The number of frames required to create a complete 360 x 180 degree spherical (virtual reality - VR) panorama is dependent primarily upon the focal length of the lens used. Panoramas can be created from a single, double or multiple rows of images. The minimum number of images is made possible by using a 180 degree field of view fish-eye lens, because a hemisphere can be captured in each image. However the need for at least 20 - 30% overlap between images, to make seamless stitching possible, means that at least three images are required. Note that this is not necessarily the best approach because each image would have to be heavily processed to create a spherical panorama.
The basic requirement for the minimum number of images is of course to achieve total coverage of the 360 x 180 degree field of view with a suitable overlap, both vertically and horizontally, between each image. The number of images can be established simply by rotating the camera through 360 degrees and noting, by observing the position of landmarks in the frame, the required overlap. If in doubt, leave 25 - 30 % overlap for each adjoining image. Once the number of the number of required rows has been established, together with the number of images for each row, set the camera in turn to each of the required elevations, and pan around to predetermined azimuth settings to capture the necessary rows of images. Where subject movement is captured in images, allow greater overlap, and if necessary take duplicate images, so that unwanted elements can more easily be removed during processing. The presence of featureless elements close to the edges of images, and significant differences in over or under exposure in adjacent frames, may also necessitate larger overlaps.
When using a lens having a higher focal length, and a narrower field of view, more images are required to complete a panorama. When using any lens other than a fish-eye, at least two rows of images must be acquired. One of these is typically taken with the camera elevated about 30 degrees above the horizontal and the other with a negative elevation of about 30 degrees (ie with the camera lowered below the horizontal). Zenith and nadir images should also be taken at the time of shooting the other images, although they are not necessarily used in the final panoramas in all cases.
The table below gives a few examples of the number of images required when using lenses of a particular focal length. Remember that the size of a camera's sensor (full-frame or APS-C) also affects the calculations. The figures quoted should be regarded only as a guide since factors such as percentage overlap and angle of elevation and declination may influence requirements.
|Lens Focal Length||Sensor Size||Number of Rows||Number of Frames|
|10 mm Fisheye||Full-frame||1||3 (+N)|
|16 mm Fisheye||Full-frame||1||6 (+Z+N)|
- "Z" indicates the requirement for a zenith image.
"N" indicates the requirement for a nadir image