Camera RAW files (CR2, NEF, ARW, DNG): what they are and how to get usable JPGs

A memory card full of .CR2, .NEF or .ARWfiles behaves nothing like a folder of photos: thumbnails may or may not appear, double-clicking opens nothing, and whatever does open them shows something flatter than the camera’s own preview. This guide explains what a RAW file actually contains, how to tell which brand’s flavour you are holding, what a JPG keeps and throws away, and a workflow that gets shareable photos off the card with nothing but a browser — while keeping the RAW as the negative.

A RAW file is sensor data, not a picture

Nearly every camera sensor sits behind a Bayer colour filter array: each photosite records one colour only, in a repeating 2×2 block of one red, two green and one blue. A 24-megapixel RAW therefore holds 24 million single-colour brightness readings — 12 million green, 6 million red, 6 million blue — not 24 million full-colour pixels. Rebuilding three colours per pixel from that mosaic is called demosaicing, and it must happen before anything can be displayed. Fujifilm’s X-Trans sensors use a 6×6 pattern instead, which is why RAF files need their own interpolation code.

Each reading is stored at 12- or 14-bit precision: 4,096 or 16,384 levels per photosite, against 256 levels per channel in a JPG. That headroom is why a RAW can be brightened by two stops without banding, and why a slightly blown sky can often be pulled back. White balance is not applied either. The camera writes its chosen multipliers as metadata — dcraw -i -v lists them as Camera multipliers— and the pixels stay in the sensor’s native colour until software applies them. The tone curve, sharpening, noise reduction and lens corrections are deferred the same way.

So how does Explorer or Finder show a thumbnail at all? Every RAW carries an embedded JPEG that the camera rendered at capture time, usually at reduced size (dcraw reports it as Thumb size; a Canon 20D file embeds a 1536 × 1024 preview). File browsers show that preview, not the sensor data. That is why a RAW “looks fine” in a folder and different once developed: the folder showed the camera’s JPG.

Which extension belongs to which camera

The extension tells you the maker and, roughly, the era. Most RAW formats are private variants of TIFF, so their first bytes are a TIFF header; Canon’s newer CR3 is an ISO media container instead — the MP4 family — which is exactly why older software cannot read it. The last column shows what the in-browser converter on this site accepts; it filters by extension before reading a single byte.

The converter’s full list is eighteen extensions: cr2, nef, arw, dng, orf, rw2, raf, pef, srw, crw, kdc, dcr, 3fr, erf, mrw, nrw, sr2 and x3f; upper-case names straight off the card are fine. To check a mystery file, read its first sixteen bytes with xxd -l 16 IMG_1234.CR2 on macOS or Linux, or Format-Hex IMG_1234.CR2 | Select-Object -First 1 in PowerShell. A file whose name says .CR2 but whose bytes begin FF D8 FF is a renamed JPEG, and no RAW converter will take it.

Why your computer will not open it, and why the camera’s JPG looks better

Operating systems ship a fixed list of RAW decoders. Windows 10 and 11 need the free Raw Image Extension from the Microsoft Store (a wrapper around LibRaw); macOS relies on Apple’s own RAW support, which grows with OS updates. A camera released last month is usually missing from both, so its files get a generic icon and Photos refuses them. Vendor software (Canon Digital Photo Professional, Nikon NX Studio, Sony Imaging Edge) supports that vendor’s newest body and nothing else.

The second surprise is that a default development looks flatter than the JPG the camera would have written. The camera’s JPEG engine applies its picture style or film simulation, a contrast curve, sensor-specific noise reduction, sharpening, and lens profiles that correct distortion, vignetting and colour fringing. A neutral converter applies none of that: it demosaics, applies white balance and a standard gamma curve, and stops.

What converting to JPG keeps and throws away

The conversion is a one-way door. Once white balance is baked in, a tungsten-orange frame stays orange; once 16,384 levels become 256, a shadow that could have been lifted turns into posterised bands when you try. Keep the RAW as the negative; JPGs are prints. TIFF sits in between: it restores nothing the JPG already discarded, but it stops further loss, so park an image you will re-save several times as TIFF between rounds. The image formats guide covers where WebP and PNG fit.

How the in-browser converter develops a RAW

The RAW to JPG converterruns dcraw — Dave Coffin’s open-source decoder — compiled with Emscripten and executed in a worker thread inside your browser tab. It is called with dcraw’s camera white balance option (-w); if a file has no usable coefficients and dcraw aborts with Cannot use camera white balance, the converter retries with dcraw’s defaults. Everything else is stock dcraw: default AHD demosaicing, sRGB output, automatic brightening, no noise reduction, no lens correction, and the camera’s orientation flag is honoured so portrait frames come out upright.

dcraw emits an 8-bit PPM, which the page paints onto a canvas and encodes as JPEG at quality 0.92. One file downloads as name.jpg with a preview; several download together as raw-converted.zip, and the list shows each frame’s finished pixel dimensions or its error text. Because the JPG is rebuilt from pixels it carries no EXIF at all: no camera serial, no timestamp, no GPS, no lens name — the RAW still has all of it. Development happens on your own device, so the files are never uploaded anywhere.

