« [ SIGMA DC 18—200MM ZOOM LENS ] | Main | [ COMET MCNAUGHT OVER S.E. AUSTRALIA ] »
February 05, 2007
[ FIX YOUR DEFECTS ]
SPEAKING OF INEXPENSIVE ZOOM LENSES lenses with bold ambitions, as we are in the review of the Sigma 18-200 lens today, even though they tend to exhibit predictable distortions there is help at hand.
Photoshop, Photoshop Elements and Paint Shop Pro all have filters for correcting the common lens distortions -- barrel, pincushion, vignetting and chromatic aberrations.
Barrel distortion is a characteristic effect from lenses used at or near the wide angle zoom limit. It shows up in pictures as vertical and horizontal lines bowing outwards, which means that it is only noticeable in photographs of buildings or book pages. You won’t spot it in photos of the cat and the kids.
Pincushion distortion occurs at the other end of the zoom range and appears as straight lines curving inwards to the centre of the image -- once again only obvious in pictures of objects with straight lines near the edge of the frame.
Vignetting is caused by uneven distribution of light onto the film or sensor and is seen as a gradual darkening of the image towards the edges.
Chromatic aberrations are usually seen in high contrast light where a hard edged object, such as a leaf, is photographed against a bright background, such as the sky. This is sometimes called “purple fringing” and has particularly plagued point and shoot compact digital cameras.
In Photoshop Elements there is a filter set that appears in the Filter drop down menu called Correct Camera Distortion. In Photoshop CS the same filter set is found under Filter/Distort/Lens Correction. In Paint Shop Pro the filter is under Adjust/Photo Fix and then select the type of distortion to be corrected.
Using the filters is easy. In the Photoshops there is a single slider that moved one way corrects barrel and moved the other fixes pincushion. The effect is displayed immediately in an interactive image overlayed with a grid of straight lines.
In the Photoshops all the lens corrections are grouped in a single dialogue box, including vignetting and chromatic aberrations. As with linear distortions the process is controlled by sliders.
There is another useful correcting filter in the Photoshop group that fixes converging verticals. This is the phenomenon of the tall building, photographed from the ground looking up, that appears to taper to a narrow point at the top, with the walls sloping away from the vertical. Once again the correction effect is controlled by sliders and the change to the image is instantly previewed.
Professional photographers correct for converging verticals with very expensive perspective shifting lenses. Mere mortals can do much the same, if not with quite the perfect results of the pros, with Photoshop.
These corrections are best done on a copy of the original image before any cropping or other changes are made. Obviously once an image is cropped the centre may be displaced which will have an effect on the way the correction filters work.
The very fact that photo editing programs include these correcting filters is an indication of how common the problem of distortion is, particularly with zoom lenses. Even quite extreme distortion, which is more or less expected in inexpensive, long range zooms, can be corrected easily -- it’s just a matter of spending time at the computer. Which means that when a review says that a lens has “noticeable distortion” that is not a killer argument against buying it.
*
Posted by terry at February 5, 2007 12:04 AM
Trackback Pings
TrackBack URL for this entry:
http://bleedingedge.com.au/cgi-bin/mt/mt-tb.cgi/1183
Comments
So suppose you wanted to spend (say) $1,000 to get a better zoom with the widest-reasonably-possible zoom range (minimising the compromises in this cheap-but-effective lens)... what would you buy? After a quick look, I haven't seen anything even remotely close (in zoom range) at any price - except for the near-identical Tamron lens.
Posted by: Richard at February 5, 2007 02:38 AM
Richard: It depends on the camera. Generally speaking the best medium range zoom lenses are made by the camera manufacturers. Canon and Nikon have two distinct types of lens in their catalogues -- the el cheapo units that they sell as "2 for the price of one" with their kits and the better quality, faster, image-stabilised lenses of modest range but superb optical and mechanical quality. Olympus make both great lenses and so-so lenses. The Sony Alpha comes with a no-name kit lens "made in China", but they also offer Zeiss lenses of superb quality. In every case you pay a lot more -- and I mean a LOT more -- for the good lens.
With Canon, for instance, the superb EFS 17-85 lens has an RRP of $1100. It gives you a film-equivalent range of 25-127mm approx which makes it both a serious wide angle optic and also ideal for portraits. It doesn't try to be both a fish-eye and a lens for chasing wildlife.It sets a price/quality benchmark and it is an absolute joy to use. Canon offer it as an alternative to their standard kit combination and they call it the "enthusiast's kit".
Posted by: Terry at February 5, 2007 10:15 PM