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01-16-2012, 04:01 PM - 1 Like   #16
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QuoteOriginally posted by liukaitc Quote
saw this pic online.since I do not have ultra-wide lens...just curious..
8mm? 10mm? 12mm?
Your question intrigued me. It seemed, intuitively, that there must be one specific focal length that would have the perspective distortion seen in this picture. Then I got an idea. Why not use ptgui, and bring the image in as an 8mm APSC equivalent photo, and then adjust the pitch until the horizon is exactly in the middle of the new panorama. If the verticals came out straight, then the focal length is correct. This is the resulting perspective corrected panorama, with the horizon as close to the middle of the photo as I could get it, where the source photo is mathematically modeled as an 8mm APSC image, with a 112.6 degree HFOV, and a 90 degree VFOV. The output image was generated assuming the source image had a pitch of 15 degrees, and the output image has a VFOV=120.5 degrees, which is 90+15*2, plus a half of a degree of margin. You can see the little black line at the top. Mathematically, ultrawide rectilinear lenses only have vertical lines as vertical if the horizon is in the middle of the image, with a zero degree pitch.

Since the vertical lines in the resulting panorama came out more or less straight, with the horizon smack dab in the middle, I think this proves the source image was made with an 8mm APS-C lens, tipped upward by 15 degrees.
- Sheldon


01-16-2012, 05:10 PM   #17
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Fantastic sheld. You're hired for the CSI team.
01-16-2012, 05:53 PM   #18
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QuoteOriginally posted by sheld Quote
I think this proves the source image was made with an 8mm APS-C lens, tipped upward by 15 degrees
Sigma 8-16mm then?

BUT

Does your math fail if it was shot on micro-four-thirds?
01-16-2012, 06:53 PM - 1 Like   #19
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QuoteOriginally posted by DaveHolmes Quote
Sigma 8-16mm then?

BUT

Does your math fail if it was shot on micro-four-thirds?
If the picture was taken on a micro 4/3rd, than the source photo would presumably have a 4:3 aspect ratio, but lets say it was cropped vertically to a 3:2 aspect ratio. Than the 35mm equivalent would be 16mm film equivalent instead of the 12mm film equivalent values that I used above, which is not nearly as wide. I didn't try 16mm film equivalent, but I did try 15mm film equivalent, which would be the Sigma 10-20mm lens at 10mm lens with an HVOV=100.4 degrees and a VFOV=77.3 degrees. To get the horizon in the center with a mathematically ideal 15mm film equivalent lens, I had to assume the camera was pitched upwards 12 degrees. Notice the resulting image does not match, since vertical lines are no longer vertical/parallel. I posted the ptgui incorrectly corrected image below. So, the photo was taken with a 12mm film equivalent focal length, which is 8mm APS-C digital equivalent. It could have been taken at 12mm, on a full frame DSLR, using a Sigma 12-24mm lens, or 8mm on an APS-C DSLR, using a Sigma 8-16mm lens. I think the results are probably accurate enough to suggest that if it was taken on an APSC DSLR, than it was a Nikon/Pentax/Sony DSLR, using an 8-16mm lens at 8mm. This is as opposed to the slightly smaller sensor on a Canon DSLR, using the same 8-16mm lens at 8mm.
- Shel



Last edited by sheld; 01-16-2012 at 07:13 PM.
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