Blocked highlights

Owned by Reading the Negative as Evidence, which restates this fault sensitometrically: the exposure slid the subject’s range along the log-exposure axis until the highlight end climbed onto the shoulder, where density stops changing with exposure. That page also separates the version that is not a fault at all — a subject brightness range longer than the film and development could hold, which is tone reproduction rather than failure. This entry was assembled earlier from commissioning the pinhole camera, which names it as one of the two failures at the ends of an exposure bracket.

What you see

An area that was bright in the subject and is now so dense on the negative that no detail can be seen in it. A white wall and the white window frame in front of it are one flat slab.

Read this on the negative and remember that a negative is inverted — what was bright in the world is dense here, and what was dark is thin. ILFORD’s description of a sound negative is the benchmark to compare against — a full range of tones, some parts almost clear like the rebates and others so dense you can only just read print through them.

Likely causes

1. The exposure carried that part of the subject past the top of the material’s usable range.
2. Development was carried far enough to move the highlights onto the shoulder.
3. The subject’s range exceeded what the material can hold.

What is happening: the chemistry and physics

Above a certain exposure a material stops adding density in proportion to what it receives. The characteristic curve turns over into its shoulder and then flattens, so two subject luminances that differ by a stop arrive at the same density, and the difference between them has not been recorded.

That is why it is not recoverable. There is no information in the area to bring back; the material stopped responding before the shutter closed, and no amount of printing can distinguish two densities that are equal.

Long exposures make it worse from a second direction. ILFORD note that contrast is increased with long exposures so that pulling the development may be required, which is why a pinhole worker meets blocked highlights sooner than the arithmetic alone suggests.

Diagnostic questions

• Is there any detail at all in the area under a bright light? If a loupe finds separation, it is dense rather than blocked and can be printed through.
• Are the shadows empty too? Both at once means the subject was beyond the material rather than that the exposure was wrong, and no single exposure in a bracket will fix it.
• Is the rebate normal? It must be, or you are looking at fog rather than at exposure.
• Recompute the exposure from the log, in the order speed, aperture, extension, reciprocity. A double reciprocity correction is the commonest arithmetic route to this fault.

!Which end to protect, and why

With one sheet and no second chance, protect the shadow detail. A slightly dense highlight can still be printed through and an empty shadow cannot be printed at all.

That asymmetry is why a bracket is read by finding the sheet where the shadows have just opened rather than the one where the highlights look best.

Corrective action

Nothing on that negative.

Where the subject’s range was the cause, the response is to change the scene — a subject with less range in it — or to accept the loss deliberately, which the art part of the course treats as a picture-making choice rather than as a fault.

Where development was the cause, shorten it on the next film. Where exposure was the cause, bracket.

Prevention

Bracket at half-stop intervals on any subject you have not photographed before, and read the bracket by the shadows.

Compute the exposure chain in order and record the metered time and the corrected time separately.

Meter or judge the brightest and darkest parts of the subject before exposing, and decide in advance which end you are willing to lose.