Bit depth for archival photography
Bit depth is how finely a picture records tone: the number of steps a channel has between black and white. Every extra bit doubles them.
| Bits per channel | Steps from black to white |
|---|---|
| 8 | 256 |
| 10 | 1,024 |
| 12 | 4,096 |
| 14 | 16,384 |
| 16 | 65,536 |
For a preservation master the question is not only how many bits the file has, but how many the camera recorded — and those are often not the same number.
What cameras record
A camera records what its sensor and its raw format allow. Most record 12 or 14 bits; some medium-format cameras 16; some drop to fewer in continuous or silent shooting. Read from the raw files of real cameras:
| Camera and file | Bits recorded |
|---|---|
| Fujifilm GFX100S, RAF | 16 |
| Sony, ARW | 14 — or 12 in continuous shooting |
| iPhone, Apple ProRAW (DNG) | 10, through a tone curve |
| An uncompressed DNG | 16 |
A JPEG from any camera holds 8 bits. A camera's own HEIF can hold 10.
The figure is not in the EXIF data. In a raw file the ordinary tags describe the small preview picture embedded in it, and software tends to report the depth it decodes to — Apple's, for one, reports 16 for every raw — rather than the depth the camera wrote. The real number sits in the raw image's own header, which is where the PHOTARCH app reads it.
Why files come in 8 or 16
There are no 12- or 14-bit image files in practice. The TIFF format allows them on paper, but image software reads and writes 8 or 16 bits a channel: Apple's imaging frameworks will not create a 14-bit image, a 14-bit TIFF is read by macOS as 16 bits, and ffmpeg will not read one at all.
So a 12- or 14-bit capture goes into a 16-bit file, which holds everything the camera recorded without loss. Nothing is added: the extra two or four bits hold no information of their own. That is why the file should say how many bits the camera recorded.
What the standards ask
FADGI asks for 8 bits at one and two stars and 16 bits at three and four; Metamorfoze for 16 or 8 at its highest level and 8 below it. Both look at the file. FADGI adds that bit depth defines how many shades a file can hold, not the quality of the information in it.
PHOTARCH's rule
PHOTARCH never changes the number in either direction. A picture photographed in 8 bits is exported in 8. A picture photographed in more — 10, 12, 14 or 16 — is exported in 16, which holds all of it without loss, and the file's metadata says how many bits the camera recorded.
PHOTARCH's own levels go by that recorded number. A study asks for at least 10 bits, recorded by the camera in a raw; a reproduction for at least 12, written into a 16-bit TIFF. A finished picture brought in as a lossless file, such as a focus stack exported as a 16-bit TIFF, is judged on the bits the file holds. An ordinary 14-bit raw reaches both, and an iPhone's 10-bit ProRAW reaches a study. Where a raw format does not say how many bits it holds, the recorded bit depth is listed as not measured — never assumed to be 16.
For flat originals, with FADGI, ISO 19264-1 and Metamorfoze turned on, bit depth follows FADGI and is judged on the file: a 14-bit raw written into a 16-bit TIFF counts as 16 there, and the bits the camera recorded are shown beside it.
What more bits do, and what they do not
More bits give finer steps between tones. That matters most when tones are stretched: lifting shadows, correcting an exposure, the colour card's correction of every picture in a sitting. A picture with too few steps shows it as bands in smooth gradients.
More bits do not add detail or sharpness, and they do not lower noise. In most captures the lowest bits record noise rather than tone. Sixteen recorded bits are worth most when the exposure uses them.
How to make the most of it
- Photograph raw, and check the camera's raw settings: the deepest mode, uncompressed or losslessly compressed where it is offered. Avoid continuous and silent modes if they record fewer bits.
- Use the lowest native ISO.
- Expose so that the white sits just under the point where it clips. Every stop of unused headroom at the top is a bit the file carries and the picture does not use.
- Keep the master as a TIFF. A JPEG is 8 bits and compressed: right for access, not for the master.
More on the whole procedure in How to photograph archaeological artefacts.