« [REVIEW—CANON POWERSHOT A650is] | Main | [REVIEW—CANON EOS 450D DSLR ] »
May 22, 2008
[PINS AND BARRELS]
The Nikon Coolpix P60 shows just about the worst barrel distortion we have encountered in a compact camera
A PLAINTIVE PLEA COMES FROM READER N. He wants to know the meaning of arcane terms that pop up from time to time in these columns.
For instance, what is “barrel distortion”? He likes the sound of it, he says, but does not make clear whether he is attracted to its voluptuousness or its connection with ale on tap.
And barrel distortion hardly ever occurs on its own in the text. It is usually accompanied by its optical companion, “pin cushion distortion”. So, what are these twin evils of the cheap, mass produced lens? And some not-so-cheap lenses, too.
Wide angle lenses – that is any lens with a focal length shorter than 28mm in film terms – are almost always affected to a greater or lesser degree by barrel distortion. The name describes the phenomenon. At and near the edges of the image straight lines begin to bow outwards, producing a ludicrous effect when the subject is a building with straight vertical and horizontal lines, as most buildings have.
Pincushion distortion is the opposite effect. It usually shows up in zoom lenses at the longest focal length and it appears as inward bowing (concave, we might say) lines near the image edge.
With fixed focal length prime lenses neither of these distortions is a problem, although any very short focal length lens will have the effect, hence the generic term “fisheye” for prime lenses shorter than about 16mm. Zooms are a different kettle of fisheyes, particularly the cheap little zooms fitted to compact cameras. Furthermore, the longer the zoom range the more likely it is that distortion will be a problem.
We are often asked our opinion of the various 18–200mm lenses that are all the rage for DSLRs. It looks like an ideal walking-about lens, giving a film-equivalent range of 27–300mm. You could use it to shoot the Eiffel Tower from close-up and still be able to look into a lion’s eye on safari in the Serengeti. It looks too good to be true, and it is. These lenses not only cover an amazing focal length range, they are also cheap.
The Nikkor 18–200mm vibration reduction lens is one of the better examples. The German lens testing site, www.photozone.de says: “Extreme zooms tend to suffer from rather hefty distortions and the AF-S 18-200mm is no exception in this respect. In fact the amount of barrel distortions (4.1%) at 18mm broke a new negative record…” Pincushioning at the long end is also extreme.
Both barrel and pincushion can be corrected in Photoshop, under Filter/Distort/Lens Correction. There is a simple slider to straighten the lines near the edge.
And while we are in this function in Photoshop there is also a correcting tool for converging verticals. You know the problem, standing on street level, looking up at the Empire State building, and the picture is of a building leaning backwards and vanishing into a diminishing point. Photoshop will correct this distortion, but the end result might show some other slight aberrations near ground level.
Optical distortion is not a deal breaker when buying a camera or lens, but in our experience the more modest a zoom range – say 3x – the better the lens turns out to be.
*
Posted by terry at May 22, 2008 12:58 AM
Trackback Pings
TrackBack URL for this entry:
http://bleedingedge.com.au/cgi-bin/mt/mt-tb.cgi/1487