Why shoot in RAW?

To understand the difference between RAW files and JPEG files and to decide whether or not you want to use RAW files you first need to understand a little bit about what is happening inside your camera.

When light enters your digital camera it falls on a sensor (which takes the place of film). This sensor is divided into separate areas called PIXELS.

The digital sensor in the majority of cameras is called a BAYER PATTERN sensor. This name comes from the arrangement of red, green and blue sensitive areas (pixels). The surface of a typical sensor is represented in the image above.

Each pixel in the sensor (represented by the individual coloured squares in the image above) responds to either red, green or blue light.

Each sensor measures the intensity of light falling on it. The green pixels measure the green light, the red pixels measure the red light and the blue pixels measure the blue light.

The number of rows of pixels multiplied by the number of columns of pixels gives the size of the sensor in megapixels. For example, if a camera sensor has 4000 columns of pixels and 3000 rows of pixels it will have a total of 4000 X 3000 = 12, 000,000 pixels (i.e. 12 MP).

A 12 megapixel camera has 12 million effective (useful) pixels on the sensor. A 5 megapixel camera has only 5 million effective pixels. The more pixels there are, the more information can be generated from the light falling on the sensor. However other factors, such as the size of each pixel, also influence the quality of the images the camera can produce.

If you have your camera set to save JPEG images to the memory card then the information from the sensor is processed through several stages before it is saved. The basic process is:

1. The amount of light falling on the sensor is controlled by the shutter speed and aperture you have selected. The digital information generated by the sensor describes the colour of the light and its intensity for each individual pixel. This is the RAW information. For each pixel, the colour can be red, green or blue and the intensity will be from 0 (no light) to 4095 (maximum light) for a 12-bit sensor. A 14-bit sensor can differentiate between 16384 different levels of light intensity on each pixel - so a 14-bit sensor generates much more information than a 12-bit sensor. (You don't need to understand the technicalities of this.)

2. The RAW information generated by the sensor passes through an amplifier that increases the magnitude of the signals from the sensor. The amount of amplification is controlled by the ISO setting on the camera. If you set your camera to a high ISO (e.g. 800) the signal from the sensor is amplified more than when you have the camera set to a low ISO (such as 100). The ISO settings are roughly equivalent to the ISO speed ratings of film - a digital camera set to ISO 400 requires approximately the same exposure settings as a film camera using ISO 400 film in order to prodce a well exposed image.

3. The amplified information from the sensor is now decoded to create an "image" file. (You don't need to understand how this magic happens.)

4. The "image file" is then adjusted according to the settings you have chosen on the camera - white balance, saturation, sharpening, contrast, etc.

5. The adjusted image file is now turned into a JPEG file by a process called "compression". You have some control over how much compression is used (how much the size of the "image" file is reduced) by your choice of the image "quality" setting on the camera. NEVER use anything other than the highest quality setting. This will produce the largest files but they will also contain the most information and produce the best quality images when you print them.

6. The JPEG file is then saved to the memory card.

If you set your camera to save RAW files to the memory card then the only "processing" of the image data that takes place in the camera is the amplification of the signal from the sensor. The amount of amplification depends on the ISO setting - just as it does for JPEG files.

Information about all the other camera settings (such as white balance) is saved as part of the RAW file but it is NOT used to change the data in the RAW file.

All the additional processing that is done "in camera" for JPEG files is done in your computer for RAW files - if you choose to do it. There are also many additional adjustments that you can make in the computer that are not possible in the camera (such as compensating for lens distortion).

1. To display a RAW image on your computer you first have to open it in a "RAW conversion program" such as Adobe Camera Raw or a proprietry program supplied by your camera manufacturer. This program "decodes" the information recorded by the digital sensor and converts it into an "image" that can be displayed.

2. You can now make whatever adjustments you want to the image (white balance, contrast, saturation, etc). You might want to start by applying the settings that your camera would have used if you had it set to save JPEG files, or you may want to start with a set of "defaults" applied by the RAW conversion program (this is what I recommend). The important point is that whatever you use as the starting point you have complete control over all these adjustments - you can change settings (such as white balance) as often and as much as you like - the process is always reversible. One great benefit of RAW files is that all the changes you make are automatically stored in a separate part of the file - you never change the original information that was recorded by your camera.

3. When you are satisfied with the adjustments you have made to the RAW image in the RAW Converter (e.g., Adobe Camera Raw) you can save it or you can open it in Photoshop (or another image processing program) for further adjustments. You can decide to save it as a JPEG file (not recommended) or a TIFF file or a Photoshop file or in any of the other file formats that are available through Photoshop.

The digital sensors in each make/model of camera are different - so they produce different RAW files. For example,the RAW files generated by a Nikon D2X are not the same as the RAW files generated by a Nikon D3.

Whatever software you are using to convert your RAW files to images that can be displayed has to be able to recognise the type of RAW file that your camera produces. That's why you might have to update your software if you get a new camera. With software such as Adobe Camera Raw this usually means downloading a "patch" from the Internet (a free and simple process).

