The Canon EOS Digital Rebel XSi/450D Companion

PHOTOGRAPHY 101

WHY THERE ARE TWO WAYS TO CONTROL LIGHT

97

In this book, we’ll use a few aperture-related terms that you’ll regularly hear pho-

tographers employ. When we speak of stopping down a lens, we mean choosing a

smaller aperture (larger f-number). Opening up the lens means to choose a larger

aperture (smaller f-number). Shooting full wide means to choose the largest pos-

sible aperture (the one with the smallest number).

is last term can be a little tricky because, on a zoom lens, the largest possible

aperture can vary depending on your current focal length. For example, if you’re

using the 18-55mm lens that Canon bundles with the XSi, then you have a large

aperture of f3.5 when shooting with the lens at full wide but a large aperture of

f5.6 when shooting with the camera at full telephoto. In other words, the ap-

erture can’t open as wide when you zoom the lens in all the way. is is simply

because of the nature of engineering a lens. (If you look at the front of the lens,

you’ll see that it says “1:3.5-5.6.” is tells you the aperture range, from full wide

to full telephoto.)

Why ere Are Two Ways to Control Light

So, you can limit the amount of light that hits the sensor by changing the amount

of time that the shutter is open, and you can limit the amount of light that hits

the sensor by changing the size of the aperture in the lens. But why do you need

two mechanisms ...