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11-12-2011, 01:15 PM   #1
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lens aperture and focal length markings

I understand the markings on a lens that says 1.4/55 - the 1.4 designates the aperature or f stop and the 50 designates the focal length in mm.
Am I right so far?
Why then does it appear on the lens as 1:1.4/50????
Why the 1: ????

11-12-2011, 01:27 PM   #2
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The aperture figure is a ratio: <focal length>:<aperture diameter>. The maximum aperture diameter of a lens marked 1:1.4 is 1/1.4x of its focal length or the focal lenght of the lens is 1.4x of the maximum aperture diameter. This also explains why the f stop numbers get bigger as the aperture openings get smaller.
11-12-2011, 02:36 PM   #3
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As jolepp said, f-stops are fractions, the ratio of the iris diameter to the focal length. So a 50mm lens with a 12.5mm aperture is 12.5/50 or 1:4 or f/4.0. We use the notation f/nn instead of 1:nn because of 1) tradition and 2) laziness.

On zoom lenses with variable maximum apertures, we see a notation like 18-55/3.5-5.6 or 1:3.5-5.6 18-55mm. This means the maximum aperture at 18mm is f/3.5 and the maximum at 55mm is f/5.6. When we stop those way down, the LCD screen on the camera reveals different minimum apertures. On the DA18-55 it's f/22 at 18mm, and f/40 at 55mm.

We use these f/numbers just so we don't have to worry about the actual iris diameters, only their relationships. We can easily control DOF while maintaining the same exposure by varying the f-stop and shutter speed in sync -- open the aperture by on stop whilst doubling the shutter speed, or close the aperture one stop and halve the speed. Same amount of light, different optical effects.
11-13-2011, 12:45 PM   #4
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Original Poster
Thanks for the clarification Jolepp and RiRico.
I always get answers on this site - love it.


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