Schewe, J & Fraser, B 2011, ‘Digital camera raw’, in Real world camera raw with Adobe photoshop CS5, Peachpit Press, Berkeley CA, pp. 1-16.
Three facets of Photoshop CS5 – Adobe PS camera Raw (plug-in to PS), Bridge (stand alone application) & Photoshop
What is raw capture?
- The sensor data from the camera
- Camera-generated metadata (data about data) (reference to Ch 8 for full explanation of metadata)
- Encoding of raw data varies with camera manufacturers – different extension – e.g. Canon uses CRW & CR2 (mine) – all are readable in Camera Raw.
The Camera Sensor
- Most cameras use the mosaic sensor – colour filter array (also some support Foveon X3 technology)
- 2-D sensor array to collect photons – (CCD – Charged coupled device or CMOS – complementary metal oxide semiconductor- mine is CMOS with 18.00 mp)
- Colour information is produced by applying colour filters to the array – so that individual elements only record the light of either red, green or blue wavelength (Bayer array, striping)
- More green filters as eyes are most sensitive to green. (Diagrams of array and Bayer colour filter array)
- Raw files are grey scale – but there is information about colour (i.e. each filter element’s data)
- Image from raw file is produced with raw converter – e.g. Camera Raw.
- Demosaicing – interpolation of the raw colour data from each pixel.
- Raw converters also control white balance (colour temperature depending on type of light source), colorimetric interpretation (the actual shade of each colour to depict – uses a calorimetrically defined colour space based on human perception), noise reduction (basic edge detection, anti-aliasing (to avoid colour artifacts), noise reduction, sharpening),
Foveon X3 (Sigms Corp)– exploits the different wave lengths of red, green and blue light. 3 layers of photosensors on the same chip. Front for (shorter) blue waves, mid for green, back for (longer) red waves. Full information for each pixel rather than a mosaic – so no need to demosaic – but Camera Raw still performs white balance, colorimetric interpolation and noise reduction.
Exposure and Linear Capture
- Camera sensor just counts photons – so the amount of ambient light will have a huge effect on the final image – different from the human eye/brain combination which edits the light information so we see well in both low and full light situations. (Article calls this ‘built-in compression’)
- Linear capture – important implications for digital capture. Camera may have 6 stops of dynamic range – ½ the levels (2048) devoted to the brightest stop , half of the remainder (1,024) to the next stop, ½ of the remainder (512) to the third stop etc. The darkest stop (extreme shadows) – 64 levels. (figure to demonstrate). Also figure of how human vision sees the different stops .
- Raw converters convert the linear capture to a gamma-encoded space to make the captured levels more closely match the way the human eye sees them. (Reality is much more complex – editing in Raw we move endpoints, adjust mid-tones, change contract. )
- Good exposure of the initial image is vital!
- In digital image correct exposure keeps highlights bright (close to ‘blowing out’). Better to slightly over expose than underexpose as information is lost with under-exposure, also risk of introducing noise in midtones and shadows. (NB Camera Raw can recover highlight detail – Ref. To Chapter 2),
- Figures to show how underexposure reduces the range of data collected. Discusses the camera histogram (which is for conversion to JPEG) – camera histogram may show blown highlights when they are not, in fact, blown
- Camera ISO speed may not be accurate (says for 125 could range from 75 to 150) therefore, lots of trial and error to determine the actual accuracy of the ISO setting.
Why shoot raw?
- Greater control over the interpretation of the image. (no compression applied, no in-camera processing)
- With raw – the only on-camera settings that have an effect on the captured pixels are ISO, shutter speed, aperture.
Using all the bits
- JPEG images only 8 bits/channel, most modern cameras – 12 bite per channel (4096 levels/channel). i.e. JPEG jettisons 1/3 of the colour data!
- Raw files can withstand much more editing than JPEG files
- Editing in PS is destructive – pixel information is changed – possible posterization, loss of detail when compress a tonal range. (figure to show how compression and expansion of tonal ranges can affect pixel values)
- The original Raw file is retained after editing and can be restored for re-working.
White Balance control
- WB control in Raw is less destructive – does not change pixel values, but gently scales one or two channels to match the third. (figure to show this)
- Raw is not constrained by the colour spaces of JPEG (usually sRGB or Adobe RGB) – but the colours captured may be outside either of these colour spaces – esp. Yellows and cyans – i.e. these are clipped for JPEG conversion.
- Camera Raw has 4 possible colour space destinations – ProPhoto RGB encompasses all colours that can be captured, and almost all colours we can see. (diagrams to show the colour spaces)
Exposure in raw conversion
- Exposure adjustments are ‘relatively lossless’ but there is less scope for exposure adjustments than white balance.
- Limitation of adding noise to midtones and shadows when trying to recover these parts of the image. (not enough data captured by the camera)
- Completely blown highlights beyond recovery – not enough camera data – but if some data (in one channel eg) there may be scope to recover 1 or more stops of highlight detail (depends on camera and white balance chosen).
- Good initial exposure is the best insurance!
Detail and noise
- JPEG reduces this in camera. In Raw the user has control over noise, sharpness and luminance noise and colour noise separately.
- Advantages – e.g. tailoring of noise reduction to different ISO speeds, application of quick, global sharpening to a set of images, conversion of images without sharpening for later localised sharpening in PS.
- Time – files need to pass through Camera Raw or other software before being viewable or put into acceptable file formats for printing. But batch conversions can reduce this.
- Huge file size – 2-4 times bigger than JPEG files.
- Longevity of formats – individual formats for each brand of camera – as yet, no generic format over the whole range. Images available only in today’s raw formats may not be accessible as new software take over.
- Recommends DNG (Adobe) – an open file format for raw captures.
Adobe Photoshop Camera Raw
- Is a universal converter for all cameras.
- A full-featured raw converter – fine control over white balance, exposure, noise reduction, sharpness, chromatic aberration (lens flaws and film masks), fine tuning of colour responsiveness for individual camera models.
- APCR can create callibration settings for each camera model, which CR applies automatically. Other settings can also be customised and saved as defaults.
- APCR is integrated with PS and Bridge.
- Bridge’s automation features can be applied to single or batches of images – e.g. web galleries, pdf presentations, virtual contact sheets.
- After CR the images go straight to PS for further editing.
The Digital Negative
- There is an imperfect analogy between film negatives and raw files as ‘digital negatives’ – i.e. the initial capture from which all other processing flows.