Three limits apply. The bundled engine is dcraw 9.27 (May 2016); no dcraw release ever learned CR3, so Canon bodies from the EOS M50 and EOS R onwards are out, and cameras launched after 2016 may fail for lack of a colour matrix. A file that keeps the decoder busy for 120 seconds is terminated with The engine got stuck on this file — it may be malformed. Other conversions still work.And the JPEG step depends on the browser’s canvas, which phones cap well below a 45-megapixel frame; when that happens you see JPEG encoding failed — the image may exceed this device’s canvas limits. The fix is a laptop or desktop.

When a quick conversion is enough, and when you need a RAW editor

• Proofs, client selects, social posts, family sharing — quick conversion, provided white balance was right in camera and exposure within about a stop.
• Wrong white balance (a tungsten preset left on under daylight) — editor. The converter has no white balance control beyond what the camera recorded.
• More than a stop under- or over-exposed— editor. dcraw’s automatic brightening stretches the histogram but has no highlight recovery and no shadow slider.
• ISO 6400 and above— editor, or accept grain. No noise reduction is applied here, so high-ISO frames look noisier than the camera’s JPG.
• Wide-angle architecture and horizons — editor with lens profiles; the converter has none.
• Prints and anything large — editor, exporting a 16-bit TIFF rather than an 8-bit JPG.

darktable and RawTherapee are free and open source; recent versions of both read CR3 through LibRaw. Lightroom and Capture One are the paid options. One trap: those editors keep adjustments in a catalogue or in .xmp sidecar files next to the RAW. The in-browser converter reads sensor data only, so it never sees those edits — export from the editor when you want them.

From card to shareable photos without a desktop RAW editor

1. Copy, do not move, the card into a dated folder such as 2026/2026-09-06_harbour-walk/raw/. Camera counters roll over at 9999 — and restart on every card if file numbering is set to auto-reset — so the folder date is what stops this year’s IMG_1234.CR2colliding with next year’s.
2. Back the RAW folder up before doing anything else — a second drive plus one copy elsewhere. Only then format the card, in the camera rather than on the computer.
3. Develop the whole folder in one go. Drop every RAW onto the converter; they are developed in sequence and arrive as one ZIP. Unzip into …/jpg/ next to raw/; these full-resolution JPGs are your masters.
4. Shrink only the ones you are sending. Run those through the image compressor. It opens each photo in an editor whose Save dialog has Compress image (smaller file, quality adjusts automatically) pre-ticked: that encodes at quality 72 and caps the long edge at 2,560 pixels. If a form has a hard limit, type it into Max file size (KB, optional) instead — the tool searches for the highest quality that fits under the cap and steps the dimensions down only when even the lowest quality is too large. An image longer than 4,096 pixels on a side is downsized when the editor opens (Large image resized to … so it can be edited and saved on this device.), so keep the step-3 masters. The compression guide explains the numbers.
5. Make TIFFs only when someone asks. A print shop or journal that insists on TIFF gets one from the TIFF converter, which encodes with ImageMagick compiled to WebAssembly and writes an 8-bit name_output.tiff. Its inputs are ordinary image formats (JPG, PNG, WebP, HEIC, PSD, TIFF), not RAW, so the order is RAW → JPG → TIFF. Select several files and pick Convert all to TIFF (ZIP) to keep full resolution (each arrives as name.tiff inside converted-tiff.zip); the single-file path goes through the editor and applies the same 4,096-pixel working limit as the compressor.

Troubleshooting

“this file could not be developed — it may not be a supported RAW format”

The extension passed the filter but dcraw could not decode the contents. Most often the camera is newer than 2018: a CR3 renamed to .cr2, a Nikon Z-series NEF variant, a body dcraw has no colour matrix for. Convert to DNG with the free Adobe DNGConverter first — leave lossy compression and “embed original” off — then drop the .dng here. If DNG Converter also refuses it, the file is probably damaged: recopy it from the card.

“Drop a RAW photo — supported extensions: CR2, NEF, ARW, DNG, ORF, RW2, RAF, PEF, SRW, CRW…”

None of the dropped files had an extension on the list. The usual culprits are .cr3, which is not supported, and JPGs or HEICs mixed into the selection — the converter silently skips those and only complains when nothing at all qualifies. iPhone HEICs belong in the HEIC guide instead.

The colours look flat, or different from the camera’s preview

Expected, for the reasons above: no picture style, no lens profile, no sharpening. If the problem is a colour cast rather than a lack of contrast, the camera’s white balance setting was wrong at capture and only a RAW editor can change it after the fact.

The JPGs are nearly as big as the RAWs

Expected: every pixel is written at quality 92, so a 24-megapixel frame lands at 6–12 MB. That is the master. Send copies through the compressor; its pre-ticked option typically lands around 1 MB.

A DNG from a phone will not open, or opens strangely

Pixel and most Android camera apps write standard mosaic DNGs that develop like any other RAW. iPhone ProRAW is different: it is a linear DNG — already demosaiced, with Apple’s computational processing in the pixels — and it uses DNG 1.6 features that postdate dcraw, so it may be refused. Export a JPG or HEIC from Photos for those. DNG Converter output is always the mosaic kind and opens normally.

“The RAW engine did not load — reload the page and try again.”

The decoder runs inside a small sandboxed frame that must load within 15 seconds. A content blocker that strips iframes or an interrupted first load stops it. Reload with the blocker paused; once fully loaded, the page keeps working offline.