Geometric Distortions - Sunwaysite

Geometric Distortion & Other



The magical zoom

24-70

6th April 2019


Is standard zoom range lens today. 24 and 70 mm wasn’t always the standard. There is a number of variants alongside them that reach to 75/85/105/120 mm or start at 28 mm, and even 35 as the range extends to 150 mm in the Tamron’s portrait zoom. The speed also got more varied, and the modern standard f2.8 is more frequently accompanied by f4, faster and slower variable aperture and even f2 versions.

The zoom is called normal, which may well be valid for the range, but there is one thing that makes it different from the other lenses in this range. It is the zooming function. It changed what was used to be normal before dramatically. That is often downplayed by the people who tend not to use it while those who need it all the time will likely overstate it.

The truth though still is that the 24-70, and any of its variants optically at that level, remains a special lens. It’s because it enables changing focal length, a great many of them, without the need for other lenses and without the need to change them. Granted, if one has a rucksack full of fixed lenses, and the time or the option to change them, they’ve got a workaround, if they accept some compromises like not being able to do this way of zooming quick enough or while shooting. The real trouble with that setup though is that that rucksack and the possibility of changing the lenses are very often ruled out.

Changing the distance to get the required framing isn’t often an option either. But there is a different snag; you are not zooming this way. Instead, you are coming closer or further away from the subject, and that changes the perspective in your photo. In other words, it changes everything, including the elements that you can mind to be changing. This turns us back to the rucksack and those non-options because changing the FL in any other way can only be done by zooming. That way you crop the view to the desired framing and maintain the perspective. This is very different, and it can lead to very different results. You can also do it fast, almost not moving and while shooting, but mainly, you can do it. That is the magic of zoom.

But that is not all the magic there is, right? Some of it must be left we feel. Yes, although using zoom vs prime gives added possibilities there is a wider question to it in photography. It’s because this can ultimately be a big plus for one to get the shot in theory and practice, but it can also be a minus in practice.

Having more possibilities takes more concentration. As it is the result that counts, we need to ask whether the distribution of our attention is optimal for getting the optimal result and even decide what the optimal result for the given moment and scenario is. There can be less than a second for this. So we combine the judgement with our intuition. This is very dynamic, and it means that we can feel rather than think in many instances. Naturally, the photographer as an individual is central to this. We have our own responses and reactions to what we see or what is unfolding in front of us, a different eye, intentions and different opinions about the results themselves.

It’s true that in practice, a restricted choice can help us. And that our concentration will prove more useful elsewhere, often to our surprise. It can also create our distinct style. On the other hand, it can and will quickly destroy the shot or make it impossible. There are obvious answers to this and less obvious as well. It’s the latter instance that rules out universal answers. It simply will always be up to you - one of the main ingredients of photography.




Vignetting

6th April 2019

Uneven illumination is another kind of distortion though it concerns light.

And its correction can also be questionable in many cases. About 95% of my subject shots (an instance where an individual subject plays a major role in the picture) shot wide open are left with full shading in effect (that includes f8 of our 200-500 VR/1.4 TC combination).

The vignetting characteristic of each lens is important and a factor I consider when looking at a lens. So far I don’t remember us adding the shading in processing; it’s always the particular lens with the full effect of the aperture or corrected to a degree or applying the correcting profile (mostly in non-subject shots that include a broad scope of the scene in sharp rendition).


2 June 2017


In September 2014 Samyang released stereographic projection UWA for 135 format that is part of fisheye lens family. The stereographic diagonal fisheye maintains principle of fisheye projection where the linear geometry is bent but unlike the classic fisheyes, it leaves objects shapes in original form. The regular projection fisheye lenses deform shape of objects at the periphery beyond recognition. This is especially apparent in the corners where the fisheye effect naturally draws attention of a viewer (due to the compression). This doesn’t have to be a negative effect but it distinctly separates rendition of a stereographic UWA from typical equisolid angle and equidistant fisheye lenses.

As there are no elements of extreme rendition with stereographic projection, it creates its own category among the UWA lenses.

Another projection useful and intuitive for object recognition (in the entire image) is perspective projection of corrected UWA. The shapes of objects are deformed but easily recognisable and intuitively correct (though exaggerated) while the linear geometry in the image does not change.

It is debatable which projection produces more natural representation of the scene. The stereographic projection preserves shapes of objects locally in remarkably truthful representation and virtually unchanged but it curves the whole space. It exaggerates “its sense” in often intuitively correct way that is in line with our natural perception.

This makes both of these projection types closely related and fit for realistic representation and distinct from non-stereographic projection fisheye.

The fact, however remarkable, has obviously no a priori bearings on artistic, photographic or aesthetic choice and preferences as apparent from the subjective nature of most applications.


26 November 2014


Most rectilinear lenses add some kind of geometric distortion to the picture as they project capture of a subject. One of the most prominent distortions is changed perspective and it is particularly noticeable with the wide and ultra wide angles. It is a kind of distortion that influences remarkably well transition from our own perception of the scene based on our brain and two lenses cooperation to a one lens capture. Prominent changes in perspective and its exaggeration are also extending the tools for a richer expression.

Another kind of geometric distortion of a lens is barrel distortion. Unlike the regular and predictable changes in perspective, this kind of radial distortions are often seen as an optical flaw of a lens and that because it is much less necessary for the design and thus more changeable from lens to lens.

Because the barrel distortion is also a perspective distortion of a sort we can see these two interacting. We will notice that the barrel distortion works its own way towards offsetting the perspective distortion effects by compressing the near corner geometry while making the central area visually more apparent by enlarging it. A mild degree of the barrel distortion in particular can be then often viewed as a beneficial and desirable effect for the overall projection of the scene. It is fine that it is retaining its reputation of a flaw as it is a matter of degree, and it is also less useful universally as a tool. I myself tune my corrections for barrel distortion for each image separately and with some care. Correction that I apply is often partial  and many times I am leaving it without any correction whereas sometimes I will choose to correct it fully.

Geometric distortion working in opposite direction of the barrel is pincushion distortion. This curves of the space in the image I correct often fully and in some instances almost always, for example, when typical wildlife is the subject. For other subjects, this I again tend to weight individually.

Moustache types of distortions that are less symmetrical work towards the elongated corner geometry and similar (compression) less effectively but even they can be found beneficial for rendering characteristics of a lens by a photographer. Their irregular curves though often command a default correction.

Observing meaningful details when processing a photograph can make a difference. And just as, e.g. careful framing where the each millimeter plays its role, the symmetrical distortions can help in creating a photograph that conveys its content precisely the way we want. Realising this can also change our view of excessive distortions that inhere in the design of affected lenses because the partial corrections using suitable software have only minimal negative impact on the rendering and resolution quality. And it can, on the other hand, add to the means for a thoughtful processing of digital images using mild distortions optimally for the intent of the final execution.




© HSunway



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