Archive for Sunday, February 14, 2010

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

February 14, 2010


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.


Brad Maestas 8 years, 4 months ago

Good info for those fledgling photogs, Mike. Kit lenses are notorious for being low-quality compared to their more expensive, wide-aperture counterparts. Build quality, image quality, amount of distortions and the fabled variable aperture are all significant shortcomings.

Mercifully, DSLR's today have much lower noise at the higher ISO's and permit decent photos at the higher settings. I mainly shoot film these days - Kodak Tri-X 400 in a Leica M6 with either a Leitz 50mm f/2 Summicron or a Voigtländer 50mm f/1.1 Nokton. Obviously not a sport setup (:D) but I mainly shoot street anyway. I only shoot in available light and most of my venue photography is, for example, a jazz band in a dark bar. I will obviously reach for my f/1.1 lens but sometimes ISO 400 is not enough so I will push my roll to 800 or 1600 and develop accordingly afterward (just means increased develop time). Tri-X has excellent underexposure capabilities and with a f/1.1 lens at ISO 1600 you can all but shoot in candlelight.

Occasionally, my client will request digital photos as well and for that, I bring 50mm f/1.4 and 24-70mm f/2.8 lenses on a Canon 20D. I also have a Contax Zeiss 50/1.4 manual focus that's on an adapter that gives great results as well but I mainly use the 24-70. The only problem with the 20D is its sensor noise. Being an older DSLR it has significant noise at ISO 1600 and above. One shoot I did was entirely in 1600 and was so noisy that I went and bought NoiseNinja software to suppress it. It does a decent job once you learn how to use it properly. Stock settings tend to soften your image significantly.

Some more tips for those parents wanting to shoot in the gym, take these equivalent exposure settings and put your camera in manual mode. No AE lock needed. If you're using Nikon, lock in your ISO as they are normally set to automatically pick the ISO for you. Another pro tip is to take a piece of printer paper with you to set your White Balance point. Set it at a 45º angle to your sensor and fill the frame with the page, reflecting the light (temperature) from above into the camera. Have your camera set its WB to that reference frame. That, combined with your locked in manual settings will give you much more consistency shot-to-shot. No more dark to light to dark or red to blue to red casts. Auto WB is usually just fine but if you want rock-solid pro consistency, manual everything is the way to go! Happy shooting!

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