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Welcome to lesson 4, where we set things straight! In photographs at least. We set things straight by shoving off distortion.
Architectural photographs do not tolerate 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.
 Pincushion distortion |
 Barrel distortion |
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.
Bars behind bars
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.
Straight walls
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.
Click for larger version and check out the roof-line.
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No news here. I have been correcting for distortion from a variety of lenses for over 5 years. What is news to me is that viewers expect a small amount of distortion in some scenes and that I should not always perfectly correct for it.
Jim Moule
Interesting point, Jim. This is step two or understanding gear. In other words, understanding what works best for you and your audience rather than just focus on numbers and specifications. Great going 🙂
I came to the point to only correct distortion if there’re straight lines directly next to the frame. Normally, my lenses are sufficient corrected and a little bit distortion can also add to the character of a lens.
With all those compact lenses for mirrorless, distortion correction is one of the first things lens designers drop. But like any software correction, it costs resolution.
Yes, and that’s still a very opaque topic, even in the high-end market. I’m pretty sure MTF curves are given for the uncorrected lens and results may be different after in-camera correction.
This issue can be compounded by shooting up or down also, I think. It is a problem if there is not a profile for a given lens. I guess if the edges are too bad, I could crop the image if necessary.
Thanks,
Micheal
Thanks Micheal,
you can photograph a grid and use that in photoshop or other software to measure the correction you need to get the grid back into shape. You can then use the same correction for all subjects photographed with that same lens, it should help. But, yes, a manufacturer profile is best!
Pincushion & barrel distortion aren’t always easy to get rid of. I have a fair swag of post-processing software programs, because I often p’process other people’s stuff and some of their cams are – well – awful. And sometimes the easiest solution is to chop it off by cropping. A grid helps – I’d have to check next time some of their shots turn up, I don’t know if I have manufacturer’s profiles for all their lenses – probably not, on the cheaper cams.
Nothing’s for nothing, and although cheaper cams suit many people, the easiest answer is better glass. And then the fun starts. Because it’s the person holding the cam who takes the photo, and the glass is only part of the process.
Doing a lot of architectural stuff, I did consider PCLs – but I have Nikons and I get the awful feeling Canon’s perspective control lenses are far better than Nikon’s, so I haven’t pursued it any further. Took the coward’s way out and bought a 24mm w/angle instead – happy to put up with a bit of extra cropping, the lens and the cam are so high quality that you can scarcely see any difference.
Despite all those comments, sometimes the cheap compacts made for amateurs & beginners take some startlingly good photos. And if someone is happy with their cam, I NEVER try to talk them out of it – not my place to spoil other people’s pleasure – it would be utter hypocrisy anyway, because I use three different formats myself.
Pete, PCL are the rabbit hole. Don’t go there or you’ll soon be drooling over such dream cameras as the Arca Swiss Rm3d https://captureintegration.com/arca-swiss/rm3di/ These should come with a health warning, they are so addictive 😉 Besides, keystoning is super easy to correct in PP, since it’s not a defect of the lens design but a geometric feromation that can be calculated and cancelled out in a couple of clics. Just shoot much wider than you think and let your coputer do the work 🙂 As for software correction of distortion, it’s always best if the software has a good profile of your lens. Not all distortion is regular and simplesliders don’t always provide perfect fine tuning. Still, only serious architectural work requires perfect correction. For that, some lenses such as the Loxia 35 are really brilliant. No distortion at all.
I so enjoy your articles. I’ve been a professional photographer all my life, now retired to photography as a hobby. Its such a joy learning from you. Please keep it up, you teach so beautifully.
FWIW: In retirement I care less about a lenses technical short comings and enjoy the character in its flaws.
I really like your willingness to try out all the combinations of equipment, nothing ever matches and you make it all work like great poetry.
I would also be interested in your views on post processing ,design and generally how you decide to shoot what you shoot. You really get it! I love looking at your photographs
Thank You, Keep up the great work
Wow, thank you very much Evan. That’s really a compliment that goes straight to my heart. We’ll be publishing more articles on composition and traveling very soon. Those are on the main blog, not in this series of lens-related articles. All the best, Pascal