Should you vary focal length when shooting a single-row panorama to reduce the "bow-tie" effect?
Asked 6/3/2013
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When stitching a very wide single-row panorama into a rectilinear projection, the final image can develop a strong bow-tie shape that forces heavy cropping. I've seen suggestions to zoom slightly wider for the center frames and use a longer focal length toward the sides to reduce this effect.
Is changing focal length during the pano actually useful? If so, how much should the focal length change between the edge frames and the center? Are there any other benefits or drawbacks to this approach?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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Firstly, I originally mentioned zooming as something you shouldn't do when shooting a panorama, then added "unless you really know what you're doing", this advice still holds, it's not recommended but there are circumstances where it could possibly be of some benefit, such as when shooting a very wide rectilinear projection panorama using only a single row of images.
Why is it not recommended? Well whilst it's mathematically possible to stitch images with differing focal lengths, you are introducing an additional degree of freedom, making the stitching software work harder and potentially make more mistakes.
To answer your question of how much to zoom, this page from Cambridge in colour demonstrates the effect (scroll down to about half way). For a 150 degree rectilinear panorama you'd want to zoom out 3x for the dead centre and 2x either side of that.
A far better solution to the problem is to shot multi-row panoramas shoot three rows in the middle, two either side of that and one row at either end. That way you can maintain focal length and make the stitching software's job much easier, whilst still maximising the size of your rectilinear panorama.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
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It can help in a narrow case: a very wide single-row panorama that you plan to render as rectilinear. Using a slightly wider focal length for the center frames can reduce the bow-tie distortion and the amount of cropping needed.
But in practice, it’s usually not recommended. Changing focal length introduces another variable for the stitcher, making alignment harder and increasing the chance of stitching errors. For most panoramas, keeping the same focal length throughout is the safer approach.
There isn’t a simple universal amount to zoom. The required change depends on how wide the final panorama is and the projection you want. As a rule of thumb from the discussion, rectilinear projection is generally not recommended once you get much beyond about 120° field of view; the wider you go, the more severe the issue becomes.
If you do try it, take the center image first as a reference, and expect trial and error rather than a fixed formula. Aside from helping preserve straight lines in a very wide rectilinear pano, there aren’t major benefits. Drawbacks include tougher stitching, possible mistakes, and loss of top/bottom image area anyway.
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