Thursday, 25 September 2014

A Guide to Digital Image Artefacts

Introduction

The simplest definition of photographic artefacts is that they are distortions of the original image.  Every distortion has the potential to influence the identification process which is why I have included artefacts as one of the five quality parameters in the Image Quality Tool.  Most artefacts are microscopic and only influence fine detail.  Many are familiar to most people but a few need a bit of explanation.  Here is a guide to the most commonly encountered digital image artefacts and their likely effects on bird images.


An image starts out as a series of parallel light waves that travel a path between the subject and scene and the camera.  The first distortions of the image happen along this pathway and include the influences of atoms and molecules in the air, Dust and other Air Pollution and lastly the interaction of light with this collection of matter.  The greater the distance, and thus the greater density of air between the subject and the camera the greater the influence of these parameters.  Intensity can also be simply a matter of the scale of the artefact - eg. the higher the temperature, the greater the Heat Haze.  The higher the humidity, the greater the Moisture density in the air.  Glare depends on the angle of the sun relative to subject and camera.  These artefacts all tend to reduce contrast and clarity in the image.  Heat haze as we know tends to defocus and distort fine detail due to the random movement of air molecules.


From the moment the image light waves hit the lens there is the potential for another series of artefacts to distort the image.  Defocus (an out of focus image) is of course the most obvious and recognisable image artefact associated with the lens and requires adjustment of the focus wheel and or the lens aperture as appropriate.  

Dust and Foreign Bodies on the lens can show up as fuzzy blobs on the final image, though most of the time they are so small and defocused as to be transparent and not noticeable. 

Chromatic Aberration is caused by refraction of the light as it passes through the lens.  A prism as we know splits up white light into a rainbow of spectral colours.  All lenses have this tendency due to the fact that each wavelength of light will refract at a different angle as it passes from one material into another.  The solution to chromatic aberration is a corrective glass element with an opposing refractive index.  The combination of crown and flint glass elements is called an achromatic doublet.  As lenses get lighter the glass elements are being replaced with plastic elements which also must be manufactured with this problem in mind.  Lens coatings can also help with the problem.




It is worth noting that, although most lenses are well constructed to more or less eliminate this problem, when it comes to IR and UV light some chromatic aberration may still occur.  This is part of the reason why cameras are fitted with an IR/UV blocking filter which absorbs this light before it reaches the sensor.  For more on this hidden light and it's influence see HERE

Image Distortion occurs as a consequence of lens design.  It is most obvious in wide-angle (fish-eye) lenses but can also occur in zoom lenses and poorly constructed or cheap lenses where it can go largely unnoticed.  The edges of an image have a different magnification to the centre (focus is unaffected).  Often, it is only detectable if one intentionally photographs a grid of equal sized boxes or straight lines.  Lens distortion can trip up the unwary, particularly when it comes to size comparisons and critical measurements within photographs.  For more see HERE.


Vignetting is a darkening of the image around it's periphery and is associated with long lenses and some inferior lenses.  Birders who have taken to digiscoping would be very familiar with this image artefact.

Atmospheric conditions continue to influence the lens in the form of Moisture such as rain and condensation on the objective or on internal lens elements.  In extreme cases ice crystals may form on lenses.  The effect can be a softening or defocus of the image throughout or in spots within the image.

Lens Flare occurs due to light scattering inside the lens, often when the sun is just offset from the camera's line of sight, or if the sun reflects brightly off something in that general field.  The effect is usually pretty recognisable but, I guess, depending on the image content, could be mistaken for something more tangible in the subject matter.  


At this point the light has made it through the lens and filters and is reaching the camera sensor.  Here is where the greatest variety of image artefacts come into play. 

Shutter speed and light intensity determine the exposure of the image on the sensor.  If the camera isn't stable, or if the subject moves during the exposure, Motion Blur gives rise to blurry images, the most familiar artefact at this stage in the process.  

The electrical charge on the camera sensor can attract Dust particles which land on the sensor's surface and appear as tiny, generally white specs on digital images.  Modern DSLRs have inbuilt dust cleaning capabilities but dust can be a real pain for those who use compact cameras.  

