Perspective correction is used to correct converging verticals in pictures of buildings, and sometimes to fix horizontal convergence too, though that’s generally less important.
Perspective correction is not necessary for most photos, but there are times when it will make the difference between an image that looks professional and one that just looks amateurish. For that reason, it’s one of a handful of basic and necessary adjustments you might need to consider.
Perspective correction is most often used for images with ‘converging verticals’. This happens when you have to tilt the camera upwards to get the whole of your subject in the frame. It’s worse with wideangle lenses because you’re generally closer to your subject and have to tilt the camera more. Wideangle lenses also exaggerate the size of objects in the foreground (the base of a building) relative to those further away (the top).
Sometimes strong converging lines can be a deliberate part of the composition, but it has to look deliberate. Otherwise, a small degree of convergence can just look like sloppy technique.
Perspective control lenses
This is the old school approach to correcting perspective. These lenses have a ‘shift’ mechanism that lets you keep the camera level but shift the lens upwards to get the top of your subject in the frame without tilting. Large format field cameras and technical cameras have movable lens plates on the front which achieve the same thing but with a greater range of movement.
You can still get perspective control lenses to fit many digital camera systems, but these lenses are expensive and quite slow to use. For most of us, it’s easier to use software perspective corrections.
Perspective correction with software
Many photo editing applications have perspective correction tools. Lightroom, Capture One, Exposure X and ON1 Photo RAW all have them built in, while DxO PhotoLab has a companion program called DxO ViewPoint.
These tools can be used to correct converging verticals, horizontal convergence or both. They may offer Vertical and Horizontal correction sliders or on-screen gadgets or lines that you position over walls or other straight lines in the scene that need to be perfectly horizontal or vertical.
Perspective correction tools work in a pretty simple way – by ‘stretching’ the top (or bottom) of the picture to make it wider, or by doing the same with the left or right sides.
These corrections do leave triangular wedges at the sides or at the top or bottom of the picture that do need to be cropped off – though the software may offer to do this automatically.
This means that perspective corrections do inevitably mean sacrificing some parts of the picture area at the edges; the stronger the correction, the more you lose. There is no way around this, and it does mean that these digital corrections work best when there is some free space around your main subject. If you’re taking a picture with the aim of correcting the perspective later, make sure you stand far enough back, or use a wide enough lens, to capture plenty of space around your subject.
Perspective correction vs lens corrections
Perspective correction and lens corrections are not the same thing. Lens corrections fix lens distortion (bowed edges), vignetting (corner shading) and chromatic aberration (color fringing). They simply make sure that lenses render straight lines as straight, and have nothing to do with perspective.
But it is important to apply lens corrections, where available, before you apply any perspective corrections, because you need properly straight lines in your photo to judge perspective corrections properly.
In the same way that perspective correction and lens corrections are not the same, lens distortion is not the same as perspective distortion. Lens distortion is where the lens makes straight lines look bowed; perspective distortion is where you’ve angled the camera sharply in a way that makes straight lines appear to converge.