Over the past year, quite a few prosumer (and all of the DSLR) cameras that have been released now provide the ability to shoot using the camera's RAW image format. This format differs slightly from camera to camera, but in general, a RAW file is a collection of the image data from each pixel before it has been processed by software in the camera (called firmware). I like to think of a RAW file as a “digital negative” and as a JPEG as a “digital print.” The quality of the digital negative (RAW file) never degrades and it can always be re-used in the future to make an infinite number of different “digital prints.”
As the RAW format becomes widely available, many digital shooters wonder if this format is “better” than shooting JPEG - of course, “better” often depends on the circumstances. This short column will describe the difference between the two formats and pros and cons of shooting one versus the other.
Filesize and Bit-Depth
Digital SLR RAW files are approximately 12 bits per channel, as opposed to 8 bits per channel for a JPEG. Many RAW formats are not compressed at all, and the ones that are compressed are compressed with a lossless algorithm, so no information is lost. On the other hand, JPEGs are always compressed with a lossy algorithm – the amount depends on the “quality” value that you set on the camera. As a result, RAW files can be quite a bit larger. As an example, RAW files from the S2pro are 12 megabytes in size, and high quality 12 megapixel JPEG's are about 4.8 megabytes. On a six megapixel camera like the 10D or D100, raw files are 6-8 megabytes and JPEGS are typically 1-3 megabyte per file.
So how important are the extra bits? The answer is that they are VERY important! Each bit represents a doubling of the recorded value, so 8 bits = 2^8 (256) possible values of information and 12 bits = 2^12 (4096). To put this in photographic terms – a bit = a stop. Since a stop equals a doubling of recorded light and a bit equals a doubling of recorded information, it's easy and correct to make this analogy. So the practical implication is that shooting in RAW you can record FOUR STOPS more dynamic range than shooting in JPEG. This is very important for shooting high-contrast scenes, such as sunbursts, whale sharks, etc. The following JPEG (while not a good shot) is a good example of a high-contrast scene.
(F8 @ 1/125 th JPEG from the S2pro)
As you can see, the shot goes from almost pure black to almost pure white in the center of the sunburst. The important feature of this shot – taken as a JPEG with the S2pro – is that there are really only 3 shades of blue in the water. With a little flash fill, there are perhaps 3 shades of gray in the detail of the coral. In total, there are only about 6-7 stops of range in this shot. Closing down to f11 or f22 would have helped darken the water a bit, but would require a much more powerful flash for foreground fill. The “take home” message is to use RAW to capture as many shades of blue as possible – resulting in beautiful smooth blue backgrounds.
RAW File Converters and Photoshop
A RAW file converter takes the place of the camera's firmware, and uses your computer's processing power to turn a RAW file into a JPEG or a TIFF. Photoshop CS now comes with a converter for many cameras, and many high-end digital cameras come with their own software (example: Nikon Capture or Fuji EX2.0). Because the converter can be set to make a TIFF file, no information is lost via file compression. Another awesome capability is exposure correction or “pushing” using the converter. Because of the great amount of information contained in a RAW file, the exposure can usually be corrected up or down by at least a stop, and many times 1 ½ to 2 stops.
Post Processing vs Camera Firmware Processing
As stated above, digital camera firmware converts the RAW sensor data into a JPEG file though a complicated process. Every camera does it differently and it's hard to say exactly what each model does. The firmware assigns RGB values to each pixel, chops off 4 bits from the image, usually in the shadows, and then applies a white balance. Next, the firmware sets contrast, color curves, sharpness, saturation, etc. Finally, a compression level is set and the file is saved as a JPEG. Since every camera performs this process differently, two different camera models with the same settings will result in very different images. Shooting RAW leaves these decisions up to you the photographer to set in post-processing. Because of the full feature set of the RAW converters available, usually it is possible to correct an image (setting all of the above parameters yourself) without ever using Photoshop. If you don't have time to process each shot, you can use the default converter values and the white balance recorded by the camera and you are no “worse off” than if you had just shot a JPEG. If/when you have more time later, you can go back to a good shot's “digital negative” and really work with the image to make it look its best.
Pros and Cons
I've put together a summary table of pros and cons of shooting the two formats:
|Exposure Latitude||RAW converter required|
|Higher Dynamic Range||Post Processing Time on the Computer|
|Limited Burst Capacity (7 or 9 shots)|
|No Preset White Bal.||Fewer Shots per Memory Card|
Noise Smoothing in software
More Shots per Burst
|White Balance Fixed|
Quick Delivery Time
|No Converter Req'd||Limited Adjustment Capability|
|Smaller File Size||Noise is “Built Into” the Image|
Hopefully this short column has cleared up some questions about the RAW file format. If you'd like to learn more, you can find many RAW Format GURUs in the Image Processing, Printing, and Storage forum at Wetpixel.