Effect of lens sharpness on panorama stitching

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.
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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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Panoramas from Slovakia

On a recent trip to Slovakia, I took a lot of pictures, many of them in panoramic format. I thought, combining the recent impressions from Slovakia with a short essay on panoramic viewpoint would make an interesting article.

I have been active in the panoramic community for almost two decades now, and the possibilities still amaze me. Originally, I started with film cameras and used wide-format fixed Fuji GX617, swing-lens Noblex, and rotational Seitz Roundshots. The specialized panoramic cameras were very expensive, so were the films and naturally, also the film and print processing. Although the stitching concepts emerged more than 15 years ago, originally we were fusing scanned slides, which was a lot of work, and often quite inferior to the large format panoramic film images. However, the panoramic stitching methods changed dramatically in the last ten years with the introduction of Canon 20D and newer digital cameras. The progress has been relentless and using the latest crop of large megapixel cameras, motorized accessories like Gigapan, and latest software stitching tools, it is now possible to construct very impressive panoramas.

Nevertheless, the challenges of composition and panoramic viewpoints remain today the same as hundred years ago. Anybody even with a simple pocket camera can now create a long and skinny print, but it takes some experience and practice to see and display the scenes in a pleasing panoramic format.

What is actually a panorama? A classic definition states that a panoramic image should be at least twice as long as high, but there are some Gigapan panoramas in huge 4:3 or 3:2 formats, assembled from hundreds of individual images, and sometimes you’ll see also very tall pictures, called vertical panoramas. And there are the screen-only-based spherical panoramas. To add more complexity to the subject, we are dealing not only with the actual physical format, but also with a panoramic viewpoint.

Simple panoramas with a single point of interest can be seen and digested with a single glance, some short-distance rotational panoramas with curved streets and corners have to be studied and analyzed, and then there are some sophisticated panoramas that are similar to old master paintings depicting large groups of people and other objects, telling complete and complex stories.

Let’s move from the theory to practice, and I will use the examples just from my recent trip. On that journey, I didn’t carry any specialized panoramic camera, and most images were taken handheld by a dSLR with a 50mm lens and stitched later together in Photoshop CS5.

The first picture on the top is of Spissky Castle in eastern Slovakia, one of the biggest medieval castles in central Europe. The ruins of this partially restored castle are photogenic enough to produce a pleasing image even in the conventional 3:2 format. However, I found the entire scene so enticing, that a panoramic format seemed more appropriate. This panoramic picture was combined from six vertical and slightly overlapped images. Picture like this really should be viewed on a huge wall-sized print, but if you look carefully, even the small screen version tells a story (click on the image to see a slightly larger version).

Initially, the viewer’s eye is drawn to the castle on the right, and then he becomes aware of the countryside around – several villages and hamlets in the centre, a road and plowed fields in the foreground, mountains in the far distance, and a beautiful sky with picturesque clouds.

The picture below in a conventional 3:2 format conveys a completely diferent feeling. The focus is primarily on the castle, and even the vilage below serves just as an underlining border to emphasize the hill with the castle. Not better or worse than the panorama, but definitely a picture with a very different feel.

The next panorama shows an apartment building subdivision in town Poprad against the backdrop of High Tatra mountain range.

In this case, the panorama is much simpler, showing four separate vertical segments – a field, city, mountain range, and a sky with clouds. All four bands are very symmetrical, spanning the entire width of the image.

The next landscape panorama is yet slightly different in structure.

A large green meadow in the middle, and a mountain range in the background. A small cabin on the right side is the focal point in the picture, and the trees and bushes on both sides contribute to a nicely balanced image.

Panoramas can be also very effective in the urban places. The following scene shows two streets with renovated buildings in the town of Banska Bystrica in central Slovakia. A short conventional picture couldn’t accommodate this amount of information, whereas the panoramic picture depicts the entire scene very naturally, almost exactly as experienced with our own eyes.

The problem in many cities, especially in Europe, are the parked cars. Often they ruin the street scene for you. Whenever possible, I look for pedestrian zones. The following picture shows the main square in the same town, devoid of any cars.

Shooting doesn’t have to be confined to the day light. Sometimes, the night lighting will provide special effects, suitable also for panoramas. Here is a night capture of the main square in Bardejov, a famous health spa town in eastern Slovakia.

Sometimes, there is so much colour in the scene, that you really need a wide canvass to show it all. Here is a picture of Banska Stiavnica, a medieval mining town, proclaimed by the UNESCO to be a World Heritage Site.

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