Tuesday, July 29, 2014
Not So Sharp?
Diffraction Distraction • Flickering Time-Lapses • The Drive Vs. The Cloud: Updated “Whether” Report
Q To get more depth of field in my macro shots, I've been using an aperture of ƒ/22. I'm seeing a larger area of sharpness, but the overall image just isn't sharp. I'm using an expensive 100mm macro lens with disappointing results. What's going on?
San Diego, California
A An optical phenomenon known as diffraction is affecting your image quality. The sharpest image is formed by straight rays of light, that is, rays that aren't impeded or bent as they enter the lens and reach the sensor. The wider the lens opening, the sharper the image. The downside of this is that, at higher magnifications, depth of field is minimal; efforts to increase it by choosing a smaller aperture result in an image that's being formed by rays that are bent as they enter the lens diaphragm.
As an example, I often use the Canon MP-E 65mm ƒ/2.8 1-5x Macro lens. At 1x, I can use ƒ/16 and get good results in sharpness. When I extend the lens to attain 5x, I need to open up the aperture to at least ƒ/4 to minimize diffraction, but I get less depth of field. If I use ƒ/16 at 5x, the image won't be sharp at any point. This is a real problem because, at the higher magnifications, I need as much depth of field as possible. The best that can be done with a single shot is to compose the image so the area of sharp focus is positioned to emphasize the message or most important feature.
A total solution is a technique called focus stacking. With the lens set at its optimum aperture, the photographer captures a series of images, moving through the subject and overlapping the depth of field at each position. A post-capture program such as Photoshop (www.adobe.com), Helicon Focus (www.heliconsoft.com) or Zerene Stacker (www.zerenesystems.com) assembles the "slices" of focus into one completely sharp image. The programs miraculously keep the sharp areas of each image in the stack and ignore the out-of-focus areas. Stacking can be used in many photographic situations, from macro to landscape, and it's a technique I highly recommend and teach in my seminars and workshops.
Q I'm into creating time-lapse movies with my DSLR, but I'm having trouble with a sort of flicker in the movie when it plays back. What's causing this, and how do I get rid of it?
A Flicker is a common problem in time-lapse (TL) compositions; it's caused by slight variances in exposure from one capture to the next. Even though your exposure and shutter speed are manually set exactly the same for each capture, the lens diaphragm doesn't open and close precisely the same amount each time it cycles from the preset aperture to wide-open between captures. A better process would leave the aperture closed down to the preferred aperture during the whole sequence, eliminating the opening between captures, but today's lenses don't do that. An older, completely manual lens (when was the last time you saw one of these?) remains at the set aperture between captures. Some TL photographers compensate for this by setting their exposure to the widest aperture possible to minimize the movement of the diaphragm. The drawback to this strategy is that control of exposure and depth of field are limited, and a fast shutter speed is required.
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