Many people just getting into photography will buy a good quality camera body and a versatile zoom lens. Usually 5hese are bundled as a “kit” and represent a good value. These lenses are designed for the consumer market, have good optical quality but a less robust build than more expensive glass. They will also have a variable aperture such as f/3.5~f/5.6. Since aperture is one of those ‘things’ that has to be learned, let’s do a short review. The aperture setting determines the amount of light getting through the lens to cast an image on the sensor (or film). The wider the aperture, the more light getting in. In the example above f/3.5 is the widest aperture. If the lens was something like the new Nikkor 18mm~140mm, the lens would have the capability of shooting at f/3.5 when set to 18mm.
But it’s a variable aperture lens. So what happens when the user ‘zooms in’ is that the lens physically extends increasing the distance that the light has to travel to reach the sensor. When that occurs, the aperture changes (varies) so that at 140mm the maximum aperture is f/5.6 which is less than half the amount of light captured at f/3.5. Thus, to compensate, the shutter speed will have be slower or the ISO will have to be higher.
The change in aperture is gradual as the local length is changed but you will notice that it is not a linear progresssion, and goes to The minimum (such as f/5.6) fairly quickly. So the messsage is to be aware of what’s going on when shooting.