Expect to hear a lot more about colour fringing in digital photos as the pixel counts for compact digicams continue to rise, while sensors become smaller. At the same time, expect to see the true reasons for colour fringing misattributed more often than not. To ‘put you in the picture’ (so to speak), if you see coloured fringes along edges in a photograph it can be attributed to one of two effects: chromatic aberration or blooming. The problem is deciding which one is the cause.
|If the picture was captured on film, deciding what caused the coloured fringing is easy: only chromatic aberration can be blamed. It’s more difficult if the shot was taken with a digital camera because both blooming and chromatic aberration can create coloured fringes. Teasing out which one is relatively easy once you understand how the fringes are produced.
Chromatic aberration is a lens-based defect that occurs because the refractive index of optical glass varies with different wavelengths. In most optical glass types, blue light is bent (‘refracted’) more than green or red light. As a result, a simple lens is unable to bring all wavelengths of light into a single focus point on the image plane. There are two main types. With longitudinal chromatic aberration, wavelengths towards the blue end of the spectrum focus slightly in front of the image plane and wavelengths towards the red end of the spectrum focus behind it. Transverse (or lateral) chromatic aberration occurs when the different focus points are side-shifted. In practice, lenses affected by chromatic aberration generally suffer from both types. It is most common in wide angle lenses of the retro-focus type and older tele lenses (including zooms).
Chromatic aberration is rare in modern, high-quality, compound lenses because the manufacturers can combine a number of lens elements with complementary refractive indices into a single lens that can bring all wavelengths to a common focus point. Achromatic lenses correct the focus at the blue and red ends of the spectrum, pulling them in to focus near the green focus point. However, they may only work at the extremes of the spectrum, leaving wavelengths between red and green and green and blue uncorrected. This will produce purplish and greenish fringes because the intermediate colours average out. Apochromatic lenses bring all three wavebands (red, green and blue) to a common focus, minimising fringing effects. A superachromatic lens corrects for four or more wavelengths and should eliminate all colour errors. Many of these corrections are achieved through use of lens glasses with low or anomalous dispersion, which is why lens manufacturers emphasise these features in their promotional materials.
Where it occurs, chromatic aberration is usually only seen towards the edges of photographs. Unfortunately it can be difficult to identify because it only shows up along the boundaries between dark and bright areas in the subject. Even with poorly-corrected lenses, little or no effect can be seen in areas with uniform colour or brightness. For this reason, it is easily confused with blooming, an effect that only occurs in digital photographs.
Although you may encounter chromatic aberration with some digital camera lenses (most commonly long telephotos), a more likely cause of coloured fringing in digital photos is blooming – especially with high-resolution, compact digicams with small image sensors. Blooming occurs when a group of photodiodes on an image sensor receive more light than they can handle. The resulting electrical charge overflows into adjacent photodiodes, which lose their ability to record a signal that is proportional to the incoming light they receive. This will produce an area in which the image data is corrupted.
Blooming is most obvious in regions in digital photographs that are over-exposed. It can appear in any part of a photograph – including the centre – and, because different colour channels react differently to over-exposure, the edges of areas of blooming can vary with different wavelengths. Hence the coloured fringes.
Most CCD manufacturers include anti-blooming gates on the sensor to drain off the overflow of charge. These ‘drains’ can limit the degree of over-saturation but may not totally eliminate the blooming effect. Unfortunately, they may also reduce the overall sensitivity of the sensor, leading to increased image noise.
Interestingly, infrared ‘light’ can produce blooming as easily as normal ‘white’ light because most CCDs detect IR wavelengths equally well and it can be difficult to separate IR-derived blooming from lateral chromatic aberration. The effects of excessive IR exposure can affect both the red and blue colour channels. The problem is most obvious along the edges of bright highlights, where it shows up as a purple fringe. When data from a group of pixels affected by IR blooming is interpolated, the resulting pixels will have higher signal levels from red and blue than from the green channel. Red plus blue produces purple; hence the purple fringing.
Smear is a related problem caused by oversaturated photodiodes, which is most commonly seen in electronic viewfinders, where the ‘bleeding’ of the signal from lights or brightly-lit areas in a subject can create vertical stripes above and below the object. In most cases, this smearing is processed out when the image is captured by the camera and does not appear in the final picture. However, it can make composing shots with an EVF difficult – and may also affect subjects viewed with a camera’s LCD screen. Smear and blooming are less common with CMOS sensors.
Several software applications are available for removing coloured fringes from digital pictures.
Picture Window Pro V3.5 from Digital Light & Color (www.dl-c.com) sells for $US89.99 (30-day trial downloads are available) and includes a wide range of editing facilities plus corrections for colour, chromatic aberration and moire.
Flo’s UnDistort Filter V0.5 (http://flofilters.free.fr/) is a freeware plug-in that works with Photoshop, Photoshop Elements, Image Ready, Corel Photopaint, IrfanView, Jasc Paint Shop Pro and many other popular applications. The freeware version can be used for images up to 800 x 600 pixels. For 15 Euros you can purchase a full working version that can be used with larger image files.