Frame-to-frame density variation

Owned by reading the negative as evidence.

What you see

Frames on one roll that do not match one another in overall density, with no consistent direction: one heavy, three normal, one thin, and so on. The rebate is the same all the way down the strip, and the frame numbers are evenly spaced and equally legible.

Printing the roll needs a different exposure for nearly every frame, which is the practical complaint that usually brings people here.

Likely causes

1. Metering that responded to a different part of each scene — the commonest by a wide margin, and not a fault of anything.
2. A shutter whose error varies with the speed selected.
3. A shutter that sticks intermittently, giving occasional very heavy or very thin frames among correct ones.
4. An aperture that does not stop down consistently, particularly on a lens that has been stored.
5. Uncorrected reciprocity on the long exposures only, which affects those frames and no others.
6. A filter used on some frames and not others, with the factor applied inconsistently.

What is happening: the chemistry and physics

There is no chemistry in this entry, and that is precisely what makes it diagnosable.

Everything the whole film shared is eliminated by the observation itself. One roll went into one tank, in one bath, at one temperature, for one time, with one agitation scheme, and came out with one rebate density from end to end. Development, the developer, the fixer, the wash, the film’s manufacture and its storage history are common to every frame, so none of them can produce a difference between frames.

What remains is everything upstream of the tank that can differ from one exposure to the next: the light, the subject, the meter’s response to it, the shutter, the aperture and the filter.

!Rebate constant, frames varying — that is the whole diagnosis

Compare two adjacent rebate areas from a heavy frame and a thin one, side by side on a light box. If they match, the process treated both frames identically and the difference was made before the film reached the tank.

If they do not match, you have something else: uneven development, a flow fault, or a film that was not fully covered by the solution — and the entry you want is mottled or uneven development or insufficient solution volume.

Diagnostic questions

1. Do the rebates match between a heavy frame and a thin one? The branch above, and it is the first thing to do.
2. Does the error track the shutter speed? Sort the frames by the speed you used. A consistent offset at one speed and not at others is the shutter, and it is testable.
3. Does it track the subject? Bright backgrounds thin, dark backgrounds heavy, is a meter reading the background rather than the subject — a metering method rather than a fault.
4. Does it track the filter? ILFORD warn that with some automatic-exposure cameras the correction for deep red and orange filters can leave negatives underexposed by as much as one and a half stops even with through-the-lens metering.
5. Are the very long exposures the thin ones? Beyond half a second the reciprocity correction applies; ILFORD give HP5 Plus as an adjusted time equal to the metered time raised to the power 1.31, so a metered 30 seconds is about 92.
6. Is one frame wildly different rather than several slightly? That is a stuck shutter or a light leak reaching one frame, not a systematic error.

Corrective action

Print each frame on its own terms. Density variation between frames costs printing time and nothing else, provided each frame is itself printable.

Where a frame is beyond rescue, the diagnosis has still earned its keep, because the same cause is on the next roll.

Prevention

Keep an exposure record. Frame number, shutter speed, aperture, filter and what you metered off. Without it, none of the six questions above can be answered, and the fault cannot be attributed however carefully you look at the negatives.

Test the shutter across its range, at every speed you actually use, and record the result. A shutter that is a stop fast at 1/500 and correct at 1/60 will produce this fault indefinitely and will never look broken.

Settle on one metering method and use it every time, so that when it is wrong it is wrong consistently and can therefore be corrected. An inconsistent method produces inconsistent errors, and inconsistent errors cannot be calibrated out.