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Matt Grum mentions in a comment on an answer related to locking parameters when shooting a panorama that it is beneficial to zoom out on the middle section of the image while shooting a single row panorama when a rectilinear projection is applied in post.

This would mean that one focal length is used for the side of the image, a slightly shorter focal length at the middle section, and again the longer focal length at the other side of the image.

The goal is to decrease the bow-tie shape that a wide, rectilinear panorama has and hence do less cropping on the final image.

How much should one decrease the focal length between the side parts of the panorama and the centre part such that the bow-tie shape is minimised and less parts have to be cropped off?

Are there any other benefits to zooming while shooting a panorama image?

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I'm afraid I haven't made myself clear in the question. I meant using variable focal lengths while shooting a panorama. See the Matt Grum's linked comment for more clarity. I will update my question. –  Bart Arondson Jun 3 '13 at 15:32
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To see the bow-tie shape that Bart is referring to, see the rectilinear projection examples here: cambridgeincolour.com/tutorials/image-projections.htm –  coneslayer Jun 3 '13 at 15:49
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I only said "zoom" in my comment but I actually meant "zoom out". You want to maximise the vertical field of view of the central images compared to the edges by zooming out. –  Matt Grum Jun 3 '13 at 16:28
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2 Answers

up vote 5 down vote accepted

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.

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Not sure about that first paragraph. I would say panorama software that are capable of stitching images of various focal-length would simply undo the zoom in when they distort images to match features, after all the whole thing has to still hold as a rectilinear projection of the scene. Now shooting multi-row would work because you would be filling the gap. –  Itai Jun 3 '13 at 17:13
    
@Itai It's not a zoom in you need for the centre images, it's a zoom out. And the panorama stitching software will indeed "undo" the zoom out, by enlarging the centre images, thus avoiding a bow-tie shaped stitched result. –  Matt Grum Jun 4 '13 at 7:25
    
In that case you would lose resolution of the final panorama. –  Itai Jun 4 '13 at 14:39
    
@Itai yes, you lose resolution but gain in terms of the vertical field of view. I never stated zooming was a particularly good idea, only that it was possible! Note you also lose resolution at the edges when producing a wide rectilinear panorama, so for the best all round resolution, shoot multiple rows in the middle, then zoom in and shoot multiple rows at the edges! –  Matt Grum Jun 4 '13 at 14:45
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When you shoot a panorama the result of the stitched image in the rectilinear mode will often looks like this:

enter image description here

The bow effect can be reduced by zooming in the photos that appears in the middle of the final image.

Personally, I don't see the benefits of this method, other than keeping the lines straight.

  1. Cropping the image in post processing will be much easier.
  2. Stitching photos that does not have the same focal length can create a lot of problems.
  3. You loose details from the bottom and the top of the frame.

Using a rectilinear projection is not recommended for images that cover more than 120°. If you are going to shoot a very wide field of view and use a rectilinear projection anyway, I suggest you'll take the middle image first (or at least as a reference image) and then shoot the side images with a zoom that will cover the same horizontal bottom and top lines of your reference image.

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I'm sorry, I haven't made myself clear in the question.It is about zooming while shooting a panorama, i.e. using different focal lengths for different parts of the panorama. Please see my edited question. –  Bart Arondson Jun 3 '13 at 15:37
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In my question I mention rectilinear mapping. An example is shown here. Cropping it would severely reduce the field of view, hence zooming into the middle section is adviced (I think at least that's the reason). Your panorama has a different mapping (spherical?) where it is easier to crop it. –  Bart Arondson Jun 3 '13 at 15:51
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It's an example of rectilinear projection, doesn't have much to do with the quality of the software. The linked result (as your 270 deg. panorama) can be obtained with Photoshop's panorama tool . I'm aware that I can do wide panoramas like you show, and have done so myself successfully. However, I want to learn more about this specific case when you use a variable focal length + rectilinear mapping to make a panorama. –  Bart Arondson Jun 3 '13 at 16:02
    
You are showing a cylindrical projection, rectilinear cannot exceed 180 degrees. Some arguments may apply but I have no idea and was curious of someone would find a good reason to zoom-in. –  Itai Jun 3 '13 at 16:03
    
I focused my answer on the rectilinear projection. –  Itay Gal Jun 3 '13 at 16:11
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