Convert NEF to PNG

JPEG re-compresses every time it is written. One save is invisible; ten are not. Edges pick up ringing, flat gradients develop blocking, and the damage is cumulative and irreversible — each save compresses the artefacts of the previous one. Anyone who has retouched a file over an afternoon, saving as they go, has seen it.

PNG has no such behaviour. Its compression is exact, so the tenth save is bit-identical to the first and the file can be reopened and rewritten indefinitely without cost. That, and not any notion of superior quality, is what a PNG derived from an NEF is for.

It is worth being precise about the starting point. The converter does not develop the NEF; it extracts the finished JPEG your Nikon wrote at capture, complete with the Picture Control and white balance the camera applied. PNG then preserves that exactly.

So the PNG is a lossless copy of a compressed original. It cannot recover the highlight detail sitting in the 14-bit sensor data, cannot undo the camera’s tone curve, and is 8 bits per channel however many PNG is capable of holding. What it can do is stop any further loss, which is a smaller claim and a true one.

Every chunk in a PNG file carries a CRC. That makes bit rot detectable: a file that has been silently corrupted on a failing drive or by a bad copy will announce itself when a decoder checks the chunk rather than quietly rendering a band of garbage. JPEG has no equivalent and will often display a damaged file without complaint.

For anything that has to survive on storage for years without being looked at, that property is worth more than a few megabytes. It is one of the reasons PNG appears in preservation guidance alongside TIFF rather than being dismissed as a web format.

If the goal is a deposit copy rather than an editing copy, TIFF is usually the format the standard names, and it is available here as a target as well. It handles multiple images in one file, it is what most cataloguing and digital preservation systems expect, and it has decades of institutional practice behind it.

PNG is the right choice when the requirement says PNG, when the file has to be viewable in a browser without conversion, or when you want the checksum behaviour without the sprawl of TIFF’s many variants. Saying so plainly is more useful than pretending this page is the only sensible route.

Nikon’s sensors have grown steadily and the arithmetic scales with them. A 24-megapixel Z6 frame is roughly 72 MB of raw pixels before compression; a 45.7-megapixel D850 or Z7 frame is about 137 MB. PNG typically brings a photograph down to half or three-quarters of that, because photographs have very little of the repetition its compression looks for.

The result is files in the tens of megabytes, sometimes above sixty. That is fine on a workstation with a fast disk and painful over a shared drive or in a folder that gets synced. Decide where the file is going to live before generating four hundred of them.

The honest comparison is not PNG against JPG, it is PNG against opening the NEF in a raw developer. Working from the raw gives exposure, white balance and highlight recovery at almost no quality cost, because those adjustments are applied to 14-bit sensor data before anything is rendered.

Editing the extracted preview means making those decisions against 8-bit output with a tone curve already applied, where a two-stop shadow lift bands and a white balance shift skews the colours. Choose this route when the raw developer is not available, when the picture only needs local retouching rather than tonal work, or when the camera’s rendering is already what you want and you are compositing rather than grading.

Producing a PNG requires decoding and re-encoding, and the EXIF block does not come through. Lens, focal length, exposure settings, capture time and any GPS coordinate are absent from the output.

For a catalogue that is a workflow question rather than a loss, because the authoritative record should be read out of the NEF and stored in the catalogue anyway. Derivative files that carry their own half-copy of the metadata are how catalogues end up disagreeing with themselves.

Batches work, and consistency is the reason to use one. Drop the whole selection, let each file extract and encode with the same settings, and take the result as a ZIP. Converting files one at a time over several sessions is how a set ends up with three different maximum widths in it.

PNG encoding is genuine work at these pixel counts, so a large batch takes real time — considerably longer than the JPG route, where the preview is copied out without being re-encoded at all. Start it and leave it rather than watching it.

Everything happens in the browser: reading the NEF, finding the preview, encoding the PNG. Nothing is uploaded, which for material under a deposit agreement or a client contract is not a convenience but a requirement.

The free tier accepts input files up to 100 MB, which covers even an uncompressed D850 NEF at around 90 MB. A file over that is listed in the drop zone but not converted, and one drop takes up to 100 files at once.