The RAW digital image format

What is the Raw format

Digital camera image sensors create an image from literally millions of tiny light sensing areas on a silicon chip. There are two main types (CCD and CMOS) but all you really need to know is that the image is built up from individual pixels. (CCD/CMOS comparisons, and a useful article on sensor designs)

Each light sensor is receptive to a broad range of colours and has a minute coloured filter in front of it. There are usually three different colours (red, green and blue). Since we see green best the filters are in a particular pattern (one called a Bayer pattern is common) with more 'green' pixels

Some manufacturers have variations on this (Fuji, Sony), and Foveon make chips with each pixel location sensitive to all three colours. Whatever techniques is used, the camera needs to take the raw image data and save it as a picture file.

The sensor can provide data at 12 to 14 bits per pixel (4096 to 16,384 levels of intensity) This is reduced to 8 bits per colour in a JPEG file (24 bits total, 8 per colour R,G and B)

The complete unreduced data is stored in a raw format file.

Technically, the camera has performed minimal processing on the data from the chip including a level of noise reduction, although this varies with different model cameras and settings.

The camera has a lot of other information available when a picture is taken. Along with the date and time it records shutter speed, aperture and its estimate of the white balance setting and other pertinent data. This is recorded with any picture.

To produce a JPEG or TIFF file, the computer inside the camera has to do quite a lot of calculations on the raw data to produce a picture file.

These are carried out once, and the result is what you see when you open the file in a picture editor program.

With a raw format file these calculations are not done and all of the original data is saved to the raw file.

How do you use raw files

Camera manufacturers will provide special software to read and convert raw files.

This is in effect using your computer to do the calculations that the camera would have done if you'd saved in JPEG. Things such as sharpening, colour settings and contrast all take place during the conversion.

Sadly, most camera manufacturers are much better (it is hoped) at making cameras than writing software for you to use. This is where specialist conversion programs and plugins for popular image editing programs come in.

The examples below are using Photoshop CS to do a conversion of an Olympus E20 raw file. The picture is the same one used in the converting colour to black and white article elsewhere on the site. There is a collection of info on other raw conversion programs at the end of this article.

With Photoshop CS you will get the RAW import dialogue if you open up a raw image from one of the supported cameras (there are regular updates to cover new models)

Notice the ORF suffix to the file name, signifying an Olympus raw file.

The picture opens up in a preview window with lots of settings. This is not a tutorial on the CS raw import feature, so I'll just point out a few of the things you can do when importing.

We also use DxO Optics Pro for processing some of our RAW files. There are several detailed reviews of DxO on the site which go into more detail.

Resizing

The native resolution of the camera here gives a 2572x1920 pixel image.

It's quite possible to do the resizing at this point, and a few years ago, I'd have always carried it out here.

Photoshop now has improved sharpening algorithms that can give better results. I've written a separate article on Resampling of raw camera files with some examples and details of my current workflow - As I say there, don't be afraid to change your workflow if a better way comes along.

White balance

You can choose to use the data suggested by the camera, or apply your own settings.

The point about using RAW files is that you get to make a choice, and can change your mind if you think you could do better.

With RAW files you can often use a camera profile to reproduce colours more accurately. This is not needed for general use, but does allow you to cope with special lighting conditions.

I have a personal dislike for 'Energy Saving' domestic light bulbs. Their colour rendition is at best poor, and will cause all kinds of issues for digital cameras.

We've an article showing how to make your own custom colour profiles for cameras that have RAW files supported by Adobe Camera Raw (ACR).

Chromatic aberration

A noticeable defect when the wide angle teleconverter was fitted to the E20 is colour fringing or chromatic abberation.

These false colours can be corrected in a JPEG image using the lens correction filter in Photoshop.

However if the correction is carried out at the time of processing the RAW data, then the results are a lot more effective.

In ACR there are adjustments you can make to do this.

By adjusting the sliders much of the effect can be removed.

In DxO Optics Pro, supported lens/camera combinations can have this distortion removed automatically

Look at the following (much magnified) samples -- before and after correction.

Another before and after sample

Quite a difference.

Even the expensive (£1400) Canon 14 2.8L II prime lens that I use on my Canon 1Ds Mk 3 has a bit of chromatic aberration at its wider settings and benefits from a slight adjustment in many shots.

So, What are the advantages of raw format.

All your conversions are done on a fast powerful computer at your convenience.
Images can be 'fixed' in ways which would be very difficult without the raw sensor data.
You get the full range of data from the sensor.
Without sharpening or compression you have not 'lost' any data.
You can change your mind about some of the picture settings after you have taken it.

February 2009 -- I've reprocessed some of my Canon 1Ds raw files from 2004 for new prints. Some were noticeably better than before, making the difference between what would make a good 27"x17" or not. September 2012 - For updating all our gallery and commercial images on the site I went back to the original RAW files. Image processing software is getting more and more powerful so I'm really glad I have all my many thousands of images in their original raw format.

