Wit the 21st Century advent of the amazingly simple digital cameras, the understanding of technical expressions such as lens aperture, and numerous other photographic general use expressions have become somewhat obsolete for the majority of digital camera clickers. One basic cause of this is the fact that nearly all general purpose digital cameras purchased in 2008 do not need such expertise as that expertise probably wont be able to be applied to a basic digital camera anyway. Not to forget the mobile phone with digital camera ability.

However that knowledge is a must for the DSLR camera photographer. Yes, with a special emphasis for those owning digital single lens reflex cameras; shortened to DSLR herein. Keen starting out amateur photographers have a need to possess an understanding of such terms as focal ratio and focal length. Specifically, they need to possess an understanding of these terms in order to get superior results. Okay, what do these expressions mean:

- Focal length means the distance from the lens to the focal plane of the imager.

- Aperture is the usable diameter of the lens opening.

You need to understand both terms...focal length and aperture. Each has a precise relationship to the other. Now, the relationship that exists between focal length and aperture is defined as the "focal ratio". The "focal ratio" (f-number) is established by dividing the focal length of the lens by the camera lens aperture .

Example:

A lens possesses an aperture, (usable diameter of the lens opening) of 50mm. The focal length of the lens is 500mm. Accordingly the focal ratio = 500/50 = 10. The focal ratio is generally recorded as f/10. e.g. this means the lens has a focal ratio = f/10

Now you understand how to calculate the focal ratio the next question maybe "So what?". Well, for the ambitious amateur DSLR camera user, and without doubt those using DSLR cameras, understanding of "focal ratio" is vital information due to a photographic fact known as "lens speed".

Precisely what is meant by "lens speed" and why is it critical?

Lens speed refers to the "maximum aperture diameter", or "minimum f-number", of a photographic camera lens system. Using a lens with an increased maximum aperture will be considered a fast lens. This is due to the fact the lens delivers increased light illuminance to the image plane, thus permitting a much faster shutter speed. In the event the lens possesses a lower maximum aperture (larger minimum f-number), then of course the lens will be categorised as "slow" due to the fact that it provides less light intensity and requires a slower shutter speed.

Camera lenses will be classified as "fast" or "slow" . This is essential feedback for DSLR camera owners as the lens speed given by the minimum f-number, or conversely maximum aperture diameter, or maximum numerical aperture, is a quantitative method to to compare lenses.

Knowing the lens speed is vital information when photographing in dim light, and even moreso when using long telephoto lenses. Understanding lens speed is also particularly vital in order to determine depth of field when undertaking portrait photography. Lens speed is a most important variable in conjunction with other photographic variables such as focal length and camera size.

The photographer who understands these concepts and knows how to apply them to their cameras and lenses in all photographic situations will produce excellent results.
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Ross H St Quintin authors articles on a range of fascinating topics. These topics include sports & fitness, digital Single lens Reflex photography, digital video technology and travel in Western Australia. Visit his sites at:
http://facts4you.net/dslr
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