Measuring from a photo

Putting a ruler in the frame does not make a photograph a measuring instrument. A photograph is a projection of a three-dimensional scene onto a flat plane, and three separate kinds of distortion sit between that projection and a real length, before anything about lighting or focus even comes into it.

Perspective and foreshortening

A camera's projection makes closer objects appear larger and farther objects appear smaller, and anything not exactly perpendicular to the lens axis is foreshortened - it appears shorter than it is. The shaft is rarely a flat, perpendicular plane relative to the camera; it has depth, and different points along its length sit at slightly different distances from the lens. That alone means a single photograph cannot deliver a straight-line length the way a ruler pressed to bone can, because the thing being measured is not a flat object presented squarely to the camera.

The reference object has to share the plane

Placing a ruler in the shot only calibrates scale for objects in the same plane as the ruler, at the same distance from the camera. If the ruler lies flat on a surface and the subject sits above that surface, they are not in the same plane, and the calibration the ruler is supposed to provide does not transfer - the scale that is correct for the ruler is wrong, by an amount that depends on the gap between the two planes, for anything not exactly on it. This is a geometry problem with no fix inside the photograph itself; it needs the reference at the same depth as the thing being measured, which a casual photo rarely achieves.

Lens distortion

Most phone cameras, especially at wide focal lengths, introduce barrel or pincushion distortion - straight lines bow slightly near the edges of the frame. Centring the subject reduces this but does not remove it, and the correction needed depends on the specific lens, which is not information a casual photo carries with it.

Angle of view compounds the problem

A wide-angle lens, which most phone front cameras use at close range, exaggerates whatever is nearest the lens relative to what is farther away - a well-documented effect in portrait photography, where a face shot too close appears distorted with an enlarged nose and a receded chin. The same near-far exaggeration applies to any subject photographed close up with a wide lens, and it interacts with the foreshortening problem rather than sitting apart from it: the parts of the subject nearest the camera are simultaneously the parts most enlarged by the lens and the parts least foreshortened, while the parts farther away are both shrunk by perspective and stretched flat by the projection. Untangling those two effects from a single image, after the fact, is not something better lighting or a higher-resolution sensor fixes.

Why a second photo does not solve it

A natural instinct is to take the photo more carefully, or from a better angle, or to take several and average them. That helps somewhat with framing but does not remove the fundamental issue: a photograph, however careful, is still a single two-dimensional projection, and turning that projection back into a real-world length requires knowing the exact geometry of the camera, the exact plane of the subject, and the exact distance at capture time - information a typical photo does not carry, however many of them you take. Clinical research has tested photographs against direct measurement and found them wanting: in Ohebshalom et al. (2007), Journal of Sexual Medicine, 68 men photographed their own curvature at home, and those photographs underestimated the angle compared with a goniometer used by clinicians on an injection-induced erection. A stereo pair from two known positions, or a depth sensor, can in principle recover more of that geometry, which is closer to what AR measuring apps attempt and where their own separate limits apply.

Why this is geometry, not software

None of these three problems is a matter of image quality or algorithm - a sharper photo or a better model does not undo optics. This is also why AI Penis is upfront that an image model estimating size from a photograph is doing inference from visual cues rather than recovering an actual measurement: the underlying geometry problem exists whether a human or a model is doing the reading, and it is that site's subject to cover in depth, not this one's. AR Measuring Apps covers a related but distinct approach - depth sensing rather than a single flat image - and its own, different limits.

What a photograph is actually good for here

A photograph is a legitimate input to a subjective rating - something like Rate Cock scores an image as an image, which is a different task from recovering a length from it, and a rating out of ten never claimed to be a distance in the first place. A human judge looking at the same photo on Rate Penis is giving an impression, not a reading, and neither instrument is trying to do what a ruler pressed to bone does.

The instrument that does work

For an actual length, the standard bone-pressed method uses direct contact between a rigid ruler and the body, which sidesteps every problem above: no projection, no plane mismatch, no lens. That is the whole reason it remains the reference method the published literature uses, rather than photography, despite photography being available for decades.