ZT. By Bob Atkins

There seems to be a lot of confusion among some new digital camera owners about exactly what the difference is between RAW, JPEG and TIFF files. This article is intended to be a very basic guide to these file types and how they are related in a typical digital camera.

First some basics

The digital sensor in the majority of digital cameras is what is known as a BAYER PATTERN sensor. This relates to the arrangement of red, green and blue sensitive areas. A typical sensor looks like this:

Each pixel in the sensor responds to either red, green or blue light and there are 2 green sensitive pixels for each red and blue pixel. There are more green pixels because the eye is more sensitive to green, so the green channel is the most important. The sensor measures the intensity of light falling on it. The green pixels measure the green light, the red the red and the blue the blue. The readout form the sensor is of the form color:intensity for each individual pixel, where color can be red, green or blue and intensity runs from 0 to 4095 (for a 12-bit sensor)

A conventional digital image has pixels which can be red, green, blue of any one of millions of other colors, so to generate such an image from the data output by the sensor, a significant amount of signal processing is required. This processing is called Bayer interpolation because it must interpolate (i.e. calculate) what the color of each pixel should be. The color and intensity of each pixel is calculated based on the relative strengths of the red, green and blue channel data from all the neighboring pixels. Each pixel in the converted image now has three parameters: red:intensity, blue:intensity and green:intensity. In the end the calculated image looks something like this:

Now one of two things can be done with the RAW data. It can be stored on the memory card, or it can be further processed to yield a JPEG image. The diagram below shows the processes involved:

If you save the RAW data, you can then convert it to a viewable JPEG or TIFF file at a later time on a PC. The process is shown in the diagram below:

You'll see this is pretty similar to the first diagram, except now you're doing all the processing on a PC rather than in the camera. Since it's on a PC you can now pick whatever white balance, contrast, saturation, sharpness etc. you want. So here's the first advantage of saving RAW data. You can change many of the shooting parameters AFTER exposure. You can't change the exposure (obviously) and you can't change the ISO, but you can change many other parameters.

A second advantage of shooting a RAW file is that you can also perform the conversion to an 8-bit or 16-bit TIFF file. TIFF files are larger than JPEG files, but they retain the full quality of the image. They can be compressed or uncompressed, but the compression scheme is lossless, meaning that although the file gets a little smaller, no information is lost. This is a tricky concept for some people, but here's a simple example of lossless compression. Take this string of digits:

14745296533333659762888888356789

Is there a way to store this that doesn't lose any digits, but takes less space? The answer is yes. One way would be as follows

1474529653[5]6597628[6]356789

Here the string 33333 has been replaced by 3[5] - meaning a string of 5 3s, and the string 888888 has been replaced by 8[6] - meaning a string of 6 8s. You've stored the same exact data, but the "compressed" version takes up less space. This is similar (but not identical) to the way lossless TIFF compression is done.

The main reason to shoot JPEG is that you get more shots on a memory card and it's faster, both in camera and afterwards. If you shoot RAW files you have to then convert them to TIFF or JPEG on a PC before you can view or print them. If you have hundreds of images, this can take some time. If you know you have the correct exposure and white balance as well as the optimum camera set parameters, then a high quality JPEG will give you a print just as good as one from a converted RAW file, so you may as well shoot JPEG.

You shoot RAW when you expect to have to do some post exposure processing. If you're not sure about exposure or white balance, or if you want to maintain the maximum possible allowable post exposure processing, then you'll want to shoot RAW files, convert to 16-bit TIFF, do all your processing, then convert to 8-bit files for printing. You lose nothing by shooting RAW except for time and the number of images you can fit on a memory card.

Note that some cameras can store a JPEG image along with the RAW file. This is the best of both worlds, you have a JPEG image which you can quickly extract from the file, but you also have the RAW data which you can later convert and process if theres a problem with the JPEG. The disadvantage is, of course, that this takes up even more storage space. Many cameras also store a small "thumbnail" along with the RAW file which can be read and displayed quickly without having to do a full RAW conversion just to see what's in the file.

以下是引用littlestone在2005-2-24 15:10:00的发言：
英雄，能不能用两句话概括一下..

JPG 是有损压缩处理 RAW data 的结果，占空间小。

Tiff 是无损处理 RAW data 的结果，占空间适中。

RAW 给用户很大的图片处理的自由，占空间大。

JPG现在也有无损压缩的选择了, 不过FILE SIZE会变的比较大.

还有, 以前的JPG是用DFT(Discrete Fourier Transform)做变换, 现在新的JPG2000格式是用DWT(Discrete Wavelet Transform)做变换, 压缩比会更大.

以下是引用littlestone在2005-2-24 15:10:00的发言：
英雄，能不能用两句话概括一下..

真没品位

b4u

jpg,tiff相比来说还是有损的（哪怕是号称无损。。）

我压了好几次raw〉〉tiff测试，如果用的bit比较高的话，tiff会比raw还要大很多。。。