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Tuesday, December 13, 2011

Make Your Panos Match Up


Wide-Angle Panoramas • See The Sharpness Get • The Most From A Lens And Camera • ND Filters • Sensor Cleaning • Missing Arches

Labels: How-ToColumnTech Tips


Wide-Angle Panoramas
Q I'm having trouble getting my wide-angle panoramas to stitch together. I'm using 24mm and 17mm wide-angle lenses. What's the trick to get these images to match up?
B. Norman
Salt Lake City, Utah



This 10-image panorama of the Los Osos Oaks State Nature Reserve was taken with a Canon EF 17-40mm ƒ/4L lens at 17mm. By rotating around the center of the lens instead of from under the camera, the images matched up when stitched.
A I've said for years that the secret for addressing the parallax problem (the displacement effect of viewing objects along different lines of sight) in wide-angle panoramas is to rotate the lens around its nodal point, where the light paths converge before hitting the film or sensor. Recent discussion seeks to move us toward using the term "entrance pupil" as the location on which you should center the rotation of the lens on the tripod. Whatever you call it, the point you're seeking is the approximate center of the lens from front to back. You'll need a slider under the camera to position the lens on the tripod properly over the rotation point. If you're rotating around the back or center of the camera, rather than the lens, the images won't line up properly. This calculation isn't particularly important when photographing panoramas with a telephoto lens; however, many telephotos have a tripod collar that helps to position the lens close to the optimal rotation point.

See The Sharpness
I just returned from teaching a Canon Live Learning Workshop (learn.usa.canon.com/live_learning/workshop_main.shtml) in southwestern Colorado. During a field session, one of the participants called me over to check a composition through the camera. My first conclusion was that the camera/lens wasn't properly focused, so I asked the student if she had adjusted the viewfinder for corrective lenses or contacts. She had not, but said she did normally wear eyeglasses, although not to photograph. The student's strategy was to rely on the camera's autofocus function to achieve sharp focus in every instance, because whether or not she was wearing her glasses to photograph, the image she saw in the viewfinder was blurry. As a result, she was never able to critically judge the sharpness of focus or the positioning of the focus in her image.

Together, we adjusted the viewfinder diopter in her camera until the image in the viewfinder was sharp when she was wearing her glasses and looking through the area of the lens corrected for distance. We possibly could have made the adjustment to achieve sharpness without corrective lenses, but that depends on the individual's vision challenges. Using her glasses, and with the diopter properly set, this photographer was for the first time able to work in manual focus mode and to critically judge the sharpness, and position of sharpness, being rendered by the autofocus function.

This isn't an uncommon situation. Either people don't know they can correct the viewfinder, or they don't want to make the effort to use this very important feature of their DSLRs. To adjust your diopter to make the best match between your vision and the autofocus, find a subject with contrast, use the autofocus to attain sharp focus, and then adjust the setting on the viewfinder to make the image sharp to your vision. If you wear corrective lenses, look through the area of your glasses that corrects for distance.

I also see a lot of photographers taking pictures with sunglasses in place. Yes, we're out in the field doing nature photography, and sunglasses, sometimes prescription sunglasses, are the eyewear of choice in those circumstances. Unfortunately, sunglasses cut the light by a factor of several ƒ-stops, depending on darkness and polarization. Whatever lens is mounted, your camera uses the maximum lens aperture (that is, wide open) for the focusing function. Otherwise, if you're shooting stopped down for critical focus, you would be looking through a very dark viewfinder. Critical focus is helped by having a lens with a wide aperture for a bright viewfinder. If you're wearing dark sunglasses, you're undermining this sophisticated function of your DSLR and diminishing your chances for critical focus.

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