In this age of the general purpose digital camera, the concept of focal length, lens aperture, focal ratios, and many other photographic technical terms have become somewhat obsolete for most digital camera users. One basis reason for this is the fact that most basic digital cameras used these days do not require such knowledge as the knowledge might not be able to be applied to a basic digital camera, let alone a digital phone camera.
The situation is very different for the serious amateur photographer. Specially those using digital single lens reflex cameras (from now on in this article shortened to DSLR). Serious starting out amateur photographers will need to understand such terms as focal length, aperture and focal ratio. Particularly they need to understand these terms in order to get the best results. Okay, what do these terms mean?
- Focal length is simply 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 as each has a relationship to the other. The relationship between focal length and aperture is referred to as the “focal ratio”. The “focal ratio” (f-number) is calculated by dividing the focal length of a lens by the camera lens aperture .
Example:
The lens system of a camera has an aperture( usable diameter of the lens opening) of 50mm. The focal length of the lens is 500mm Therefore the focal ratio = 500/50 = 10. The focal ratio is generally written as f/10. e.g. the lens has a focal ratio of f/10
Now you understand how to calculate the focal ratio the next question should be “So what?”. Well, for the serious amateur photographer, particularly those using DSLR cameras, the “focal ratio” is essential knowledge due to a photographic concept referred to as “lens speed”.
Okay…what is meant by “lens speed” and why is it important?
The term lens speed refers to the “maximum aperture diameter”, or “minimum f-number”, of a photographic lens or lens system. When using a lens possessing a larger maximum aperture (that is, a smaller minimum f-number) the lens would be nominated as a fast lens. The reason being it delivers more light intensity (illuminance) to the focal or image plane, permitting a faster shutter speed. If the lens has a smaller maximum aperture (larger minimum f-number) then the lens would be classified as “slow” because it delivers less light intensity and needs a slower shutter speed.
Camera lenses are classified as “fast” or “slow” . This is vital information for photographers as the Llns speed given by the minimum f-number, or conversely maximum aperture diameter, or maximum numerical aperture, is a vital quantitative method in order to compare lenses.
Knowing the lens speed is essential information when photographing in dim light, and moreso when using long telephoto lenses. Being aware of lens speed is also particularly important in order to determine depth of field when undertaking portrait photography. Lens speed is a major variable in tandem with other photographic variables such as focal length and camera format size.
The photographer who understands these terms and knows how to apply them to their qw in varying photographic situations will produce excellent results.
Autor : ROSS ST QUIN TIN
