Uneven f-stop test-strip bands

Owned by f-stop timing and print exposure, which teaches the series and the rounding rule, and by commissioning the enlarger, which measures the illumination uniformity this fault most often turns out to be.

What you see

A test strip exposed in an f-stop series should step by a visibly equal amount from band to band, all the way across. This one does not: two adjacent bands look nearly the same, or one step is obviously larger than its neighbours, or the whole strip is even at one end and crowded at the other.

The temptation is to blame the timer. It is usually the enlarger or the tray.

Likely causes

• Illumination falloff across the strip. A long strip laid across the baseboard samples different parts of a cone of light. If the corners are down on the centre, the bands at the ends carry that difference on top of their exposure difference.
• The card moved wrongly on a cumulative strip. Each band on a cumulative strip is the sum of every exposure that preceded it, so one mistimed card move corrupts that band and every band after it.
• Increments rounded one at a time. Rounding each ideal increment to the timer’s resolution and adding them makes every band inherit every earlier rounding.
• Development that is not the same for every band. A strip that lies proud of a shallow tray, or is agitated by rocking one end, develops unevenly along its length — and the pattern looks exactly like an exposure error.

What is happening

Only the third cause is arithmetic, and Part XVII quantifies it so that it can be ruled out rather than suspected. On a five-band third-stop strip from a base of 12 seconds, rounding each increment separately puts the middle band — the one you read the base exposure from — 12 millistops high; rounding the cumulative first and taking differences leaves the worst band 8 millistops out, with no compounding as the strip grows.

!The arithmetic cannot be what you are looking at

A third of a stop is 333 millistops. The worst rounding error above is about a fortieth of that, and neither version is visible on a print.

So if you can see the unevenness, the rounding rule is not your fault — and a timer whose firmware implements it, as the course’s does in integer milliseconds, has not saved you from anything you were about to see. Look at the enlarger and the tray instead. The rounding rule earns its place on a twenty-band strip, where the errors would otherwise compound, not on a five-band one.

Diagnostic questions

1. Turn the strip end for end and expose another. If the same place on the baseboard stays dark rather than the same band, the fault is illumination, not timing.
2. Is the strip cumulative or single-band? Give each band its own exposure with everything else masked. A single-band strip cannot inherit an earlier error, so if the unevenness survives, the card was not the cause.
3. Does the uneven step move when you change the base exposure? A fixed band position points at the easel or the lamp; a fixed step size points at the series.
4. Is the tray deep enough, and is the agitation even along the strip’s length? Develop a strip given one uniform exposure across its whole area: it should come out uniform. If it does not, the tray is the fault and no exposure test will be readable until it is fixed.

Corrective action

Measure the illumination uniformity as density on paper, the way Part XVI’s commissioning sequence does, before trusting any strip laid across the baseboard. Where the falloff is real, put the strip near the centre and accept a shorter one, or stop down.

Expose single-band strips when the reading matters. They cost the same paper as a cumulative strip and cannot propagate a card error.

Give every band the same development: a tray deep enough that the strip stays covered, and agitation that moves solution across the whole strip rather than rocking one end.

Prevention

Commission the enlarger before printing on it, and re-run the uniformity map after any change to the head or the lens. Where a strip has to be long, expose it in the centre of the field and move the easel between bands rather than moving the card across a falling-off cone of light.