number rating of the lens. Coupled with a tripod to eliminate hand shake and vibrations, a very long exposure (even upto or more than a second) will result in a crisply detailed postcard-like scenery shot.

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