Depth of Field: Two Subjects

Name the two things that must be sharp and read the apertures back. It says when no aperture works, and which input would open one again.

Say what has to be sharp.

Most of these tools ask for one distance and hand back a range. Real frames have two things in them — the rock in the foreground and the ridge behind it, the front petal and the back of the flower. Give this one both distances and it works the other way round: where to focus, and which apertures actually reach from one to the other.

The exact requirement, not the nearest stop.

If a frame needs f/9.6, that is what it says, along with the first stop on the ladder that satisfies it and the one below that does not. Thirds of a stop are on the ladder, so a value the whole stops skip over is still yours to use.

No model list to be missing from.

Twenty-four formats, from sheet film down to a phone.

Choose a sensor by its measurements, or type the two numbers off the maker's spec sheet. There are no brands and no models anywhere in this app, so the table cannot go out of date and your body cannot be absent from it.

The sharpness standard is yours.

Everything here turns on the circle of confusion, and that number is set by you: name a print size and the distance you will stand from it, or count pixels instead. The micron it produces is shown with the arithmetic that made it, and it is printed beside every verdict.

It says when no aperture works.

Diffraction is measured against your standard, not against a fixed stop, so the ceiling moves when your standard moves. When the requirement passes the ceiling, the two walls cross and the answer is that there is no aperture — followed by which of three inputs would open one again.

Sent as a card.

The verdict goes out as a picture and as text: both distances, focus, the window, the exact requirement, the ceiling, and the standard it was all judged at.

Geometry only. Nothing here promises a result, and nothing leaves this device.

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