Why the same mistakes keep happening
Most photography exposure errors are not arithmetic errors — they are assumption errors. The formula is right, but the input is idealized: a round number, an optimistic rate, a best-case month. Below are the mistakes behind most bad results, grouped by the calculator where they bite.
Exposure Triangle Calculator
Thinking f/4 to f/5.6 is one stop of "a little bit" — it is exactly one stop (√2 ratio).
Compensating in the wrong direction after a change; the delta sign matters.
Chasing ISO-minimalism at the cost of shutter drag — a sharp noisy photo beats a clean blurry one.
Depth of Field Calculator
Stopping to f/16 for landscapes instead of using hyperfocal — diffraction softens the whole frame past ~f/11.
Forgetting that subject distance dominates: at macro distances, DoF collapses regardless of aperture.
Confusing background blur (bokeh) with DoF — a thin DoF at 200mm blurs backgrounds more than the same DoF at 35mm.
ND Filter Calculator
Trying to compose through ND1000 — compose first, filter second, always.
Forgetting that long exposures need tripod discipline and (on some cameras) a covered viewfinder against light leaks.
Buying variable NDs for critical work — the cross-polarization darkening at max strength is real.
Star Exposure Calculator
Trusting autofocus at night — manual focus on a bright star, verified by test shot, always.
Shooting past the rule "because ISO will fix it" — trailing is geometric; no ISO undoes it.
Forgetting the intervalometer and spare batteries; night sessions consume both.
The habit that fixes all of them
Write down the assumption you are least sure about every time you run a number. If the answer matters, test it: change that one input by ±20% and see whether the decision flips. If it flips, the assumption — not the math — is your real problem, and it deserves the research time.