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ANSWERS: 2
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The number is the fastest aperture setting the lens has. For instance, your lens should be able to go to 18, which basically has the aperture close the lens to such a level that very little light comes in. 1.8 on the other hand, is your lens's aperture without it having to move to shut out light. The opening itself is 1.8, and that's what the number means.
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It refers to the size of the diaphragm aperture, commonly referred to as the f-stop. Lower f-stop numbers (e.g., f/1.4, f/1.8) indicate a larger aperture diameter. Conversely, a larger f-stop number (f/16, f/22) indicate a smaller diameter. Large apertures admit more light than small apertures - it's a bigger window. The f-stop number is mathematically related to the focal length of the lens and the diameter of the aperture. If you have a lens with a focal length of 50 mm, as yours has, an f-stop value of f/1.8 means the physical diameter of the aperture is about 27.8 mm (50 mm divided by 1.8). If you stopped your lens down to f/16, the diameter of the aperture would be 50/16 mm or about 3.1 mm. A lens rated in the manner you describe means that the largest f-stop available with the lens is f/1.8.
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