Welcome to lesson 4, where we set things straight! In photographs at least. We set things straight by shoving off distortion.
Distortion is easy to recognize, as it alters the shape of the image as a whole, but it is difficult to explain, but here goes 😉 Often, lens designers introduce stops in their designs in order to limit some aberrations such as astigmatism and spherical aberration.
A stop is a circular aperture such as the iris of the lens.
When the stop is right next to a lens, oblique rays going through the stop hit the lens near its center. But if the stop is placed further away from the lens, that oblique ray hits the lens away from its center, and isn’t bent with exactly the same way. So that oblique rays appear to have gone through a lens of different power to the axial rays.
Zoom lenses, in particular those with broad focal ranges, are usually far more afflicted than primes (because of all the elements moving closer or away from the stop through the focal range). But not all primes are immune and wide-angle lenses are difficult to make without any distortion.
These days, most native lenses are corrected in-camera. The Zeiss Batis 1.8/85, for example, is a superb lens with (huge) 3% distortion. The designers let the distortion fly away in order to create an otherwise excellent lens in a small (AF-compatible) package, knowing that the Sony A7x camera would correct this almost perfectly. So, testing is particularly important for legacy & third-party lenses.
Straight lines in scene are rendered curved on the sensor. The main types of distortion are :
Straight lines running through the image center remain straight.
Find a subject such as window bars, glass buildings with regular window frames and place yourself as squarely as possible to it.
This sort of grid will display any distortion very easily. Roof lines, building edges are also great for this. Placing lines towards the edge of the frame will reveal distortion more easily than in the center.
If you’re using a zoom lens, it’s important to test at several focal lengths. Both extremes and a middle point are good start. If you find an inversion in distortion between any two these points (for instant pincushion at the wide end and barrel at the middle point) try again between the two. No need to segment the zoom range in extra small portions, but it’s interesting to know the approximate point at which distortion inverts, if it does.
Since focusing a lens changes the spacing of its elements, it can also change the lens’ distortion. So, if you often use a lens at close focus, it’s a good idea to test it at that setting rather than at infinity. Or both, if both settings are important to you.
Since distortion is not related to lens aperture, it is easily corrected with a lens profile, even when the lens is third-party (and the Exif doesn’t tell the post-processing software what aperture was used). So if you software enables profile corrections, this is your number one option.
If not, pure barrel distortion and pincushion distortion are usually very easy to correct using the dedicated sliders in most software. Just adjust the correction until the scene appears straight. If you do this with your test picture, you’ll be able to calibrate your lens and dial in the same correction for every photo made with that same lens. If you’ve tested the lens at several focal distances, you’ll know whether you need distinct correction settings at different focal lengths.
Only wavy distortion is a real issue. See the roof line below. Some cases are far worse than this and preclude the use of the lens for any type of architectural work. If this is an issue for you and you cannot find a lens profile, then the lens is not suitable for your use.
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