a paper from loupe

Your camera raw files

What loupe does with a raw file, from the moment you upload it to the moment someone sees it, and why.

A raw file is the closest thing a digital camera has to a negative. It records what the sensor saw, before anything has been decided about how it should look. This paper follows one raw file through loupe: what loupe reads from it, which picture it shows, what it keeps, and what it will never do to it. It ends with how other kinds of tools treat raw files, and what we have not settled.

1. A raw file is a negative

Straight off the sensor, a raw file is not yet a picture. It is a set of measurements of light: dark, flat and slightly green if you looked at the numbers directly. Every program that opens one has to develop it, choosing a brightness, a contrast and a color before anyone has edited anything. Each program has its own way of doing that, so the same raw file looks a little different in each one.

Two more things follow, and they shape everything loupe does.

Your edits are not in the raw file. When you crop, brighten or recolor a photograph in your editing program, the program writes down what you did. Those notes live in the program's catalog, in a small .xmp file beside the raw, or, for some programs, inside the raw file itself.¹ They are instructions only the program that wrote them knows how to carry out exactly. One open-source program's own manual says of its import of another program's settings that it "will never give identical results".² The only file that carries your finished picture everywhere is one your program made from the raw: an exported JPEG or TIFF, or a full-size picture your program saved inside a DNG.

Every raw file carries a small picture of its own, and it can be mistaken for the photograph. When the camera takes the picture, it also makes a JPEG and tucks it inside the raw file. That is what the camera's screen shows, and what photo browsers show first. Its size varies enormously from camera to camera:

Camera (year)	The photograph, as recorded	The picture inside	Share of the photograph
Nikon D2H (2003)	2496 × 1648	570 × 375	5%
Canon EOS-1Ds Mark II (2004)	4992 × 3328	1536 × 1024	9%
Canon EOS-1D Mark II (2004)	3504 × 2336	1536 × 1024	19%
Canon Digital Rebel XT (2005)	3456 × 2304	1536 × 1024	20%
Nikon D70 (2004)	3039 × 2014	3008 × 2000	98%
Nikon D3 (2007)	4288 × 2844	4256 × 2832	99%
Sony A7 IV (2021)	3516 × 2344	3504 × 2336	99%

Measured by loupe on sample raw files from a public archive dedicated to the public domain, one file per camera.³ A wider survey of that archive, 535 files in six raw formats, found that it depends on the maker and the camera's age: most Nikon cameras from about 2004 on and most Canon cameras from about 2008 on carry a full-size picture, Sony's from about 2020, and Fujifilm's and Olympus's raw files none at all.

The small picture looks finished on screen. A tool that hands it out as the photograph may hand out a tenth of what the camera recorded, or less, and if the raw file is then let go, nobody notices until a print is wanted. Keep the raw file.

2. Bringing raw files in

What loupe takes. Twenty-three kinds of raw file, among them Canon's CR2 and CR3, Nikon's NEF, Sony's ARW, Fujifilm's RAF, Olympus's ORF, Panasonic's RW2, Pentax's PEF and the open DNG, up to 300 MB each. loupe knows a raw file by its name, because a web browser often calls a raw file something else, and a program that believed it would open only the small picture inside.

Its fingerprint, before it leaves your computer. Your browser works out a fingerprint of every file before it is sent: a long number that changes if even one byte of the file changes. loupe keeps it, lists it in your copy to keep, and checks the stored file against it each time it makes the photograph's large picture and thumbnail. If they ever disagreed, loupe would make no copies and say so.

Your .xmp, read first. Choose the .xmp beside each raw file along with it. loupe pairs them by name, reads the .xmp before anything inside the raw file, and keeps it exactly as it came. From it loupe offers your title, caption, keywords, date, place, credit, rights, who is pictured and your star rating. Nothing goes into your record until you have said yes. The develop settings in it are kept, and never carried out: see §4.

Other programs' settings, kept. Some editing programs write their own settings file beside the raw instead of an .xmp (.pp3, .dop, .cos, .nksc), and two keep it in a folder of their own. Choose it with the raw, or drop the whole folder and loupe finds those two folders. It is kept exactly as it came, in the folder it came from, and loupe carries none of it out. It reads only what such a file is known to carry about the photograph: its star rating, keywords, caption and title, where that kind of file carries them. They are offered as an .xmp's are, after the .xmp's own, and nothing goes into your record until you have said yes.

Stars. A rating in your .xmp, or in another program's settings file, is read and kept. Stars stay out of sight until you turn them on, and loupe never writes them into a file it makes for someone else.

A raw file and your finished picture, as one photograph. If you upload a raw file together with the picture you finished from it, with the same name (IMG_1.CR2 and IMG_1.jpg), loupe keeps them as one photograph. Your finished picture is the one shown; the raw file is kept with it, untouched, and never cut into copies. If the raw file comes later, add it on the photograph's own page, under its camera raw; loupe then reads the two together, as if they had arrived at once. If your finished picture, exported from your editing program, came out noticeably smaller than the raw file, loupe asks you, once, on the photograph's page: show the finished picture, or show the whole raw. You can change your mind later with show this one instead; nothing is copied or lost when you do.

