Tables an charts base the circle of confusion, the Case A 50mm lens focused @ 2 feet aperture f/8 circle of confusion 1/1000 of focal length = 0.05mm. Span of D of F = 0.39 feet.
Case B 100mm lens focused @ 4 feet aperture f/8 circle of confusion same 0.05. Span of D of F same 0.39 feet = 4.7 inches
Case C 100mm lens focused 4 feet criteria for D of F on1/1000 of the focal length sets circle size at 0.01mm. Span of D or F = 0.79 feet = 9.5 inches.
No need to apply this rule of thumb because the display magnification for both case A and B will be the same. Conclusion: Case C not applicable because images case A and B will be displayed at the same magnification so span f D of F for A & B are the same.
Focal length 50 mm
Point focused upon 2.00 feet
Distant point sharply defined 2.22 feet
Near point sharply defined 1.82 feet
Span 0.39 feet
Diameter of circle of confusion 0.05 mm
f/number 8.00
Focal length 100 mm
Point focused upon 4.00 feet
Distant point sharply defined 4.21 feet
Near point sharply defined 3.81 feet
Span 0.39 feet
Diameter of circle of confusion 0.05 mm
f/number 8.00
Focal length 100 mm
Point focused upon 4.00 feet
Distant point sharply defined 4.43 feet
Near point sharply defined 3.64 feet
Span 0.79 feet
Diameter of circle of confusion 0.10 mm
f/number 8.00