Near-limit softness

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 3.

What you see

One object is soft and everything behind it is as sharp as the camera ever gets. The soft object is the nearest thing in the frame.

This is the only softness in the pinhole set that depends on subject distance, and that dependence is what identifies it.

Likely causes

1. The nearest subject is closer than the near limit for the plate and focal distance in use.
2. No near limit was ever computed for that configuration.
3. The framing was chosen for the picture and the distance was a consequence.

What is happening: the chemistry and physics

The blur circle in a pinhole camera is

b = d(1 + f/u)

Blur circle against subject distance

where d is the hole diameter, f the focal distance and u the subject distance. The bracket grows as u falls. At infinity a point becomes a disc exactly the width of the hole; at u = f it becomes two hole widths.

There is no focus to lose, so this is not “out of focus” in the sense a lens user means. It is the geometric blur getting larger because the cone of rays leaving a near point is still spreading when it reaches the film.

A worked case makes the size of it concrete. At f = 120 mm with a 0.30 mm hole, the distant geometric blur is 0.30 mm and the diffraction blur is 0.44 mm, so the total is 0.74 mm and geometry contributes 41 per cent of it. Bring the subject to 300 mm and the geometric part becomes 0.30 × (1 + 120/300) = 0.42 mm, so the total goes to 0.86 mm — 16 per cent worse. At 150 mm it is 0.54 + 0.44 = 0.98 mm, 32 per cent worse, and that is visible.

Diagnostic questions

• Is the distant background sharp? This settles it in ten seconds. Optical blur from the aperture affects the whole frame equally; only the near-limit blur is distance-dependent. If the near object is soft and a wall thirty metres away is at the camera’s usual standard, this is the answer and nothing else on the tree is.
• Is the soft thing the nearest thing? It must be.
• What was u, and what is d(1 + f/u) for that configuration? The arithmetic is one line.
• Is the softness the same in every direction? If not, it is movement and the near limit is a red herring.

Corrective action

Move back, or accept it as a property of the picture rather than as a fault.

A smaller hole reduces the geometric term in proportion, but the diffraction term rises in inverse proportion, so below the optimum you lose more than you gain. This is the one case where the obvious lever makes things worse.

Prevention

Compute the near limit for each configuration once and write it on the frame in white pencil — the distance at which the total blur has grown by whatever fraction you decide is your limit. For the camera worked above, a 20 per cent limit puts the near limit at about 250 mm.

Record u in the exposure log for close work, so that a soft foreground can be checked against the arithmetic rather than argued about.

Keep the near limit in mind at the framing stage, where it is free, rather than at the reading stage, where it is not.