Lens EXIF Data

A modern lens carries a small amount of electronics and a set of contacts on its mount. When you attach it, the body queries the lens over those contacts and gets back an identifier, the focal range, the aperture range, and — while you shoot — the focal length the zoom ring currently sits at and the aperture the diaphragm closed to.

The body writes those answers into EXIF at capture time. It does not measure them. That single fact explains almost everything below: a lens with no electronics reports nothing, an adapter that does not pass data through reports nothing, and a lens that reports the wrong identifier gets that wrong identifier written faithfully into your file.

On phones, there is no removable lens at all. The camera app fills the same fields with the fixed characteristics of whichever camera module fired — which is why an iPhone frame has a FocalLength of a few millimetres and a FNumber that never changes.

FocalLength is the physical focal length in millimetres at the moment the shutter fired. On a zoom it changes from frame to frame; on a prime it is the same in every file the lens ever produces.

FocalLengthIn35mmFilm is the same optical situation restated for a full-frame sensor, so that a number means the same angle of view regardless of what camera took the shot. A 25mm lens on a Micro Four Thirds body and a 50mm lens on a full-frame body frame the scene identically, and both write 50 here.

Not every camera writes the equivalent. When it is absent, a viewer has no way to compute it, because the sensor size is not recorded in any reliable standard field either. Some bodies write FocalPlaneXResolution and its unit, from which the sensor width can sometimes be inferred — but that is a reconstruction, not a reading.

Reading LensSpecification

This one tag describes the lens rather than the shot. It is an array of four rationals, in order: minimum focal length, maximum focal length, maximum aperture at the minimum focal length, and maximum aperture at the maximum focal length.

• 24, 70, 2.8, 2.8 — a constant-aperture 24–70mm f/2.8 zoom.
• 24, 105, 4, 5.6 — a variable-aperture zoom that loses a stop and a bit as you zoom in.
• 50, 50, 1.4, 1.4 — a 50mm f/1.4 prime. Both focal lengths match because it does not zoom.

A 0in either aperture slot means "unknown" — common with manual and adapted lenses, where the body knows the focal range because you typed it in, but was never told the maximum aperture.

There is no ImageStabilization field in the EXIF specification. If you want to know whether IS, VR, OSS, or IBIS was switched on for a shot, the answer is in the MakerNote — the manufacturer-defined block that each brand structures however it likes, and that the standard says nothing about.

The MakerNote is also the first casualty of editing. Export a JPEG from most software and the block is dropped or rewritten, taking the stabilization state with it. See EXIF data by camera brand for how each manufacturer structures this block.

An empty LensModel is normal far more often than it is suspicious. In rough order of likelihood:

• The lens has no electronics. Vintage glass, fully manual lenses, and most cine lenses have no contacts to report through. The body records the exposure it metered and nothing about the lens. You will often see FNumber of 0 as well, because the body cannot know what you set the aperture ring to.
• An adapter is in the way. A dumb mount adapter passes light and nothing else. A smart adapter passes data, but sometimes only partially — a common result is a correct focal length with an empty or generic lens name.
• The body does not write the tag. LensSerialNumber in particular is written by some systems and simply never populated by others. Its absence says nothing about the lens.
• A third-party lens identifies itself loosely. Independent manufacturers report an identifier the body maps to a name from its own internal table. When the body has no entry, you get a blank, a generic string, or — occasionally — the name of a completely different lens that happens to share the identifier.
• The file was processed. Social platforms, chat apps, and many exporters strip EXIF wholesale. A photo pulled from the web usually has no lens data because it has no metadata at all.

Confirm which case you are in by opening the original file — not a download or a re-export — in the EXIF viewer and checking the All tab. If Make and Model are present but every lens tag is missing, the camera wrote metadata and had nothing to say about the lens. If everything is missing, the file was stripped.

With that established, here is what to do. Take your own test frames on the actual lens — not photos the seller supplied — and open them in a viewer. Then look for internal inconsistency:

1. Does LensModel name the lens you were sold? A mismatch is worth an explanation. So is a suspiciously generic string on a lens sold as a first-party flagship.
2. Does LensSpecification match the lens barrel? A lens engraved 70–200mm f/2.8 that reports 70, 200, 4, 4 is not what the engraving says.
3. Can it actually reach the aperture it claims? Shoot wide open. If FNumber never goes below f/4 on a lens sold as f/2.8, the diaphragm or the reported specification is wrong — and it does not much matter which.
4. Does FocalLength stay inside the specified range? Values outside the range in LensSpecification mean one of the two fields is lying.
5. Does LensSerialNumber match the barrel? Only if your system writes it. Where it is written, a mismatch between the engraved serial and the reported one is a strong signal that the lens is not what it presents itself as.

For a photo someone else supplied — a sample frame in a listing — the questions are different, because you are now assessing the file rather than the lens. A Software tag naming an image editor, a missing MakerNote on a file that claims to come straight out of a camera, or an ApertureValue that disagrees with FNumber all indicate the file was rewritten at some point. That is not proof of dishonesty — exporting a photo does all three — but a listing photo should not need it.

Metadata is one input. Optical tests, the mount's fit and finish, the serial on the barrel, and the seller's standing matter more. Use the EXIF to raise a question, then answer it somewhere other than the EXIF. If you are checking a body rather than a lens, the brand pages under camera brands cover what each manufacturer records.