Convert DNG to PNG

That is the question worth answering before converting anything. DNG exists to be the file kept: documented, standardised as ISO 12234-2, not dependent on a manufacturer continuing to support a discontinued camera. If somebody converted an archive to DNG, they were making a preservation decision.

The PNG is therefore not a second master. It is an access copy — the thing a viewer displays, a catalogue thumbnails, a colleague on a machine with no raw support can open. Treating it as anything more leads to two files claiming to be authoritative and a slow drift between them.

For most access purposes JPG is the better answer and it is worth saying so on a page about PNG. Browsing a catalogue, sending a colleague a picture, illustrating a record — none of these are affected by lossy compression, and the files are a fraction of the size, which matters when there are forty thousand of them.

PNG earns its place when the derivative will itself be worked on: annotated, measured, compared between versions, or re-saved repeatedly. JPEG re-compresses on every write and the damage accumulates; PNG does not, so a file that will be opened and saved a dozen times stays exactly as it started.

The converter does not develop the sensor data. It finds the JPEG stored inside the DNG and re-encodes it as PNG. In a camera-native DNG — a Leica, a Pentax, a drone, an iPhone shooting ProRAW — that preview came from the device at capture.

In a DNG converted from a CR2, NEF, ARW or ORF, it was generated by the converting software using whatever raw settings applied at the time, and Adobe DNG Converter offers it at medium or full size. So the appearance and the dimensions of your access copies were decided by whoever ran the conversion, possibly years ago. It is worth checking one file before generating the set.

Every chunk in a PNG carries a CRC, so a file corrupted by a failing disk or a bad copy announces itself when a decoder checks it rather than rendering a band of noise. JPEG has no equivalent and will often display damaged data without complaint.

For a derivative set that sits on storage for years and is read occasionally, that is a practical property rather than a theoretical one. It does not replace fixity checking across the whole collection, but it means a damaged access copy is noticed at the moment somebody tries to use it.

The arithmetic is the constraint. A 12-megapixel phone DNG yields roughly 36 MB of raw pixels, and PNG typically brings a photograph to half or three-quarters of that. A full-resolution preview from a high-resolution camera is several times larger again.

Multiply by the size of the collection before starting. Access copies are usually better generated at a fixed maximum width — 2,000 pixels serves almost any viewer and most catalogue front ends — which keeps the set predictable and an order of magnitude smaller than full resolution.

The re-encode drops the EXIF, so the PNG carries no camera model, no exposure data, no timestamp and no GPS coordinate. That sounds like a loss and is mostly a discipline.

The authoritative metadata is in the DNG and should be read from there into the catalogue record. Derivatives that carry their own partial copy are how a collection ends up with three slightly different capture dates for one image and no way to decide which is right.

A DNG can carry sensor data at up to 16 bits per channel, and PNG can store 16 bits per channel, which makes it tempting to assume the depth carries across. It does not. The extracted preview is an ordinary 8-bit JPEG, so the PNG is 8-bit.

If a derivative with real tonal depth is required — for reproduction, for tonal analysis, for anything that will be adjusted heavily — it has to come from software that demosaics the DNG. Adobe Camera Raw, Lightroom and darktable all read DNG properly and export 16-bit TIFF, which is what that requirement usually names anyway.

Drop the whole selection, use one maximum width for the set, and take the result as a ZIP. Consistency is the point of doing it in a batch: a collection converted over three sessions with different settings is a collection that has to be regenerated.

PNG encoding at these pixel counts takes real time, considerably more than the JPG route where the preview is copied out without re-encoding. Input files are accepted up to 100 MB each on the free tier, which covers phone and drone DNGs easily and can be tight for 16-bit conversions from high-resolution cameras.

Everything runs in the browser: reading the DNG, locating the preview, encoding the PNG. Nothing is transmitted, which is the difference between a usable tool and an unusable one for anything under an access restriction, a donor agreement or a research approval.

It is verifiable rather than promised. Open the developer tools, watch the network tab through a conversion, or disconnect the machine entirely and convert anyway — the page keeps working because there was never anything for the network to do.