3. What loupe tells you arrived

A raw file can carry a good deal besides the photograph: the camera's own picture, a record of an edit, adjustments saved by the camera maker's own software. loupe reads the file's structure for these, and each raw photograph's page has a line headed what arrived, only where there is something worth saying. After an upload of several raw files, one sentence sums them up: Of 12 raws: 9 carry the camera's own full-size picture; 3 were developed by loupe because the picture inside is small.

Some of the sentences you may see:

"It carries the camera's own picture at full size, which is what you see."
"The picture inside this file is small, about a tenth of the photograph, so loupe developed the raw itself, as the camera saw it."
"The photographer edited this in Lightroom: cropped it, brightened it a little and added contrast. Loupe can't show those changes, so you're seeing the photograph before them. Ask the photographer for the finished picture."
"The photographer adjusted this in Canon's own software. The adjustment is recorded in the file, but only Canon's software can show it."
"The camera was set to black and white. Its own picture is in black and white; loupe's development of the raw is in color."
"It was opened in Lightroom; its record shows no change."
"No editing record came with this file."

That last sentence is worded with care. loupe can tell when an editing record came with a file. It can never tell that a photograph was not edited, because edits often stay inside the photographer's own program and never travel with the file. So loupe never says "not edited".

4. Which picture you see

For each raw photograph, loupe shows the truest picture it has, in this order, and says which one it chose:

What came in	What you see
Your finished picture, uploaded with the raw file	Your finished picture (§2).
A raw file carrying the camera's own picture at full size (within a tenth of it)	The camera's own picture: what you saw on the camera's screen. If the camera was set to record in Adobe RGB, loupe converts it properly for the screen rather than reading it as ordinary color.
A DNG carrying a full-size picture made by your editing program	That picture, with your edits in it.
Anything else	loupe develops the raw file itself (§5).

A small picture is never passed off as the photograph. If the picture inside is smaller than that, loupe does not show it. It develops the raw file instead and tells you why.

Your develop settings are kept, and not carried out. loupe does not try to repeat another program's edit. A copy that looked nearly, but not exactly, like your finished picture would be worse than an honest one that is plainly the photograph before your edits; and only the program that made the edit can make it exactly. For finished work, upload the finished picture beside the raw file.

5. When loupe develops a raw file itself

As shot. loupe develops a raw file the way the camera recorded it: the camera's own white balance, and no setting from any program applied. The result is loupe's development of your raw file, and loupe calls it that. It is never presented as your picture.

A familiar starting look. Since September 2026, loupe develops a raw file to look the way photographers are used to seeing it before they begin to edit: natural color, a gentle contrast, and the same starting point whatever the camera. Before that, loupe's development was flatter and grayer. How it is done, briefly:

the camera's own color measurements, blended for the light the picture was taken in;
the camera's own brightness, taken from the file where it says, from a small table of cameras where it does not, and with the stop added back for cameras that were set to hold a stop in reserve for bright highlights (Canon's Highlight Tone Priority, which loupe reads from the file);
a tone curve and a color look measured by loupe from real photographs, shared by every camera unless the file carries its own.

The version of the look is written into each photograph's own record, so that when the look improves, every copy made with an older one can be found and made again.

Each raw file in a room of its own. The code that reads raw files, like all such code, parses whatever file it is given. So loupe never develops a raw file in the part of loupe that serves your pages. Each one is developed in a separate process of its own, with a limit on its time and memory, and none of loupe's keys or passwords. A damaged or hostile file fails alone, and everything else carries on.

When it cannot. A few kinds of raw file cannot be read yet, among them Nikon's High Efficiency raw files and DNGs compressed with JPEG XL. loupe says so in plain words and asks for the JPEG you exported instead. A damaged file is called damaged, not unknown. If the starting look ever cannot be applied to a file, loupe develops it the plain way it always has, and the photograph's record says so. A sensor that sees only in black and white is developed in black and white.

6. Showing it, and handing it out

Your file is never changed. loupe makes its own copies for the screen from the picture it chose in §4: a large picture, a thumbnail, and, for a large photograph, the tiles that let a visitor zoom in. They are converted to the color space every screen understands, and loupe records how. They are loupe's copies, made again whenever they need to be, and they are not yours to keep.

What you download. Your raw file comes back to you exactly as you uploaded it, and so does the file kept beside it.

What a visitor downloads. Nothing, unless you allow it. If you allow the file to print from for a raw photograph, the visitor receives the picture loupe shows: your finished picture as you uploaded it, if you uploaded one; otherwise a full-size JPEG made as it goes out, at high quality with its color profile, and never stored. No one is handed a raw file to print from, because almost no one could.

