Basic Lens Specifications
Millimeters (mm): Millimeters are a measure of length. Lens focal lengths are measured in millimeters. There are 25.4mm to an inch, and in the 1950s and before, lens focal lengths tended to be measured in inches.
When you set a fire with a magnifying glass, the distance between the magnifying glass and paper is the magnifying glass' focal length. Camera lenses are more complicated, but it's the same measurement.
(To start a fire with your camera lens, use the longest, fastest tele you have. Hold the back of the lens about 2" from the paper, adjusting it just as you would a magnifying glass untill you get te smallest, sharpest dot from the sun. Be sure to use the lens wide open (f/4, not f/22), and if using a Nikon G (gelded) lens, be sure to hold the diaphragm pin open with your finger.)
In our simple example, light comes out the back of a lens at the same angle at which it entered the lens. A subject that occupies a certain angle of view will have the same angle projected from the back of the imaginary center of our lens. As the focal length increases, since the angles remain the same, the size of the image on our film or sensor increases.
A focal length tells us a lens' magnification. Longer focal lengths, like a 200mm lens, magnify more than a 50mm lens, because the imaginary center of our camera lens is further away from the film or sensor.
Zoom lenses are very complex, so you can't really see this changing outside the lens. Some zooms appear to get longer when set to wider settings, but rest assured, the focal length range refers to its magnifying power as measured from imaginary points called Nodal points.
f/number: f/stops tell us the diameter of the clear part of the lens.
f/stops really are fractions. The "f" in an f/stop is the focal length, and the resulting number is the diameter of the clear opening as seen from the front of the lens.
f/4 means that the diameter of our magnifying glass is 1/4 of its focal length. f/2 means a lens is half as big around as its focal length, or twice as big around as an f/4 lens of the same focal length.
Camera lenses are more complex than magnifying glasses. f/stops refer to the clear area you see through the front of a lens.
A 200mm f/4 lens will have a hole (200mm/4) or 50mm in diameter, or 2 inches. A 200mm f/2 lens will have a clear opening of (200mm/2) or 100mm, or a four-inch aperture.
A 50mm f/4 lens will have a clear opening of (50/4) or 12.5mm (1/2 inch). An 18mm f/3.5 lens will have a clear opening of (18/3.5) or 5mm (1/4").
f/numbers are usually expressed as fractions with a slash, like f/5.6, when writing them and reading EXIF data. Lenses themselves are often marked with a colon, like 1:2.8. It means the same thing.
Faster f/stops are lower numbers. f/1.4 and f/2 are fast, or wide, f/stops, while f/11 and f/16 are slow, or more narrow, f/stops. The smallest number o a lens, like f/2.8, is called shooting wide open, which has nothing to do with the angle of view; it refers to the diameter of the opening of the clear part of the lens.
Faster lenses always cost more and are bigger and heavier. They have more glass! Faster lenses also let in more light for use in the dark, and can have narrower depth of field.
Fast zooms are f/2.8, while slow zooms are f/5.6. Fast fixed (non-zoom) lenses are f/1.4 and f/2, while slow fixed lenses are f/2.8 or f/4.
Zoom lenses are very hard to design, so they are slower if all else, like price or size, remains the same.
Normal lenses are the easiest to make. They will be the fastest, smallest and least expensive.
Wide angles and telephotos are more difficult. They will be slower, bigger and more expensive.
Filter Size: This is the mechanical dimension of the threads on the front of the lens. These threads are almost always bigger than the clear aperture calculated from the f/stop; if the filter thread is smaller, someone's math is off.
These are basics. If you want to know much more, read Sidney Ray.
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