A set that can be resubmitted and compared

A photo set is comparable to a future set only when every image fills a defined slot - a fixed angle, distance and light - that a later set can fill again. Images told apart by role, not just content, are what let two sets months apart mean the same thing.

What a slot is

A slot is not "a photo of the subject." It is "the front-on photo, taken from about arm's length, level, in window light" - a specification detailed enough that two different photos taken to it, months apart, are directly comparable to each other.

Distance earns its place in that line because it changes geometry, not just framing. Ward and colleagues (2018) modelled face photos and found a shot from 12 inches enlarged the nose by 30% in men and 29% in women against a distortion-free projection, while 5 feet produced "essentially no difference." Two photos taken to the same slot at different distances are, by that measure, not the same photo.

A set of three slots might be front, three-quarter, and side. A set of two might be front and a close crop. What matters is not the count but that each slot is defined before the photos are taken, not assigned afterward by picking whichever three images turned out best.

How many slots make sense

More slots capture more of how the subject reads from different angles, which is useful given how much a per-angle result can vary within one set. But every additional slot is one more condition to hold constant on every future occasion, and a slot you cannot reliably repeat is worse than not having it - a partially-filled set with an inconsistent slot introduces exactly the ambiguity a mixed-condition set suffers from.

Two to three slots is enough to see angle-dependence without making the set fragile. A single slot is really the one-photo case with extra steps. More than four starts to compete with the number of angles you can actually hold level and re-light identically on a normal day, and the incremental slot rarely earns its own upkeep.

Documenting a set so it can be repeated

Each slot needs its own line: angle, distance, light source, crop, background. Four or five words per slot is enough - "front, arm's length, window light, waist-up" - written down once and reused, not reconstructed from memory each time.

The document is only useful if it is specific enough that someone who has never seen the previous set could stand in the same spot and take a matching photo. If a slot's description would let two different people produce two different photos, it is not specified tightly enough yet to call the set comparable.

What a comparable set buys you

Once the slots are fixed, a future set is not a fresh attempt at getting good photos - it is an attempt to refill the same slots as closely as the last time. That is the difference between a set that improves your understanding of the subject over a longitudinal series and a rotating pile of photos that happen to look similar.

It also gives a tool's own combination behaviour somewhere to be checked against. If the score a multi-photo tool returns moves between two sets that filled identical slots, the movement is either genuine change or tool noise - both interesting, and both only visible because the sets were built to be compared in the first place.

Rate Cock's own submission flow treats each upload as a discrete entry with its own timestamp rather than merging a set into one blurred record, which is the structural precondition for slot-based comparison to work at all - a service that keeps entries separable is one you can actually build a slotted set against.

The geometry a slot is trying to hold still is the same one covered by how field of view changes what a given distance captures - a slot's distance and angle only stay comparable if the lens behind them does too, which is a reason to keep the same device across a set rather than whichever phone is closest to hand.

A defined slot is also legible to a person, not just a tool. Sending a judge a set with named angles rather than an undifferentiated pile of photos is a small courtesy that happens to be the same discipline a comparable set requires anyway - specificity about what each image is meant to show.

None of this requires special equipment - a normal phone camera is enough for every slot described here, provided the slot's conditions, not the hardware, are what gets held constant from one set to the next.