Depth of field
What stays sharp in front of and behind the subject, from the aperture, the focal length and the distance.
Depth of field is a decision, not a measurement
Only one plane in front of a lens is actually in focus. Everything else lands on the sensor as a small disc, growing as it moves away from that plane. We agree to call a point “sharp” while its disc stays under a certain size — the circle of confusion — and that size is a convention someone chose.
Which is why two calculators can hand you different numbers and both be right. Ours divides the sensor diagonal by 1500 and prints the result, so you can compare it with whatever your other tool assumed instead of guessing which one lied.
H = f² ÷ (N × c) + f
The rule everyone repeats, and when it is false
“One third in front, two thirds behind” holds around the middle of the range and nowhere else. Focus close and the split is nearly even. Focus far and almost all of the depth falls behind the subject — until you cross the hyperfocal distance, where the far limit becomes infinity and the ratio stops meaning anything at all.
So the tool prints the split it actually computed for your settings. If it says 43 % in front, that is what your shot does, whatever the rule says.
Frequently asked
Why do two depth-of-field calculators disagree?
Because depth of field is not a physical edge — it is a convention. Only one plane is truly sharp; everything else is a disc that grows as you move away from it. We call it “still sharp” while that disc stays smaller than a circle of confusion, and different tools pick different circles. Ours is the sensor diagonal divided by 1500, which is the usual one, and we print the value so you can check.
What is the hyperfocal distance for?
It is the focus distance at which everything from half that distance to infinity falls inside the zone. It is the single most useful number in the whole calculation for a wide shot: set it once and stop worrying about focus.
Is “one third in front, two thirds behind” true?
Only in the middle of the range, and it is one of the most repeated half-truths on set. Up close the split is nearly half and half; far away almost all the depth falls behind the subject, until it reaches infinity and the ratio stops meaning anything. The tool prints the real split for your settings instead of quoting the rule.
Does the sensor change the depth of field?
Through the circle of confusion, yes — a smaller sensor tolerates a smaller disc because the image is enlarged more to reach the same screen. But the bigger effect is indirect: on a small sensor you reach for a shorter focal length to frame the same shot, and that shortens the focal length in the equation. That is why phones struggle to blur a background.
Is this exact?
The formulas are exact for a thin lens: H = f² ÷ (N × c) + f, and the near and far limits follow from it. Real lenses depart from a thin lens at very close distances, and focus breathing shifts the focal length as you rack. For preparing a shot, this is the right order of magnitude.