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RAW Files - What are they?

 RAW Files – What are they and should I use them?

This article refers to Camera Raw in Photoshop CS3 - CS 2 has less options, CS 4/5 has more and CS6 has a greatly improved Raw Converter.

 

More and more digital cameras now capable of capturing RAW files, but what are they and are there any benefits in using them?

A digital RAW file is a true record of the of the raw data recorded by the camera sensor. In other words it is a record of the luminance value for each pixel in the sensor and although it is a greyscale image it also contains colour information so that a raw converter, such as the Camera RAW plug-in in Photoshop, knows which colour a given pixel in the raw file represents. It does not, however, contain any information that humans can interpret as colour. Each RAW file has metadata attached to it which contains information about the way in which the image was captured, including the ISO setting, shutter speed and aperture value, white balance, colour space and so on.

When an exposure is made by a digital camera the raw image data is loaded into memory and sent to the onboard processor where the data is processed according to the way in which the camera is set up to output images. After processing it is written to the memory card (Compact Flash, SD, Microdrive, etc.) for subsequent transfer to a computer or, in some cases, directly to a printer using PictBridge.

If a JPEG output is selected the raw data is processed with reference to the metadata to produce a colour image with the user specified parameters for colour space, sharpening, tone and contrast control, etc and the degree of compression. The problem with JPEGs, even if no user controlled processing is carried out in the camera, i.e. no sharpening, etc is in the compression. In this respect the image suffers rather less if Large Fine JPEG is selected (modest compression) as opposed to Small JPEG (High Compression). When an image file is compressed in JPEG to produce a smaller output file size pixels of very similar tone, such those which would be found in highlight or shadow areas, are treated as being the same during compression and when the file is expanded again in, say Photoshop some detail will be lost. As the degree of compression increases so does the amount of fine detail that is lost and it is preferable to select the highest JPEG quality in order to minimise the detail lost. In the subsequent editing of any JPEG large changes to tone and colour tend to exaggerate the 8 by 8 pixel blocks which are the foundation of JPEG compression. While JPEG does a fair job of preserving luminance data it really hammers the colour leading to problems with skin tones and gentle gradations. RAW files, are losslessly compressed, i.e. no tones are discarded or are considered to be the same as very similar adjacent tones. When you shoot raw you capture on the memory card everything that the camera sensor can deliver – nothing is lost as is the case with JPEG. Most cameras capture at least 12 bits per channel for a possible 4096 levels in each channel, but the JPEG format is limited to 8 bits per channel which effectively means that the camera’s built-in conversions discard one third of the data. A raw file can withstand a great deal more editing in Photoshop than can an 8 bit per channel JPEG.

All of the forgoing is somewhat on the ‘heavy’ side, so what are the real life, practical aspects of RAW files and have they a downside?

The benefits of shooting raw may be summarized quite simply – control over the interpretation of the image. The only in-camera settings which have an effect on the captured pixels are the ISO speed, shutter speed and aperture setting. Everything else is under your control when you convert the raw file, including the white balance, the colour space, the tonal response (brightness and contrast), and the detail rendition (sharpness and noise reduction). You can even alter the basic exposure or resize the image and any changes made in the raw state are far less destructive than similar changes in Photoshop.

There is a downside – the penalty for gaining a huge amount of control in the conversion process is an increase in processing time. Raw files are much larger than JPEG files, typically between two and four times as large (5-6 Mb on a EOS 10D and 25-35 Mb on the full frame EOS 5D Mk II) which also implies the need for larger memory cards and more storage space on the computer hard drive – a raw file from the EOS 5D Mk II expands to ~120 Mb when transferred into Photoshop at 16 bit.

The format of raw files varies between camera makes and between models in the range and consequently the Camera RAW plug-in is constantly updated by Adobe. It is important to note that the raw plug-in for CS3 supports only those cameras available during the lifetime of CS3; for cameras such as the Canon EOS 5D MK II, released around the time CS4 was introduced, the RAW files will only open in CS4. There is a workaround to this dilemma and that is the use of Adobe DNG, a free download from Adobe, which converts RAW files to the Digital Negative Format which can be opened in any version of Photoshop capable of dealing with RAW files, i.e. RAW files from the Canon EOS 5D Mk II for example which will only open directly in CS4 will open in CS2 or CS3 when converted to .dng files.