Some of the RAW file extensions used by camera manufacturers are:

Canon .CRW or .CR2
Nikon .NEF
Fuji .RAF
Kodak .DCR
Olymus .ORF
Pentax .PEF
Sony .SRF
Minolta .MRW
Sigma .X3F

Adobe has created the DNG raw format as a public archival format for digital camera raw data. Raw files from almost any camera can be converted to this format in Adobe's free Digital Negative Converter. A few camera manufacturers (e.g. Leica) have adopted this format for some of their models. When you import RAW files to your computer with Adobe Photo Downloader (part of Adobe Bridge) you have the option of converting them to the DNG format. Some reasons why you might want to do this are explained at this web site: www.adobe.com/products/dng/

The main reasons for using RAW files are:

1. RAW files contain the maximum amount of information about each image that your camera is capable of recording.

2. If your camera is saving JPG files you have to commit yourself at the time of exposure to several of the most important aspects of image quality, namely white balance, overall contrast and colour saturation. If you record yoour images as RAW files you are free to make decisions about these settings at your leisure at any time after you have taken the photograph (and opened the file in a RAW converter).

3. You have maximum control over how and when your images are converted to JPEG or TIFF.

4. All the changes you make (exposure, contrast, cropping, white balance, etc) are reversible. The original file information is never changed.

5. In Photoshop you can use 16-bit mode to edit the full 12- or 14-bit image generated from the RAW file rather than the 8-bit image that you are forced to work with in JPEG. A useful tutorial on the benefits of 16-bit editing is available at www.photoshopessentials.com/essentials/16-bit/

6. RAW files are tagged with whatever the camera's white balance setting was (either that which was manually set or via auto-white-balance), but the actual data has not been changed. This allows you to set any white balance that you want after the photograph has been taken, with no image degradation. This is not the case with JPEG files - trying to change white balance is a complicated, inexact and often frustrating business with JPEG files.

7. Even very modest computers have much more computing power than the processors used in cameras. So the conversion from RAW to JPEG can be done more efficiently and in a more sophisticated way in a computer.

8. With RAW files it is possible to correct some lens-based problems such as vignetting and chromatic aberration.

The reasons that some people choose to have their camera save JPEG files rather than RAW files are:

1. RAW files are typically 3 to 6 times larger than JPEG files of the same image. This was a concern for some people when camera memory cards were expensive. These days good cards are cheap (excellent cards are still expensive).

2. Not all image editing software will read RAW files. If you are using a low-end image editor you may not be able to open RAW files. This is not a problem if you are using a recent version of Photoshop or Photoshop Elements.

3. RAW files have to be processed BEFORE being opened in Photoshop and this adds an extra step to the image editing process. However, many image editing steps can be done quicker, easier and more effectively in Adobe Camera RAW (or other RAW converters) than they can in Photoshop - so you save time in the long run.

4. Some low-end cameras cannot save files in RAW format and some that can save RAW files operate quite slowly in RAW mode.

The above two images are both from the same exposure (the camera was set to record both the RAW file and a JPEG file). The two files were then opened in Adobe Camera RAW using the default settings. (CS3 allows you to open JPEG and TIFF files in Adobe Camera Raw). The JPEG image is on the left.

The histograms displayed in ACR are shown below each image. Note that they are not identical - the two images do not contain the same luminance information in the red, green and blue channels. Some of the information from the darkest tones and some of the information from the lightest tones has been lost in the JPEG image.

Each of the images was adjusted in Adobe Camera Raw to try to achieve the best result. The adjustments included contrast, clarity, tone curve and sharpness. The two images were cropped in ACR to produce a 10x8 inch image of the bug. The two cropped images were then opened in Photoshop and displayed at 100% size. The results are shown below.

I had tried to get each image "the best possible" in ACR, but clearly the RAW file produced richer colours. The JPEG image has the appearance of being over-sharpen even though the two images were sharpened by the same amount in ACR. (The main reason for this is that the camera applies some sharpening to JPEG files.)

The RAW image appears to have retained more detail in the highlights and shadows, as was expected from the original histograms.

Of course, the differences I have shown here will not be so obvious in all images. I started with a macro shot of a very small bug (about 5mm long) and enlarged the image to the point where the bug would be about 20cm long if printed at 300 dpi.

To further illustrate that the RAW images saved by your camera contain more information that JPEG images, consider the image above. In the background beneath the centre tree there is a metal pole - refer to the red circle.

This image was saved in-camera as both a RAW file and a JPEG file (some cameras give you this option). When the two images are enlarged to the point where the individual pixels are visible, the metal pole looks like this:

The image on the left is the JPEG image, the one on the right is the RAW image. There is clearly much more detail and tonal variation in the RAW image.

If you save your images as RAW files rather than as JPEG files in the camera you will be able to achieve better image quality and have more creative control over your final images.

JPEG is a lossy format. To make smaller file sizes (which is the whole point of the JPG format) some data is thrown away - and it can never be recovered.

If you save your images as RAW files in the camera you can convert them to JPEG (or other formats) at any time in the future. If you save your images as JPEGS you canmot reverse the process and change them to RAW files.

Comments on this tutorial are welcome. You can make them directly on this web site or send me an email.

Roy Killen
Updated 17th April 2009
Email: roykillen@mac.com