It's the aperature rating of the lens. The smaller the aperature or f5.6 the slower the lens which means less light is permitted into the lens and thus more time is required to expose the image on the sensor, whereasfast lenses; f1.2 for example, have a very wide maximum aperture and so shorter time periods are required to expose the image sensor.


This is why you will see lenses labeled for example 70-200 f2.8 IS USM vs 70-200 f4 IS USM. The 2.8 lets in 2x more light than the f4 lens. One has to think in opposites when referring to aperatures.


Hope this helps or at least answers your query?!


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