7. Keeping it

Your copy to keep is loupe's complete copy of your catalog, for you to download and keep yourself. For a raw photograph it holds:

the raw file, exactly as you uploaded it;
your finished picture, if you uploaded one with it;
any other program's settings file for it, exactly as it came, back in the folder it came from;
your .xmp, with loupe's record written in beside every setting it held. Your develop settings, your program's own notes and your camera's details all stay; where loupe's record and your file both say something, loupe's record is the one written down;
a list of every file with its size and fingerprint;
a provenance record for the photograph, in the vocabulary archives already use.⁴ It records your file and its fingerprint; your finishing, where an .xmp recording it came with the file, as your own act as the photographer, with the settings named by the fingerprint of the file that carries them; and everything loupe did, as loupe's. An archive reading it can always tell your hand from loupe's.

A raw file is worth keeping, and keeping well. The Library of Congress lists DNG and camera raw formats as acceptable for keeping photographs, though not its preferred formats.⁵ Our paper What to keep says where each of your files belongs, and why loupe should never be the only place any of them lives.

8. What loupe will never do

Change, recompress or re-save the file you uploaded.
Show a small preview as if it were the photograph.
Carry out another program's edit, or claim to.
Call a photograph "not edited".
Develop a raw file anywhere but in a room of its own.
Write your star ratings, or anything you have switched off, into a file it makes for someone else. (An original handed out to print from carries whatever your own program already wrote inside it.)

9. How other tools treat raw files

Tools that hold photographs are built for different jobs, and they treat raw files accordingly. None of the differences below is a fault; each follows from what the tool is for. We describe kinds of tools, not products, and the sources give the details.

Website and portfolio builders are made to show pictures, and most take only the formats a web page shows: JPEG, PNG, GIF, WebP and the like. They do not take raw files or TIFFs, and they limit a single file to somewhere between 15 and 25 MB. A platform built for galleries and artists says plainly, in its own documentation, not to use it as an archive for your images.⁶ With these, you develop and export every picture yourself first.

Services built for photographers often do take raw files, and keep the original exactly as it was uploaded. They differ in the details: one accepts raw files from some camera makers but not others, and another does not yet read the .xmp files that carry a photographer's captions, keywords and ratings.⁷

Editing programs develop raw files best of all, with their own starting look, and keep your edits in their own catalog or sidecar files, where they are fully meaningful only to that program.^1,2 They are where your finished picture is made. loupe is not an editing program, and it does not try to be one.

Some quick converters and viewers show or extract the picture inside a raw file, because it is fast. When that picture is full size, that is reasonable; when it is not, it is the danger described in §1.

loupe's place among them: it takes your raw files as they come, reads everything useful from them, shows the truest picture it has and says which, keeps your raw file and your settings exactly as they came, and hands you everything back.

10. What we could not settle

The starting look is measured, not borrowed, and it is young. It was fitted on photographs from a handful of cameras. Colors it has not yet seen are left as the camera's measurements give them. It will improve as we measure more cameras, and each improvement carries a new version number (§5).
Cameras loupe does not know. For a camera whose file does not state its own brightness and that is not in loupe's table, loupe uses a typical value. Such a photograph may come out a little lighter or darker than your editing program would show it.
Sharpening and lens corrections are not applied when loupe develops a raw file. Most editing programs apply both by default, so loupe's development can look a little softer, and on some lenses slightly curved at the edges.
Edits loupe cannot see. Edits kept only in an editing program's catalog never reach loupe. We cannot know they exist, and we say so (§3).

If you know the answer to any of these, or find something here that is wrong, we want to hear.

Sources

Checked in September 2026. Descriptions of other services are taken from each service's own published documentation; the sources name the publishers so that you can check them, and the text does not.

Adobe, XMP Specification Part 3: Storage in Files (sidecar files beside a raw file) — adobe.com.
darktable user manual, “Lightroom import”: the import “will never give identical results” — darktable.org.
raw.pixls.us, an archive of sample raw files released into the public domain (CC0) — raw.pixls.us. The sizes in the table were read from one file per camera by loupe, September 2026; the wider survey covered 535 files in six raw formats.
Library of Congress, PREMIS Data Dictionary for Preservation Metadata, version 3.0 — loc.gov. loupe's records are checked against the published schema.
Library of Congress, Recommended Formats Statement 2025–2026, §II, Photographs — Digital — loc.gov; and Sustainability of Digital Formats: Digital Negative (DNG) — loc.gov.
Squarespace, image formats and size (20 MB; JPG, GIF, PNG, WebP) — squarespace.com; Cargo, images (25 MB; JPEG, PNG, GIF, SVG, WebP, AVIF) — cargo.site; Artlogic, image specifications (15 MB; JPEG or PNG; “We do not recommend that you use your Artlogic Management System as an archive for your images”) — artlogic.net.
Format, supported file types (raw formats from most makers; some excluded) and image storage (“stored as uploaded”) — format.com; PhotoShelter, supported file types (“we never alter the original image uploaded”; XMP sidecar files “not currently supported”) — photoshelter.com.

loupe · Your camera raw files · September 2026