These are optical flaws produced by camera lenses and which are largely unavoidable except in the most expensive lens designs. They include distortion, chromatic aberration (color fringing), vignetting (corner shading) and edge softness.
No lens is optically perfect. In the ideal world, every lens would produce a sharp, undistorted, evenly illuminated image that’s sharp from edge to edge. Some lenses come close, but the fact is that lens designers have to work within the limitations of the laws of physics and the glass materials they have available, and almost all lenses have some or all of the following aberrations from the ‘perfect’ image.
Distortion
Distortion is where lines which should be straight appear slightly bowed. This effect grows stronger near the edges of the frame. Wideangle lenses tend to suffer from barrel distortion, where straight lines appear to bulge outwards – especially zoom lenses at the wide-angle end of their zoom range.
Telephoto lenses (especially telephoto zooms) often suffer from the opposite effect, pincushion distortion, where straight lines near the edge of the picture appear to bend inwards.
Zoom lenses suffer more from distortion than fixed focal length prime lenses because the lens designers are having to deal with a much more complex set of optical elements and have to choose compromises between optical performance, size and weight, price, zoom range and more. It’s possible to apply very effective distortion correction using software.
Chromatic aberration
Chromatic aberration is a very common aberration. It shows up as colored fringes around the edges of objects and becomes more noticeable away from the center of the picture.
Lateral chromatic aberration is the most common type, producing relatively narrow but quite prominent magenta, blue or green edge ‘halos’ around objects. Axial chromatic aberration is also quite common but less noticeable. This produces soft colored ‘halos’ around out of focus objects. It can show up if you use long focal length lenses and shallow depth of field for strong background blur. This can not easily be fixed in software and has can only be corrected effectively by advanced optical designs.
Vignetting
Vignetting is sometimes called ‘corner shading’, and it’s where the edges or corners of the image are darker than the centre. There is a tendency for this to happen with simple lenses so it’s the lens designer’s job to counteract this with the lens design.
Even so, many lenses exhibit some vignetting. It doesn’t always harm. The composition and can sometimes help focus attention on your main subject. Vignetting tends to be worse with cheaper lenses at or near their maximum aperture settings.
Diffraction
Diffraction is sometimes treated as a lens aberration, but it’s not quite the same as the others. Light is diffracted (bent) when it passes the edge of the diaphragm blades in the lens’s aperture mechanism, and at very small lens apertures a greater proportion of the light is diffracted, which leads to a drop in resolution. Generally, this effect becomes noticeable from lens apertures of f/11 and smaller – though it starts earlier with smaller format cameras because the lenses and apertures are physically smaller.
Some camera makers attempt to correct this softening effect. Fujifilm has an LMO (Lens Modulation Optimizer) feature which digitally corrects this diffraction softening.
Lens corrections
It is possible to correct many aberrations with a suitable lens design, but often not all of them, especially when customers also want long zoom ranges, wide apertures, light weight and low cost. As a result, lens design is usually a set of compromises and no lens is ‘perfect’. It often comes down to cost. Good optical performance is not difficult to achieve, but achieving excellent optical performance quickly becomes expensive.
Increasingly, lens makers and photographers are turning to digital lens corrections. Lens correction profiles may be applied automatically in-camera as the image is processed, embedded in RAW files for use by RAW conversion software, or applied automatically by software by matching the lens uses against a database of correction profiles.
This has become a cost-effective way to correct common lens aberrations and successfully eliminate distortion, chromatic aberration and vignetting to a degree that would be impossible or just too expensive with advanced optical lens designs.