Plugged In: Depth of field key for sweet fireweed photos

Photos courtesy of Joe Kashi.

Photos courtesy of Joe Kashi.

By Joe Kashi, for the Redoubt Reporter

Nothing else feels so much like high summer on the Kenai Peninsula as a brilliant field of fireweed, and the fireweed has been spectacular this year, only now reaching a peak along the side roads north of Homer’s Diamond Ridge and Skyline Drive.

On the central Kenai Peninsula, the fireweed is already near the top of the stock due to our unusually warm days, and the city of Kenai’s lovely field of flowers across the Kenai Spur Highway from Walmart is poised to burst into constant and intense bloom.

As I’ve made fireweed photographs over the past few weeks, I’ve been reminded of many lessons.

Some might think it a stretch to call most fireweed or other flower photos fine art, but so what? Not every photo has to have a lofty purpose beyond our pleasure in capturing the moment. Everyone’s taste varies, of course, and what works for one person may not be the only correct approach. So, vary your technique and try to see a very common Alaska subject in a way that’s uniquely yours.

Several focus challenges commonly occur when photographing a field of flowers. Depth of sharp focus — termed “depth of field” — may either be too shallow to sharply capture a subject that’s both expansive and deep or, depending on your subject and composition, may not be shallow enough to properly isolate a strong foreground subject by blurring out the background.

Cameras built around smaller sensors use lenses with a shorter focal length to achieve the same magnification. Those shorter focal length lenses have inherently greater depth of focus compared to the longer focal-length lenses used for larger cameras when both lenses are set to the same aperture. For that reason, a Micro Four-Thirds camera mounting a wide-angle lens set to a small aperture like f/8 will show more depth of sharp focus from front to background than a full-frame camera using a larger sensor for the same apparent subject magnification.

OLYMPUS DIGITAL CAMERAOn the other hand, if you’re trying to isolate a single brightly lit cluster of fireweed from background clutter, the best way to do that is to greatly blur the background with a full-frame camera and longer focal-length lens set to a bright aperture, like f/2 or f/2.8. In this instance, the depth of field is razor thin. Assuming your focus is accurate, only the subject will be in focus. The sharpness of the subject contrasted with the soft background provides good visual isolation and emphasis.

Thus, smaller-sensor cameras using shorter focal-length, wide-angle lenses and smaller, less-bright apertures result in more depth of field. Larger-sensor, full-frame cameras using telephoto lenses set to very wide, bright apertures result in a very narrow depth of focus, strong foreground subject emphasis and more background blurring.

Intensely blurred backgrounds are something of a fad recently and that seems to be a major reason why amateur photographers are buying expensive, full-frame cameras and lenses. Personally, I believe that this fad will pass. Achieving a sufficient depth of field to ensure that enough of the subject remains in sharp focus is a significant problem with full-frame and medium-format cameras, but one that’s often not well understood before that major purchase is rung up.

If you’re photographing a broad field of flowers, or any other subject that’s deep from front to background, finding the correct point of focus can be difficult. Generally, depth of field is maximized when your point of focus is set to a point that’s the front one-third of the total depth that you want in sharp focus. At that focus point, one-third of the sharply defined area will be in front of the focus point and two-thirds behind it.

Photos often look rather bad when blurred foreground objects intrude into the image. There are several ways to work around this. Change your angle and location so the nearby blurred foreground objects are no longer visible in your image, or focus on the nearest objects and let the farther portion of the photo gradually lose sharpness and blur. Alternatively, you might change your camera’s eyelevel angle to a much lower or higher point of view, altering the composition to emphasize the nearby objects, then making them the point of focus. A camera with a tiltable rear screen is very convenient when photographing from low and high points of view.

Highly detailed, complicated subjects like our field of flowers often fool a camera’s autofocus mechanism into locking on to some obscure background or foreground object that happens to fall within the generally large autofocus area chosen by a camera. When that occurs, your subject will not exhibit the critical sharpness that can make or break outdoor photographs like these. I’ve found that it’s easy to incorrectly blame that lack of subject sharpness on other common problems, like a poor-quality lens, camera shake or a breeze causing that field of flowers to move, rather than on subtle autofocus problems.

Sensor-based contrast detection autofocus mechanisms used in compact-system cameras tend to be more susceptible to being fooled by obscure, high-contrast backgrounds, while the phase detection autofocus hardware used with digital SLR cameras frequently drifts out of calibration, resulting in a dSLR whose focus and sharpness are always subtly off.

More modern compact-system cameras like the Olympus E-M5 Mark II, can be set to use only specific, much tighter autofocus areas, resulting in more precise focus, or to use manual focus with electronic image magnification and focus-peaking aids. Digital SLR cameras using mirrors and direct optical viewfinders can be retrofitted with aftermarket focusing screens that include split-image and micro-prism focusing aids for precise manual focus. All of these methods result in more precise, and thus sharper, focusing when used carefully.

Once focus is nailed precisely, it’s time to consider how to expose for best effect and how different light conditions can radically alter how these subjects appear in the final image, and that’s the subject of a future article.

  • Don’t forget the Redoubt Reporter’s summer photo contests, with entries due by 11:59 p.m., Aug. 21. For more information and entry forms, point your browser to www.artspaceak.org.
  • If you happen to be in Anchorage this Friday, stop by the Conoco-Phillips Gallery of Alaska Pacific University between 5:30 and 7 p.m. for the opening reception of my new solo photo exhibit, which will be up through the end of August.

Local attorney Joe Kashi received degrees from MIT and his law degree from Georgetown University. He has published articles about computer technology, law practice and digital photography in national media since 1990. Many of his articles can be accessed through his website, http://www.kashilaw.com.

1 Comment

Filed under photography, Plugged in

One response to “Plugged In: Depth of field key for sweet fireweed photos

  1. jack

    one could also use the photomerge function in photoshop to combine
    different focus points into one image.

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