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Tuesday, April 24, 2012

Eagle Eyes

Reaching For Sharpness • Bad Vibrations • The Apple Of My i

Extreme Outreach. George Lepp used a Canon EOS-1D Mark IV and a Canon EF 800mm ƒ/5.6 lens with both the EF 2X and EF 1.4X tele-extenders attached (2912mm total with the 1.3X crop magnification) to photograph this bald eagle on the nest. The critical focus was attained by using the Live View function of the camera and a Hoodman Loupe on the camera's LCD. A sturdy Gitzo GT3541 tripod was essential, along with a Really Right Stuff BH-55 ballhead. Exposure was 1⁄1000 sec. at ƒ/16 and ISO 400.

Reaching For Sharpness
Q I photograph wildlife with a Canon EOS 40D, a 500mm ƒ/4 IS lens and often a 1.4X matched tele-extender. I consistently apply technique to maximize sharpness: I use a tripod with a Wimberley tripod head, sometimes shoot with remote release, and I try to obtain at least 1⁄500 second and ƒ/8, adjusting ISO up to 800, if necessary, to maintain the shutter speed in lower light. Image Stabilization (IS) is on and set to position #1. Despite these efforts, my results are mixed. How can I get more consistent results when using long lenses?
A. Sachanowski
Emigrant, Montana

A Working with the big-gun lenses can be very rewarding, but it's a lot of work, and I know a few successful pro wildlife photographers who never use big glass. Everyone who does will have issues with sharpness at one time or another. But I'm always excited by the opportunity to get closer to wildlife subjects without endangering them or myself, as evidenced by the photograph accompanying this column.

In your specific case, you're working with 700mm when you combine the 500mm lens with the 1.4X teleconverter, and effectively at 1120mm when you add the 1.6X crop factor of the 40D's APS-C sensor. That's a lot of magnification.

Four aspects of sharpness are in play whenever you take a photograph of a wildlife subject in the field. The available depth of field (DOF), maintaining precise focus, prohibiting camera movement and mitigating subject movement are all critical variables. The DOF (maximum area of focus, from near to far in the subject) is determined by the focal length and the aperture; the longer the lens and the larger the aperture, the shallower the DOF. So when you're working with a long lens and want to let in more light for a fast shutter speed, you're effectively reducing the area in which you can achieve sharp focus. There's really not a lot you can do about it. DOF is minimal at 700mm using ƒ/8: at 150 feet, you have approximately 48 inches; at ƒ/16, you'll double the DOF from 150 feet to about eight feet. The DOF doubles with the stopping down of two full ƒ-stops. The closer you get to your subject, the more critical focus becomes; at 20 feet, you have only 0.75 inches of DOF! No matter how you look at this, at 700mm you have a very small range in which to apply the second variable, precise focus; that's often the reason for soft results with long lenses.

While we may think we can manually focus through the viewfinder in precisely the right spot 150 to 200 feet away, it's really very difficult. Autofocus can be more precise, but you can easily find yourself working beyond the capability of the 40D's ƒ/5.6 limit for AF. With a 1.4X tele-extender on the 500mm lens, you're capable of autofocus, but only the center focus point would be active. More accurate positioning on stationary subjects can be attained by managing framing and focus in Live View mode on the LCD screen, a feature that's available on your 40D and many other cameras from several manufacturers. Live View can be magnified by 5X or 10X for pinpoint focus. A Hoodman Loupe (www.hoodmanusa.com) will help you to view the LCD screen for critical evaluation, especially during the bright daytime. Attach the loupe to the camera with a Hoodman Cinema Strap or Crane; these free your hands to position and operate the camera.

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