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Tuesday, September 14, 2010

Enigmatic Autofocus


Focusing Is For The Birds • The Dark Side Of Long Exposures • Big Images From Small Files • To Tilt Or Not To Tilt

Labels: How-ToColumnTech Tips

This flock of shoveler ducks was taken with a Canon EOS 7D set to ISO 400 and an EF 500mm lens with a 1.4x tele-extender (700mm + 1.6x). The exposure was 1/1000 sec. at ƒ/11. I had preset my focus by setting it on a stationary subject at the distance I expected the birds.

Focusing Is For The Birds
Q When photographing birds with long telephoto lenses set to autofocus, I’m having problems with the lens searching first to the closest point of focus, then out to infinity, before locking onto the subject. Since the birds are usually far away, I want the lens to focus to infinity first. Waiting for the lens to go through most of its focus range while the bird flies away is frustrating! Is this a lens or a camera problem? Can the camera firmware be changed to focus first at infinity?
R. Rockwood
Portland, Oregon


A Autofocus is a cooperative function of the camera and the lens. Even if you could, you wouldn’t want to change your camera’s firmware to always seek focus at infinity first because infinity with long lenses is a long, long way away. You would compromise your autofocus capability for the majority of subjects. There’s a better solution.

Most long telephoto lenses have a limiting switch that gives the choice of enabling either the full range of focus or only a distant range of focus. I hope your lenses have this capability; if not, you might want to consider a new purchase. Anytime I’m photographing birds in flight (or any subject moving quickly at a distance, such as runners, race cars or aircraft), I set the lens to the longer range to prevent exactly the problem you describe. It significantly speeds the lens’ ability to attain the subject quickly because if the lens is focusing first at its closest point, the subject in the distance is so far out of focus as to be undistinguishable to both the photographer and the lens/camera.

A technique I use to narrow the range even further when photographing birds in flight is to find a stationary object at the approximate distance at which I anticipate photographing the birds. I preset the focus on that subject, and then when I activate the autofocus on the birds themselves, the adjustment is small enough from the outset to enable a fast lock.

The Dark Side Of Long Exposures
Q I’ve seen images where water or surf appears very milky and smooth, sometimes even cloudlike. I’ve read that a neutral-density filter makes it possible to get such an effect. However, when I go beyond three stops using a Vari-ND filter, the image appears very dark in the viewfinder of my Canon EOS 7D. How does one use the Vari-ND filter, or any other filter, to achieve that dreamy look with water?
S. Kekre
Marietta, Georgia

A Neutral density (ND) filters limit the amount of light entering the lens, allowing a long exposure even in bright conditions. There are a number of single-density ND filters you can use to achieve the effect you want; they’re generally offered in two-, three- or four-stop increments, meaning that the filter cuts the light by the equivalent of a two-, three- or four-stop reduction in the lens aperture. The Singh-Ray Vari-ND offers a range of two to eight ƒ-stops of neutral density.

Here’s the process. First, set your composition and focus without using any filtration; if using a single ND filter, remove it. If using the Vari-ND, set it to its lightest setting (two stops).

Next, determine the shutter speed you need to render the water movement. With an ocean scene such as you describe, I might choose a several-second-long exposure to cover several wave patterns. With a waterfall, I might set the exposure in the area of 1/8 to 1/4 sec. Choose the rest of your camera settings (ƒ-stop and ISO) to support the shutter speed you want. If you’re using a single ND filter, you won’t have a lot of range to work with; only one ƒ-stop and ISO setting will probably support your shutter speed. The advantage of the Vari-ND is significant. With that filter on my lens, I can first set the exposure and shutter speed I want, then dial the filter to the setting that works with my combination.

To check and fine-tune the exposure, take a picture; look at the resulting image on the LCD screen. Then check the histogram to confirm you’ve achieved the correct exposure. Pay particular attention to the white end of the histogram to be sure that the milky water, usually the brightest part of the scene, isn’t blown out. If you have pixels up against that right wall, you’re overexposed.

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