What does f 1 8 mean?

Here is what the site says about that.

From Lenses: Aperture: The f-number says how wide the lens opens. Smaller numbers let in more light: f/1.8 lets in about ten times as much light as f/5.6, more than three stops. A wide aperture means faster shutter speeds in dim light and more background blur. Zooms with a constant aperture, like a 24-70mm f/2.8, keep the same exposure throughout the zoom range; cheaper zooms get darker as you zoom in.

From Sensor sizes: The main sizes: - Full frame (36 x 24mm, the size of a 35mm film frame). The reference size: when people quote a focal length "in full-frame terms", this is what they mean. - APS-C (about 23 x 15mm). Crop factor about 1.5x (1.6x on Canon). Most entry and mid-range mirrorless cameras. - Micro Four Thirds (about 17 x 13mm). Crop factor 2x. Small, light systems from OM System and Panasonic. - 1-inch ...

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