Too little light reaching the sensor leads to Underexposure while too much leads to Overexposure.  Both of these could be considered artefacts but one must bare in mind that, depending on the luminosity of different elements in the scene small parts of the image may naturally tend towards under or overexposure.  Dynamic Range is a related factor.  On very bright days the dynamic range of the ambient light may be completely at odds with that of the camera and under those circumstances High Dynamic Range might be referred to as an artefact.  However I would consider it more of a technical deficiency rather than an image distortion.  The important point is whether or not the subject is correctly exposed.  There are two additional artefacts commonly associated with exposure.  

Noise is the fine dark or coloured grain associated with underexposed and high ISO images.  ISO adjustment is merely an amplification of image data, or, put another way, an increase in the sensitivity of the camera sensor.  The greater the amplification the greater the signal to noise ratio (SNR) and therefore the greater the amount of noise in the image.  

At the other end of the scale, overexposure gives rise to another artefact called Blooming.  Blooming occurs when photosites overflow with charge and stray electrons spill into adjacent photosites.  The effect is much like atmospheric glare from the sun or lens flare.  For more on exposure and related artefacts see HERE

When the image is initially formed by the sensor it starts out as a "Bayer Raw" mosaic of green, blue and red pixels.  The process of turning this raw mosaic image into a raw colour image is called Demosaicing and gives rise to a number of different artefacts.  Depending on the type of demosaicing interpolation used, the image may suffer Defocus (blurring), Aliasing (which is like a smoothing of  of jagged image edges) and/or may display image Sharpening Halos, particularly around contrasting edges.  These artefacts could be described as a singular demosaicing artefact or individually.  The use of image sharpening tools result in the creation of the same type of artefacts. Interpolation is also used when an image size is adjusted.  For more see HERE.



Demosaicing involves reconstruction of an image from a mosaic. Deinterlacing is much the same.  It is used to reconstruct an image when Interlaced video images are captured as grabs.  Interlacing is a technique used to increase video frame rate without increasing bandwidth.  Essentially it takes two interlaced images to create a single reconstructed video grab.  As there is a short time delay between the capture of two consecutive interlaced images a problem occurs due to the movement of subjects between the two interlaced frames.  Combing is a particularly recognisable artefact produced by deinterlacing video.

Image format and resolution have some more artefacts associated with them.  Images created from the Raw image data are compressed in different ways.  Lossy compression tends to compress files the greatest but as the name implies it results in a permanent loss of data.  Another consequence of over-compression is the phenomenon of Lossy Compression Artefacts.  These are the tiny blurred and pixellated blobs and appear most notably in highly compressed JPEG images.

Moir√© is an artefact associated with image resolution.  It can be produced wherever two regularly occurring patterns overlap.  For more see HERE,




Purple Fringing is a broad term to describe a number of different but similar artefacts. It is commonly caused by aliasing where a dark edge comes in contact with a bright background, such as the sky.  A simple demosaicing interpolation will give rise to this and can be avoided by processing the image from raw using a more complex interpolation (for more see HERE).  Purple fringing may also be symptomatic of chromatic aberration or may even simply be noise associated with dark areas and shadows of an image.



A White Balance Error could be considered an artefact if it hasn't been corrected.  White balancing is required in order to try and replicate a facility in the human brain that allows us to filter out minor changes in light colour temperature.  Unfortunately the camera is not particularly good at correcting white balance on its own, hence white balance errors are common.  Note, in the image above there is a yellowish cast to the bird and tree.  This is in fact the correct white balance for the scene as the sun was setting at the time the image was taken.  Removal of the yellow cast for a more neutral white balance could be considered an acceptable alteration of the image.  An obvious white balance error on the other hand would be a strong bluish cast, which wouldn't make sense given the scene lighting.  It takes considerable practise to develop an eye for white balance errors and their correction.  For more see HERE.

Conclusions

Hopefully this brief summary covers all the commonly encountered image artefacts.  If I have missed any please drop me an email and I will update this posting and page.  At first this collection of strange anomalies might seem a bit daunting but one should bare in mind that most artefacts have little or no impact on identification and we really need only concern ourselves with the ones that we encounter when they directly challenge a difficult identification.



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