It appears to be based on the diagonal of the format (which is also what I think the camera/lens makers use) but clearly the different format proportions mean that in practice the results are indeed only roughly equivalent. With his mathematical bent, I think
@Andysnap would probably prefer to use Soret's tables
. Slightly more seriously, you can use the tables to calculate the angle of view for long and short sides of the format.
1. Soret’s Table of angles of view
At normal photographic distances, the image size is directly proportional to the focal length of the lens. If you double the focal length of the lens, the image becomes twice as large, and so you include less of the subject. Double the size of the recording surface, though, and the status quo is restored. Put another way, the ratio of the focal length and the length of the format is constant for a given angle of view.
The normally quoted form of the table gives the length of the format in terms of the focal length of the lens, but it would normally be easier to eliminate this and just provide the ratio, which is what I have done here. Mathematical accuracy would require that the angles have both degrees and minutes of degrees; but as a practical table for practical photographers, this is an unnecessary degree of accuracy, and I have omitted them except in a few cases where the adjacent values are sufficiently close together to require the greater detail.
A few worked examples should illustrate how to use it.
If you have a 50mm lens on a 35mm or full frame digital camera, the diagonal of the frame is approximately 43mm. Dividing 43 by 50 gives 0.86; from the table, this corresponds to an angle of view of 46°- which is the figure usually given by the lens makers. With an APS-C format camera, the diagonal of the format is 25.4mm (for Canon; Canon is slightly smaller than Nikon, Sony and Pentax) and the ratio becomes 0.50. From the table, the angle of view has now decreased to 28°.
Focal length/format length Angle of view
2.67 106°
2.5 102°
2.33 99°
2.25 97°
2.0 90°
1.75 82°
1.67 80°
1.5 73°
1.33 67°
1.25 64°
1.2 62°
1.17 60°
1.14 59° 28’
1.125 58° 42’
1.1 57° 37’
1.0 53
0.9 48° 27’
0.875 47°
0.86 46° 24’
0.83 46° 11’
0.80 44°
0.75 41°
0.67 37°
0.5 28°
0.33 18° 36’
0.25 14°
