Rectilinear

Rectilinear

Not rectilinear, but spectacular: the Millennium Bridge on the River Tyne show tih a 16mm Minolta fisheye lens.

And for the letter R, we’re back with the Ilford Manual of Photography and lenses, even though the book doesn’t actually define the word explicitly. Going to an ordinary dictionary, rectilinear means ‘bounded by straight lines’ – spot on for our purposes.

And the Manual provides a definition by exclusion: it details curvilinear distortion, where there is lateral (or sideways) distortion giving variation of the magnification of the image over the field: the image of a square object appears with the sides bowed in or out. We call these effects pincushion and barrel distortion respectively, and the terms conjure up an instant mental image, unlike the technical description or the explanation!



The header shot, fully corrected by Photoshop - and the composition is more than slightly compromised, I think.

And there’s also a diagram explaining how it happens – and one of those throwaway phrases that people who understand something complicated use. I’m sure it will make perfect sense to me one day. Curvilinear distortion (literally, lines curving) happens because we use lenses instead of pinholes: the very nature of lenses is that they have thickness and curvature of both surfaces.

Therefore, light going through them at an angle and not through the centre of the lens will be deflected. That’s fine when the ray of light is not at the centre of a parallel beam of light – indeed, that‘s the whole point of a lens, because we want it to bring parallel rays of light from a distant object together at one spot in a focussed image. But where the central beam is also bent, it means that the image is deflected inwards or outwards from the spot where we want it.

In practical terms, pincushion and barrel distortion haven’t been high on my own list of problems, because of my preference for prime lenses, which are far easier to correct than zooms. Looking for an example of a problem led me to get out a Makinon 28-80 varifocal lens that I bought in around 1981. I don’t use it often…



Noticeable pinchshion distortion at the long end of an Eighties 28-80mm lens.

The simplest cure is one of those things that makes you think ‘ah, yes’ and then wonder: did I actually understand that? And the answer is to use a symmetrical arrangement – two identical lenses back to back will actually give equal and opposite distortion, so that the net effect is an absence of distortion.

Now, here’s where it gets interesting and relevant to what you do…

First, advanced digital imaging has made correction of lenses less important: all common editing suites include the ability to correct for pincushion and barrel distortion, up to and including straightening the lines that a fisheye lens gives! And, these days, processing software includes a profile for many lenses, so that the software can automatically correct distortion without the user having to do anything at all, other than enable automatic corrections! Beware, though, because this can mean that the finished image – the DNG file that Lightroom saves to your hard drive – has dramatically different framing from what you saw in the viewfinder. I only discovered this last year, with some fisheye images of the Millennium Bridge over the River Tyne. I switched off autocorrection (which was, I think, the default mode) at once!

I thnk that the final part of hte jigsaw, jsut for the moment, is that some lenses carry full correction data in their chip, so that the software doesn't need a profile: indeed, there's the possibility of the camera giving a fully-corrected view on the screen (and in the viewfinder, for mirrorless cameras).



This view from the centre of the Millennium Bridge doesn't seem distorted: shot with hte same 16mm lens, with full correction.