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 most variable-aperture zooms shot wide open, DOF will be greatest when zoomed out because the focal length is shorter. The effect of focal length is greater than that of aperture because it is squared in the formula.

• At macro reproduction ratios, DOF is still controlled by aperture, but DOF can be so narrow, regardless of aperture, that it is worth considering techniques such as focus stacking.

  • Pat David: Focus Stacking Macro Photos (Enfuse)

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.

• Does amount of background blur change with focal length given equal framing?
• How can I maximize the "blurry background, sharp subject" (bokeh) effect?