Image formats (RAW, JPEG, HEIF) and compression

RAW files

A RAW file is what the name suggests: raw, unprocessed data. It contains the image data exactly as captured on your camera sensor. Any white balance, Picture Style or other settings that you might have applied are only appended to the image file. This means they can be changed later using RAW processing software such as Canon’s Digital Photo Professional (DPP) or Adobe® Photoshop® (with up-to-date Adobe Camera Raw plug-in), among others.

A RAW file is often referred to as “digital negative” because the data can be processed and printed in different ways to produce different results, just like the negative from a film camera. Also like a film negative, the RAW file never changes. When you open a RAW file in a software application, process and edit it and then save it, this creates a new file on your computer (usually your choice of a JPEG or TIFF). The original RAW file is unchanged, and can be opened again at any time and worked on to produce a completely different result.

The letters RAW do not stand for anything – it’s just a convention that RAW is usually written in capital letters – and the names of RAW files from Canon cameras do not end in .RAW. Instead, until the DIGIC 8 processor was introduced with the EOS M50, Canon cameras saved RAW files in the .CR2 format. Some cameras also offered the option of smaller, lower-resolution “medium” (M-RAW) and “small” (S-RAW) files. These two types of files have most of the advantages of a RAW file but because they are lower resolution they take up less storage space.

The DIGIC 8 processor enabled a .CR3 file format, with a C-RAW option that captures the same resolution but produces 35–55% smaller files, saving storage space on your memory card. (To do this, however, C-RAW uses lossy compression – that is, it discards some image information. More about this shortly.)

The RAW files from different camera models are not exactly the same, even if they are the same file format (CR2 or CR3). For this reason, RAW processing software such as DPP is regularly updated to support new camera models, so if you have a new camera, do check for updates to DPP and download the latest version.

A number of EOS cameras give you the option of processing RAW images in-camera, which is great if you want JPEGs to share and prefer to customise settings such as white balance, brightness and noise reduction yourself rather than just using your camera’s built-in JPEG settings. Processing RAW files on your computer instead of in-camera, though, gives you the advantages of a larger screen and greater processing power.

Advantages of RAW

• Maximum editing flexibility
• Wide range of settings can be modified after capture
• 14-bit file – records widest range of colours and tones

Disadvantages of RAW

• Larger file size
• Needs processing (usually on a computer)

An appealing landscape shot – but this is a JPEG file, which means it has been processed in-camera, discarding most of the colour and tonal information that was initially captured.

The same shot saved as a HEIF file contains more colour and tonal detail – four times more, in fact. The difference between these two images is not a matter of exposure or contrast settings – there is simply more information and therefore more image detail in the HEIF version, most notably in areas such as the sky and clouds.

JPEG files

JPEG stands for Joint Photographic Experts Group, the body that initially defined the JPEG standard. All JPEGs are the same universal standard format, whatever their size and quality.

If you set your camera to save your shots as JPEGs, the camera processes the image information it has captured and saves a compressed file. It can be saved at different image sizes (Large, Medium or Small) and quality settings (levels of compression) to give different file sizes – selecting Large and Fine Quality produces the best quality JPEGs, while using Small and Normal produces the smallest files, so you can fit more shots on your memory card. However, even if you choose highest quality JPEG, the camera actually discards most of the data it initially captured.

When it processes the image, the camera also applies the camera parameters, Picture Style and other settings. Once the JPEG has been saved, these settings cannot be changed – they are “baked in”. You can of course open a JPEG in your image editing software and adjust colour, exposure and so on, but JPEGs are 8-bit files – that is, there is less information there than in the 10-bit, 12-bit or 14-bit files offered by EOS digital cameras – which means you have less editing headroom. More about this shortly.

This may not be a problem if you are making relatively minor edits and printing at sizes up to A4, but it might be significant if you want to make larger changes or bigger prints. Also, a JPEG file is recompressed each time it is edited and saved, meaning it can lose some data each time.

Advantages of JPEG

• Smaller file sizes – more images can be stored on a memory card
• Universal format – no special software is needed to read them
• Images are easy to view, share and print

Disadvantages of JPEG

• 8-bit – reduced colour depth and resolution
• Reduced post-processing flexibility
• Lower quality settings can result in degradation of image quality, artifacts, etc

Another example of the difference between JPEG and HEIF images. If you got this JPEG straight from your camera, you’d be quite satisfied that it captured the bright colours of this display.

By comparison, however, a HEIF contains perceptibly more colour detail, capturing much greater subtlety in areas of colour gradation where the JPEG contains relatively flat colours.

HEIF files

HEIF stands for High Efficiency Image File Format. It’s a format introduced with the EOS-1D X Mark III in 2020 and also available in the EOS R5 and EOS R6, released in the same year. The format can be used to contain data for several different types of media, including images. As in a JPEG, the effects of camera settings such as white balance and Picture Style are “baked in”, but Canon HEIF files are 10-bit, meaning they contain four times more colour and tonal information than JPEGs, which are 8-bit. As well as giving you more headroom for editing, this makes HEIF images a good option for high-res images you want to view on an HDR-standard monitor, such as a 4K reference display. (More about bit-depth shortly.)

Despite containing four times the colour data, HEIF files are typically about the same size as JPEGs, because HEIF compression is 50% more effective than JPEG (hence the “high efficiency” part of their name). The compression algorithms are also more modern than those used in JPEGs, which should prevent the artifacts and colour banding common in highly-compressed JPEGs.

Advantages of HEIF

• 10-bit – more tonal and colour information than JPEG
• More post-processing and editing flexibility
• Small file sizes despite containing much more information than JPEG, and less image degradation

Disadvantages of HEIF

• No perceptible difference from good quality JPEG on most monitors
• Printer support not yet universal – photo printing kiosks, for example

When you photograph a sunrise, the image you get from your camera might look something like this.

If it’s saved as a JPEG and you then lighten it in your image editing software, you might see more detail but also risk introducing banding, visible in the sky here, where the image does not contain enough tonal detail to show smooth gradations of colour.

These enlarged details compare the degree of banding in 8-bit, 10-bit and 12-bit versions of the same part of the lightened image.

A graphical representation of the difference that bit-depth makes. An 8-bit file (such as a JPEG) can contain much less colour information than a 10-bit file (such as a HEIF file), and a 14-bit file (such as a RAW file) contains even more. This means that, although you won’t necessarily see obvious stepping between colours in a spectrum in JPEGs, gradations of colour and tone are much smoother in files with greater bit-depth.

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