Tag Archives: sRAW

File size on the Canon 5D2

RAW file size

At its highest settings, the Canon EOS 5D Mark II produces 21 Mpixel RAW images. These are about 26 MByte according to the manual (page 55). This article will concentrate on RAW rather than on JPG.

File sizes versus sensor resolution for various Canon cameras

File sizes versus sensor resolution for various Canon cameras

The diagram compares the file sizes of various Canon digital SLR cameras. The solid red line is the 5D Mark II. At its highest resolution, it  produces 26 MByte files, but it also has 2 lower resolution sRAW modes (10 Mpixel and 5 Mpixel) which will be discussed later. 26 Mbyte files is slightly larger than the RAW files from the Canon 1Ds Mark III (also 21 MPixel), and significantly larger than images from Canon’s lower resolution models.

26 Mbytes for 21.1 Mpixel equals 10.3 bit/pixel. The sensor itself produces more data: 14 bit/pixel (remember that a camera pixel captures only one color per pixel – essentially a marketing decision to get you large numbers). The compression from 14 bit/pixel to 10 bit/pixel is in principle lossless – meaning that typical image files will shrink compared to the original data without any visible loss of quality (“lossless”). As usual for lossless encoding, some types of images will compress better than others. Some un-photolike images may even grow rather than shrink.

Information density of images generated by various Canon EOS camera

Information density of images generated by various Canon EOS camera

From the above data (based on the file sizes quoted in the Canon Instruction Manuals) we can conclude a few things:

  • A RAW file at the camera’s native resolution is roughly 10 bit per pixel, or about 1.25 MBytes per Mpixel.
  • JPEG files are (at their higher quality setting) about 2.5 bit per pixel, or 4× smaller than RAW. You can opt to make them 8× smaller instead of 4× smaller by using a lower quality setting (not shown) although most photographers will prefer the highest quality JPGs these cameras can produce.
  • The per-pixel quality of JPEG files seems to shrink with increasing resolution. Maybe simply because there is less difference between adjacent pixel readings in a high-res image (due to the subject, or due to lens limitations).
  • The size of RAW files (at the camera’s native resolution) actually increased in terms of bit/pixel (or MByte/MPixel). The camera models 10D → 30D → 40D → 450D → 5D2 show an increase in bit/pixel which goes 7.5 → 8.9 → 10.4 → 10.7 → 10.4.  All-in-all, more than a 35% increase, which can be partly explained by the transition from 12-bit to 14-bit pixel accuracy. So it might also be an increasing emphasis on quality. JPEG files, in contrast, are said to be limited to 8-bit values due to the JPEG standard.
  • The new lower resolution sRAW files require more storage per (Mega)pixel than the full RAW files. This is because they have use a very different internal technology despite the use of the same .CR2 file extension. Thus, the sRAW 2 format is more than half the size of the full resolution RAW while having 4× fewer pixels.

Storing 26 Mbyte images

26 MByte images can become a problem for processing (get a faster CPU with more RAM), storage (get a larger hard disk) and communications (get a faster network).

The storage part is easiest to solve: 1 TByte drives cost below €100 and hold 40 thousand 25 MB images. Large drives are often faster than smaller ones (more bit per track). I already took measures to prepare for the storage part: dual Samsung SpinPoint F HD103UJ 1Tb in a NAS.

Moving 26 Mbyte images

Ideally faster communication involves using local drives rather than network drives, or using a gigabit LAN to central storage (my choice). The LAN still doesn’t achieve 1 Gbit/s speeds, but say at 200 Mbit/s it still takes 1 seconds to download each image. Avoid wireless links as they are 10× slower. At 200 Mbit/s, the CPU rather than the LAN may be the bottleneck. A drive attached to USB 2.0 or Firewire will also give you about 200 Mbit/s.

Processing 26 Mbyte images

Processing is the tricky part: Lightroom and Aperture (Mac) show you the modifications you make but using lower resolution previews of images. In fact, they do not generate high-resolution images of any image files at all:  they just record what modifications you did, and repeat these when needed. They rely on lower-resolution “cached” images generated once in a while from the full resolution images. In Lightroom you can see this happening: when the full resolution image is fetched it display “Loading” or “Loading preview”. In contrast, typical image processing in Photoshop or intermediate processing in DxO Optics Pro involve reading and writing of full-size images.

There is obviously a lot to say about performance differences between different computers. For modern image processing software, having multiple cores helps. Having enough RAM (cheap nowadays) always helps. I am currently using a 1.8 GHz dual core Intel system with 2 GBytes of RAM.

The sRAW option

Photographers who feel that 10 or less Mpixel gives enough image quality have the option of creating and storing in “sRAW 1” (10 Mpixel -> 15 MB). This sRAW option is newish and not supported by some software that does support regular RAW. Photoshop Lightroom and Canon’s own software support sRAW. DxO and Aperture currently do not support sRAW. Photomatix Pro (HDR software) plans to support sRAW soon.

Although sRAW files have the same .CR2 file extension as traditional Canon RAW, they are technically very different. Analysis by Gao YangDave Coffin and Laurent Clevy has shown that sRAW does not store the original Bayer RGB matrix, but instead contains a 15-bit luminosity value per each pixel location, and 2 or 4 color components per block of 2×2 pixels (in the reduced resolution).

sRAW 1 (half the pixels) is suitable for many applications (e.g. portraits), but the limited third-party software support can be a problem. It is also not known yet how sRAW 1 compares in quality to a hypothetical RAW file from a camera with the same resolution. sRAW 1 files from the Canon 5D Mark II are on average 15 MBytes (12 bit/pixel) – meaning that you get a 1.7× reduction in file size in exchange for a 2× fewer pixels. The 2× pixel reduction should give 2× faster processing in programs like Photoshop because these internally use their own data formats.