Focal length changes between photo and panorama  

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by KJJones1974 » Sat Apr 02, 2011 12:54 pm

GURL wrote:I'm curious to know which projection you are using for your non-360x180 panos.

Spherical Projection, but the panoramas i take are usually only one or two shots high (vertical) and multiple shots across (horizontally), as you may expect, so cylindrical is a suitable (and maybe more appropriate substitute, depending on how the panorama is viewed).

To maybe help you understand where I can coming from... The panoramas I am taking are to represent the views from various locations (as printed panorama shots, ususally on A3 sheets) for use in visual impact assessments of proposed new build developments - the professional guidance for which states that the focal length used should be 50mm (35mm format equivalent), but I have a zoom lens so i get as close to the 50mm value as i can, and i must state the 'viewing distance of a print, such that it would represent the scale of the view with the naked eye when the print is held at that distance'. I can then use this print to determine the size of objects at a given distance in the photograph (for example). I have attached an example output.

To calculate the latter I need to know the focal length. The EXIF data is not very accurate and the calculated focal length in APG is obviously different each time. As I am using the APG render for use in the printed shot, i was using the calculated APG focal length to determine the 'viewing distance' for the prints... but i have come to the conclusion that that is at least as inaccurate (for my purposes) as the EXIF data, especially as it is different in different versions of APG!

The overarching question I am trying to find the answer to is:

'What focal length value should I be using to calculate the correct viewing distance? The original (actual) or a recalculated value (depending on what is altered by APG)?'

Either way, if i can get the calculated focal length (in APG) to very closely match the actual (which I may need to do some tests on my camera to find out exactly) then I will have a lot more confidence in the accuracy of the viewing distance, FOV etc.


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