The aperture is the iris opening of the lens through which light passes and strikes the image sensor (there has to be a less awkward way to word that!). The size of the aperture is called the f-number or f-stop and the number is inversely proportional to the amount of light that strikes the sensor -i.e. the larger the f-number, the smaller the opening and less light gets through to the sensor, and the converse is true as well. I have to say I’m rather embarrassed to admit how long it took me to get that into my head, but it’s one of the most essential nuggets of photography knowledge you need to take consistently beautiful photographs. I find myself primarily shooting in aperture mode these days since most of what I shoot is outdoors and I need the ability to fine tune the exposure to suit the changing light. The range of apertures depends on the lens you use – my 50mm lens’ smallest f -number is 1.8, and my normal lenses have either a 4.0 or 5.6 as the smallest f-number.