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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4731
Loc: N.E. Ohio
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Re: Strange Results in Eyepiece FOV Comparisons
05/17/08 10:32 AM
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Here's a formula to calculate eyepiece field distortion (FD), given that the eyepieces field stop (FS) and focal length (FL) are reliably known and trusted:
FD = [(FS/FL * 57.296)/AFOV] - 1
To represent FD as a percentage, just multiply the result by 100 and take the absolute value (ABS) of the result.
For the example of the 24 mm Panoptic (FL = 24 mm, FS = 27 mm):
FD = [(27/24 * 57.296)/68] - 1
FD = -0.0521
The percentage of pincushion (plus any FL induced error, if present) is therefore:
ABS(100 * -0.0521) = 5.21%
And since this is pincushion distortion (FD is negative), we can conclude that the TFOV for this eyepiece will be 5.21% less than would be indicated by the old formula:
TFOV = AFOV/Mag
PS: For those who are confused by the term "distortion", the one thing that needs to be made apparent is that pincushion, barrel, and/or angular magnification distortions do not involve such things as sharpness, resolution, contrast, transmission, color purity, lateral color, ghosting, etc... so they are not generally what first comes to mind when one thinks of image "distortion".
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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