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Say I want shoot a butterfly. My goal is to make sure the butterfly fills the frame. I can do this in two ways...I can use zoom or I can simply move the camera closer to the subject.
Which will have a greater DOF? Max zoom or min-zoom + closer camera?
Here is the standard Depth of Field formula for reference:
DOF = 2 u2 N C / f2
N = aperture F-number
C = circle of confusion
u = distance to subject
f = focal length
When aperture and subject size within the frame are constant, DOF will not change because changes to focal length (f) and distance (u) will be proportional to each other and cancel out.
On a variable-aperture zoom shot wide open, DOF will be greatest when zoomed in because the F-number is larger.
At macro reproduction ratios, DOF is still controlled by aperture, but DOF can be so narrow that it is worth considering techniques such as focus stacking.
Although not very relevant to macro photography, people often refer to DOF when they want background blur. The amount of background blur does change with focal length, even when subject size and aperture are kept constant. Different focal length and aperture combinations can be compared by using f/N to estimate background blur.
On variable-aperture zooms, maximum background blur is usually at max focal length, rather than max aperture with minimum focal length, because zoom ratios are usually greater than 2, while the max-aperture ratio is usually less than 2.
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
