Posts Tagged ‘sensor aligment’

Recently, as I was culling my latest batch of images, I noticed a focal asymmetry in one of the images photographed by Nikon D300 and Nikkor AF-S 70-300mm VR-G lens. The building in the image with its bricks and windows makes for a very good test subject. I was standing parallel to the building, across a river about 100m from it, and although its corners are technically slightly further from the camera than its middle portion, the DOF at that aperture should still portray everything in the focus.

The EXIF data shows the following info: ISO 200, 110mm, F5.6, 1/1000s. Shot under almost ideal conditions, one would expect a perfect picture. Captured at the minimum/base ISO, the zoom lens was in the middle part of the barrel, aperture close to the lens sweet spot, and the shutter speed more than adequate for this focal length. To my surprise, the left third of the image was badly blurred relative to the middle and right portion of the image (see below – and click on the images to viwe them in full 100% crop size).

Left Image Segment:
notice the blurred bricks and windows, with the blur getting stronger toward the left edge. The building behind, although not that much further, is totally out of focus.

Middle Image Segment – everything sharp

Right Image Segment:
As sharp, if not sharper than the centre portion.
Notice the sharp bricks and windows, and even the buildings behind are still in focus.

Obviously, there is a problem with the actual lens or alignment of the camera sensor. Very likely, both. Focal microadjustment by LensAlign won’t help in this case, since the center of the image is already sharp. But that is a topic for another article.

As disturbing as this situation is, it is not that uncommon. Many lenses will show a dropoff of the sharpness towards the edges. As a matter of fact, one reason why the rotational panoramic cameras take very sharp pictures even with less than stellar lenses, is the fact that they utilize just a narrow slit in the middle of the lens that is always sharp (unless you use a really bad copy of a lens).

What does all that have to do with panorama stitching?
Let’s assume, you just shot three consecutive segments in horizontal orientation that you plan to stitch. Well, most stitching programs will attempt to use the leftmost image as the left portion and the righmost image as the right portion of the panorama. The middle image will be inserted somewhere in the centre, not necessarily along straight vertical lines. Makes sense, n’est ce pas?

Unfortunately in a situation, as described above (the lens is less sharp in the left third), if you use an automated stitching program, you’ll be dealt an unsharp half of the left image and possibly also an unsharp left part of the middle image. If the lens is sharp on the right, the right side of the panorama should turn out OK. Now, if you take five or seven segments instead of three, you may get slightly better results, but not that much, since that first (leftmost) image will be used almost in its full size anyway.

So what to do if you want a sharp panorama? F8 or F7.1 aperture may yield some improvement when it comes to the optimal lens aperture, slightly greater DOF, and lower diffraction (depending on the format of the camera), and of course, factory calibration of the lens and camera is in order. But nevertheless, most lenses will exhibit less than ideal sharpness on their extreme edges, so the basic problem remains.

One practical solution is to shoot more segments (i.e. 5-10 or even more, rather than just 3-5), overlap them generously at 30-50 percent, and crop all these images on both sides, so that you retain only the sharpest middle portion of each image. Now YOU are in control! Feeding the sharpest portion of the images to the stitching software will result in a much sharper pano. Just be careful not to lose the overlap when cropping.

Another trick is to compose the first (leftmost) segment in such a way, that its left third will be to the left of the planned edge of the panorama and consequently trimmed off (planned throwaway). Coincidentally, we had to do it also in the old days of rotational cameras to allow the motor to ramp up to its proper speed.

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