Technique PHOTO SCHOOL
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Everyone has to start somewhere, even pros, so in Photo School we look at the core skills every beginner needs. Thismonth, controlling the distortion of wide-angle lenses and fixing optical flaws in Lightroom Camera class
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BARREL DISTORTION FIXED
Keystoning can even be useful, as the same principles that make buildings ‘fall over’ when shooting close to them and angling up, enlarges the foreground detail when angling the camera down. Controlling distortion To minimise barrel distortion, try shooting less wide; with large subjects or expansive views, the temptation is often to use your widest angle of view, but move back a little, zoom in and you’ll get a less distorted view, though the scene will become more compressed (see above right). To find where in the focal range barrel distortion is occurring (which you can then avoid if the subject doesn’t take to it) find a plain brick wall, shoot it straight on, then examine the amount of ‘bow’ at various focal lengths; if you decide it looks fine at, say, 28mm, then make a mental note of that. If you need to shoot wider, make sure that any in-camera lens correction is turned on (it’s applied to JPEGs, but not Raws). The only problem with this is that it crops the image slightly. Keystoning can be addressed in a similar way, by changing the focal length, your distance to the subject and the angle you’re shooting at. Again, back away from the subject and zoom in which means that less of the scene is recorded so the effect is less pronounced. If you have to shoot ultra-wide, try shooting dead level and don’t angle the camera upwards; the further off the level it is, the more lines will converge. Minor keystoning can also be corrected in software (see below).
Words & pictures by Kingsley Singleton
When shooting wide-angle (that being anything less than about 35mm on a full-frame DSLR) you’ll get distortion. This is inevitable and it shows up as either bowing of straight lines (barrel distortion) or converging verticals (keystoning), where upright lines are angled more than you’d like, making buildings seem to ‘fall over’ backwards. Both are avoidable, but while the amount of distortion can depend on the quality and type of wide-angle you’re using, it’s more often caused by the way you’re using it. If you have an 18-55mm or 24-70mm lens, you may notice a little barrel distortion at the lower end of the focal range, but extreme wide-angle lenses like a 10-20mm or 12-24mm produce even more. Fisheyes are specialist lenses where distortion is left uncorrected and so extreme that it actually becomes part of the draw. Keystoning on the other hand, occurs at all focal lengths though it’s more pronounced at wide- angle, simply because more of the scene is visible. Keystoning is produced by your angle of shooting, ie you point the camera upwards, and is exaggerated when focusing very close to a subject. Both barrel distortion and keystoning are most noticeable when shooting subjects with straight lines like buildings, or where the horizon is visible, so although the effect is still there, no matter what the subject, it’s less noticeable on natural scenes. Software skills Lens corrections ADOBE LIGHTROOM While it’s best to control distortion as much as you can when shooting (as this will retain the maximum image quality), it’s also possible to control lens distortions in Lightroom. These can be applied automatically (based on the camera model and lens), tweaked from those presets, or applied manually. The final route is vital if you’re using kit that’s not recognised by the software, and lets you take control if the automated results aren’t what you desire. Lens corrections should be made before any other changes to the picture, because as you straighten or rotate the pic, parts will be cropped out; any creative cropping is pointless until you know what you’re working with and the same goes for changes to the exposure. So, before making any other changes to the picture, head to the Develop module and open the Lens Corrections tab...
In the uncorrected version, shot at 16mm you can see barrel distortion in the pews and columns, but by stepping back and zooming in to 28mm, it’s straightened up.
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KEYSTONING FIXED
Shooting at 16mm here, tilting the camera up slightly causes obvious keystone distortion, but keeping the camera level produces straight vertical lines.
AUTOMATEDOPTIONS In Lens Corrections tab, you’ll find four headings: Basic, Profile, Color and Manual. Under Basic you can tick Enable Profile Corrections which uses data about the camera, lens and focal length used to reduce barrel distortion, fringing and vignetting. You can also use automatic options further down the tab to level up the picture and avoid keystoning, but the Manual route is more successful. Under Profile you should see the kit you used, but if there’s a problem, a body and lens can be selected from a list. Under the same heading, the Distortion and Vignetting sliders tweak those presets. The Color heading is used to manually control fringing in the image, but this is usually removed by the Remove Chromatic Aberration box under the Basic heading. MANUAL LENSCORRECTIONS Under the Manual heading there are sliders split into two groups: Transform and Lens Vignetting. These control the distortion and are helpful in fixing keystoning and other angular problems. Helpfully, whenever the sliders are used a grid appears on the picture as a guide. Click Constrain Crop before you use them or you’ll get blank white sections appearing at the edges. The Distortion slider pinches or bloats the picture to correct for barrel distortion (or pincushion if you’re using a telephoto lens), but if your lens has been identified under the Basic or Profile tabs it’s not necessary to use it (the same goes for the Vignetting options). The Vertical and Horizontal sliders angle the planes of the image and with the grid as a guide you can line up straight lines in the shot. Rotate can also be used along with the guide to set the horizon straight, although both this and the Scale option, which reduces or enlarges the pic, are more easily applied using the Crop Overlay tool. The final slider, Aspect, stretches the picture horizontally or vertically, compensating for any squashing that occurs when using the Horizontal or Vertical sliders.
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MANUAL LENS CORRECTIONS
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Issue 21 | Photography News
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