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Thanks to Erik Mathy for providing the formula.
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Formula:F = (f1 x f2) / (f1 + f2 - d)f1 is the front lens, f2 is the back lens, d is the distance between the lenses.
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BFL = (f2 x (d - f1)) / (d - (f1 + f2))F is the focal length, BFL is the distance between the back lens and the recording media (film, sensor)
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Lens Formula 1 two-element lens
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Front Lens1500While not necessary, it is best if both the front and back lenses are the same.
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Back Lens1500While not necessary, it is best if both the front and back lenses are the same.
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Lens Separation100
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Focal Length775.862069
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Back Focal Length724.137931
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All above units are mm
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Please leave the formula unchanged so others can use the formula and change the yellow values and see the blue values change accordingly.
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Lenses can be purchased at www.surplushed.com
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THE FORMULA BELOW DOES NOT WORK FOR A DOUBLE GAUSS IGNORE IT FOR NOW
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Formula for each pair:F = (f1 x f2) / (f1 + f2 - d)
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BFL = (f2 x (d - f1)) / (d - (f1 + f2))
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Lens Formula 2 Double Gauss *
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Front Pair
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Enter First Lens FL104First Lens focal length
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Back Lens104Second Lens focal length
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Lens Separation5Gap between first two elements
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Focal Length53.28078818
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Ignore this line50.71921182
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Third Lens
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Carryover From Above
53.28078818Results from above. Do not manually enter anything here.
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Enter 3rd Lens FL-158Third lens focal length.
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Lens Separation20Gap between first pair and second pair.
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Focal Length67.49853859
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Ignore this line42.16162414
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Fourth Lens
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Carryover From Above
67.49853859Results from above. Do not manually enter anything here.
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Enter 4th Lens FL69Fourth lens focal length.
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Lens Separation10Gap between second pair.
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Focal Length36.81781003
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Back Focal Length31.36320156
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* A double-Gauss lens has 4 elements with a Positive Meniscus and Negative Meniscus in a pair
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at the front and and another pair at the back flipped so the negative meniscus
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is inside and the positive is outside. It should look like this: (( )) (= Positive (=Negative
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