8-bit, 24-bit, 48-bit, 64-bit: bit depth explained for scans
What the bit numbers on scanners and files really mean, why they are confusing, and when more bits make a visible difference.
Scanner boxes and file settings are full of numbers: 8-bit, 16-bit, 24-bit, 48-bit, even 64-bit. They sound like a ladder of quality, but some of them describe the same thing in different ways. Here is what they actually mean.
What a "bit" is doing
Every pixel in a scan stores a brightness value for red, green and blue. Bit depth is how many steps each of those values can have. More bits means finer steps between dark and light.
- 8 bits give 256 steps.
- 16 bits give 65,536 steps.
Why the numbers are confusing
Sometimes the number describes one colour, and sometimes all three added together:
| What you see written | Per colour | Also called | Possible colours |
|---|---|---|---|
| 24-bit colour | 8 bits | "8-bit" RGB | about 16.7 million |
| 48-bit colour | 16 bits | "16-bit" RGB | about 281 trillion |
| 64-bit | 16 bits | 48-bit colour plus a 16-bit infrared layer | as 48-bit, plus the dust map |
So "24-bit" and "8-bit" usually mean the same thing, and so do "48-bit" and "16-bit". Black-and-white scans have only one channel, so an 8-bit or 16-bit greyscale scan is exactly that.
The 64-bit files some film scanning software creates are 48-bit colour with the scanner's infrared layer stored alongside. That extra layer is what allows dust and scratch removal to be done later, rather than at the moment of scanning.
Can you see the difference?
Usually not, at first. Almost every screen, phone and printer works with 8 bits per colour, and 16.7 million colours are more than the eye can separate in a normal photo. A good 24-bit scan and a 48-bit scan of the same print look the same side by side.
The difference appears when you edit. Brightening a dark photo, rescuing a faded print or correcting a strong colour cast stretches those steps apart. With only 256 steps, smooth areas such as skies, walls and skin can break into visible bands. With 65,536 steps there is plenty in reserve, and the result stays smooth.
Where bit depth matters most
- Faded and very dark originals, which need big corrections.
- Negatives and slides, where the scanner has to capture a wide range from the densest shadows to the brightest highlights.
- Restoration work, where a photo may be adjusted many times.
- Raw scans, which are always 16 bits per colour so that all the correction can be done later.
Bit depth and file formats
| Format | Bit depth |
|---|---|
| JPEG | 8 bits per colour only (24-bit) |
| TIFF | 8 or 16 bits per colour (24-bit or 48-bit), and 64-bit with infrared |
| RAW (DNG) | 16 bits per colour |
This is one of the main reasons keen photographers ask for TIFF or RAW files. See our guide to JPEG, TIFF and RAW for more.
The cost: file size
More bits means bigger files. For a 6×4 print scanned at 600dpi, before any compression:
| Bit depth | Approximate size |
|---|---|
| 24-bit | 26MB (a JPEG compresses this to around 2 to 4MB) |
| 48-bit | 52MB |
| 64-bit | 69MB |
A note on scanner marketing
A scanner advertised as "48-bit" can store 16 bits per colour, but its sensor may really capture 12 or 14 bits, with the rest filled in. That is normal and still gives far more room than 8 bits. The quality of the scanner's optics and sensor matters much more than the number on the box.
So what should you choose?
For viewing, sharing and printing your family photos, 24-bit JPEG files are all you need. If you plan to edit seriously, restore photos yourself or keep a master archive, ask for 48-bit TIFF or RAW files, and see our questions & answers, or just ask Dee.
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