Is there a downside?

Raw files are bigger
JPEGs are quick and easy to use.
Special software is required to do the conversion.
Not all cameras support raw formats and may offer reduced functionality when using raw.

Summary

Raw files are essentially 'digital negatives', or perhaps more accurately 'digital exposed but undeveloped film'.

You get to carry out much of the work that the camera would do, on your own computer.

You can decide how you want the image to look.

If new software comes out with revolutionary new capabilities, you can go back and do a new conversion (see Keith's review of DxO Optics Pro for an example of this).

The added dynamic range (16 bit files) gives you much more flexability in subsequent editing of images (particularly in bringing out detail in shadows)

Sure, the files are bigger, but disk and card costs are getting cheaper by the minute.

Other views

With good conversion software I leave my cameras set at raw most of the time.

However I should recount a photo job (in 2004) where I had 500 items of men's clothing to shoot for a trade catalogue. The images were going to be cut out from the background and printed at 30-40mm high.

The lighting was consistent and full colour accuracy was not required. 8 bit JPEG files were more than good enough for the client's needs - and took up far less space.

That was then - now I'd use raw and do a bulk conversion with Photoshop :-)

There is often debate about the merits of RAW shooting, and I've heard some utter nonsense on both sides.

The following quote points out two differing views and is from Farzal Majid's article on the Nikon D70 raw format (NEF)

"The first is people for whom the creative moment is when you press the shutter release, and who do not want to be involved in post-processing. This was Henri Cartier-Bresson's approach. This group also includes professionals like wedding photographers or photojournalists for whom a streamlined workflow is an economic necessity (even though the overhead of a RAW workflow diminishes with the right software, it is still there).
The second type of users is composed of perfectionists who want complete control over the image. In the age of film, they would spend long hours in the darkroom getting their prints just like they want them. This is the approach of Ansel Adams. The RAW file is the negative."

I guess my own preference tends to the second for my landscape work ;-)

For some types of work you might find that the speed and ease of post processing trumps any extra control over raw file conversion, however I don't do sports or press photography.

Oct 2005 - I've written an article about the technical image processing and resizing issues that I came across when producing 29 prints (up to 78"x43") for a large exhibition of my work. One more reason I'm glad I have the original RAW files to go back to if need be.
May 2012 - Raw files and working at 16 bit were an important aspect of making a 14 metre long print of Leicester City Centre at dusk.

Raw conversion resources

Some software to get you started...

http://store.adobe.com/products/photoshop/cameraraw.html
Feature list from Adobe about the RAW importer mentioned in the article (Mac/win)
Also has downloadable pdf files about RAW capture and other image processing related info
http://www.dxo.com/en/photo
Raw file processing software from DxO - Keith has written a lengthy review (11/04) and now uses it for some of his best work
2005 - review of DxO 3.5
2007 we've a comprehensive V4 DxO Optics Pro review
2009 DxO V5 - even better conversions
2012 DxO V7.2
Photo Ninja - an all new RAW file converter
Bibble Pro (now Corel AfterShot Pro)
Capture One is renowned for its workflow management. Used by many professionals who shoot a -lot- of raw images. (Mac/win)Trial versions available
Breezebrowser (win only). Trial version available.
RAW Developer (Mac) Powerful raw conversion s/w - lots of adjustments and settings to play with. Trial version available.

Raw conversion techniques

http://visual-vacations.com/Photography/hybrid_raw_conversion.htm
Making use of linear raw conversions to improve dynamic range.
ColorChecker Passport - Keith reviews a simple way of making DNG camera profiles for Lightroom and Photoshop.
The software is available for free and offers Adobe Camera Raw (ACR) calibration if you use a standard colorchecer card
Keith has written more about calibrating cameras in a review of using the SG ColorChecker Card from GretagMacbeth
SpyderCheckr - Keith's review of Datacolor's colour correction system for RAW file processing.
http://www.creativepro.com/story/feature/21351.html
Alternative to creating profiles for getting accurate colour with CS raw import, by Bruce Fraser (see his excellent book on colour management)
http://www.russellbrown.com/tips_tech.html
Dr. Brown's Image processor. Javascript utility for automating raw processing in PS CS. Includes tutorials
Real World Camera Raw with Photoshop CS - good book by Bruce Fraser (see our books section)
http://www.normankoren.com/digital_tonality.html
A very comprehensive article of getting the best quality and tonality from your digital images. Part of a site that should be in any digital photographers bookmarks.
Lightroom from Adobe
http://www.adobe.com/products/dng/main.html
Digital Negative or DNG is an open format for raw camera files. Hopefully this will make it easier to use raw format and address the problems of how you are going to be able to read the current proprietary files in years to come.
http://owl.phy.queensu.ca/~phil/exiftool/Canon_raw.html
Far more information about what lives in a raw file than you probably wanted to know.

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