Head height gets all the attention because it appears on every checklist. The eye line is quieter: it is in the specification, it is checked, and most people have never heard of it.
What the rule says
The eyes have to fall inside a horizontal band measured from the bottom edge of the photo. The US, for example, asks that the eye line sit between roughly 1 1/8 and 1 3/8 inches from the bottom of a 2 inch square. Other authorities express it as a percentage of the frame height, usually somewhere between 50 and 65 percent up from the bottom.
It is a second, independent constraint on the vertical crop. You can satisfy head height and still fail it.
How that happens
Picture a correct head height positioned slightly too high in the frame: a little too much space above the crown is impossible, so instead the chin ends up close to the bottom edge and the eyes ride high. Head height passes. Eye line fails.
The reverse is just as common. Crop generously at the top so the head is comfortably inside the frame, and the eyes drift below the band while the head height is still technically legal.
The two rules together pin the face in place both in size and in position. Satisfying one is not evidence about the other.
Level eyes and a level head are different things
There is a related check that trips people constantly. Faces are not symmetrical - most people's eyes sit at very slightly different heights, and almost everyone holds their head with a degree or two of tilt without noticing.
Straightening the photograph so the head looks upright often leaves the eye line at an angle. Straightening so the eyes are level is what the specification asks for, and it can leave the head looking imperceptibly tilted. The second is correct.
This is why rotation in tenths of a degree is worth having. A full degree of rotation is visible; the correction you usually need is smaller than that.
What this means when you crop
- Level the eyes first. Rotation changes every subsequent measurement, so doing it last means measuring twice.
- Then set head height. Chin to crown, inside the band for your document.
- Then check the eye line. If it is out, the fix is to move the crop up or down, not to resize it.
- Re-check head height after moving. On a fixed-aspect crop it will not have changed, but it is worth confirming.
If the eye line cannot be satisfied at a legal head height, the original framing is the problem and no amount of cropping will rescue it. Take another photo with the camera at eye level - which is, not coincidentally, the most common cause of the whole issue.
Camera height is the root cause
A camera held above eye level and angled down puts the eyes high in the frame and foreshortens the chin. A camera below eye level does the opposite and, as a bonus, photographs the underside of the nose.
Level with the eyes, about a metre and a half away, is the geometry every passport specification quietly assumes. Get that right and the eye line usually lands in the band without anyone having to think about it.
IDShot levels the eyes automatically, draws the eye line with the allowed band beside it, and measures both rules independently - so a photo that passes one and fails the other says so, rather than showing a single pass or fail.