Behind the Lens: Mechanics of photography: The ISO is exposed!

At the end of last week's column, I was demonstrating how differences in aperture affect exposure by reducing or increasing the amount of light that enters your camera. Shutter speed affects exposure by controlling the length of time light comes into the camera. The other leg of the exposure equation, ISO, controls your camera's sensor or film sensitivity. I'll go into greater detail about shutter speed and ISO in future columns. Together, in multiple combinations, these three settings can provide correct exposures for any situation. To demonstrate the relationship between these camera controls, here are five exposure settings that could be used at Free State High School's gym to yield a correct exposure.

1/500th second - f2.8 at 1600 ISO

1/250th second - f4.0 at 1600 ISO

1/500th second - f4.0 at 3200 ISO

1/125th second - f5.6 at 1600 ISO

1/125th second - f8.0 at 3200 ISO

All are correct exposures. I've often used the second setting to shoot basketball. I have to use a minimum of 1/250 shutter speed to freeze the action. Set any slower and I would see blurring in my photos. I use f4.0, because that is the largest aperture I have on my two lenses. With those selections determined, I'm left to set an ISO for a proper exposure. From experience I know that to be around 1600 ISO.

If my lenses had a maximum aperture of f2.8, which lets in twice as much light as f4.0, I could change my shutter speed to an even faster, action-stopping 1/500th. If you have similar camera equipment and use any of these settings you, too, should get proper exposures. From reader e-mails about dark and blurry action shots, my guess is that we don't have similar equipment. A reader inquired how to take better sports photos of their daughter in low light. They use a Nikon D40, with 18-55mm and 55-200mm lenses.

The important information is missing. What is the largest or maximum aperture for these lenses? With that information we could determine a correct exposure from the chart above. But there's a problem. Many zoom lenses, on point-and-shoots and as interchangeable lenses, have a variable aperture. This means that as you zoom out from the shortest to the longest focal length the maximum aperture decreases. No way, man! Way.

Check out your lenses. You may have one that reads 55-200mm f4.0-5.6. This means you have a maximum aperture of f4.0 at 70mm but at 200mm your aperture has dropped to f5.6. If you make no other exposure corrections to compensate, you've unknowingly lost half your light while zooming in on your star athlete. Your photos will be dark. If I was photographing with this lens at a FSHS basketball game I would probably keep it zoomed out at the 200mm f5.6 setting and arrive at a new exposure equation to add to the ones listed above.

1/250th second - f5.6 at 3200 ISO.

Having an awareness of your aperture settings, and its relationship with the other exposure controls, can help you avoid the unexpected dark photographs. Next week we'll talk more about shutter speed and avoiding blurry images.

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