 

Image Adjustment in Camera RAW

When a RAW file (or .dng file) is opened either in Photoshop or through Bridge it opens in the Camera RAW plug-in (screenshot) – the CS3 plug-in is shown with the default settings.

 

 

At the base of the Camera RAW screen is a link leading to a dialogue box for the selection of Colour Space and Depth (8 or 16 bits/channel) and for the image size and resolution – normally these should be left at the default values unless changes are specifically required in these parameters. The image adjustment  controls are to the right of the screen with the histogram at the top.

When using the plug-in for the first time it is recommended to set the Preferences for Sharpening to ‘Preview images only’ (Click on the  icon at  the top of the Camera Raw window) and to apply sharpening in Photoshop rather than in Camera Raw.

Starting with the Adjust tab (default) from top to bottom the image adjustment controls are :

 

White Balance – the default is ‘As Shot’, but other options are available through the drop down menu. Changing the White Balance will also change

Colour Temperature which may be adjusted independently if required.

Tint – alters the colour bias of the image.

Exposure – this slider adjusts the original exposure by up to +/- 4 stops.

Recovery – moving the slider to the right attempts to recover details from overexposed highlights.

Fill Light – moving the slider to the right attempts to recover details from the shadows without brightening the blacks.

Blacks – specifies which input levels are mapped to black in the final image.

When adjusting either the Exposure, Recovery or Blacks slider holding down the Alt key shows a Clipping Display which allows you to see exactly which pixels are being clipped or lost, i.e. which areas are losing detail as the sliders are moved (Exactly the same as holding down the Alt key when making a Levels adjustment in Photoshop). When adjusting Exposure you should aim for the point at which the image just becomes completely black and when adjusting Shadows for the point where the image just becomes completely white.

Brightness and Contrast are self explanatory, but note that holding down the Alt key while making adjustments does not show a Clipping Display

Clarity – Adjusts local contrast on an automatic basis

Vibrance – Adjusts the saturation so that clipping is minimised as colours approach full saturation.

Saturation – Adjusts the saturation of all image colours equally from -100 (monochrome) to +100 ( double the saturation).

 

The Tone Curve tab provides for tonal adjustment of the image using the conventional tone curve or by the use of sliders

 

The Detail tab allows for control of the degree of sharpening applied to the image or image preview and there are sliders to adjust the Luminance and Colour Noise if required.

 

The HSL/Greyscale tab provides for the adjustment of Hue, Saturation and Luminance of individual colours and ther is a checkbox to convert the image to Grayscale.

 

The Lens tab allows for the correction of chromatic aberrations arising from the use of non-APO lenses and a very useful Vignetting control to compensate for dark/light corners in the image arising from lens defects.

 

The Camera Calibration tab enables corrections to be made to the camera profile as your camera may differ very slightly from the example used by Adobe in setting up the basic profile – it is unlikely that you will need to make any adjustments here.

 

The Presets tab lists any settings which have been saved as a Preset.

 

When you are satisfied with the result of the adjustments made in Camera Raw transfer adjusted image into Photoshop; hold down the Alt key – the Open Button changes to Open Copy and click on this button and a copy of the adjusted file is opened in Photoshop. This ensures that the original raw file is not altered in any way when viewed in Bridge. If you click on Open the adjustments made in Camera Raw will be shown in the thumbnail/preview in Bridge  - they can be removed by right clicking on the thumbnail and selecting Develop Settings > Clear Settings.

The image will open in Photoshop with the same file extension as the original RAW file, e.g. _MG_1722.CR2 (for Canon), but when the File > Save command is made the Save As dialogue box opens with .psd as the default format. It is important to remember that the image adjusted in Camera Raw is only the starting point for the final image. There is no provision in Camera Raw in CS3 and below to make localised adjustments as one would with an Adjustment Layer and Layer Mask in Photoshop, but a major advance in Camera Raw in CS4 provides the option for localised adjustments using an Adjustment Brush or Graduated Adjustment mask albeit not on an adjustment layer as such, but on the image itself.

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