Optical flaws and aberrations

Pekka Buttler, June-November 2021

Introduction

JAPB offers a series of articles describing the various optical flaws and aberrations that affect many lens designs1. Together, these articles constitute JAPB’s definitive guide of optical flaws and aberrations.

In case you’re asking why JAPB has decided to embark on such a task, scroll down and read the rationale.

Index of articles on optical flaws and aberrations:
Distortions (optical)
Vignetting
Veiling and Ghosting (flare)
Atypical ghosts
Spherical aberration
Focus shift
Comatic aberration (‘Coma’)
Astigmatism
Chromatic aberrations
Field curvature
Uniform colour casts
Partial colour casts

Related articles (which are not really flaws or aberrations):
Diffraction (even though it’s not an optical flaw or aberration)
• Analysing bokeh balls (not an optical flaw or aberration, but they help analysing lenses)

Have we forgotten something or left a particular aberration unmentioned? Tell us about it, or leave a comment.

Rationale

Let’s start by taking a few steps back (prime shooters call it using ‘leg zoom’) and trying to grasp the wider picture.

There are a number of factors that contribute to that owners of interchangeable lens cameras need so many lenses.
1) Focal lengths (fields-of-view). Sometimes you want a wide field-of-view and sometimes you want your lens to give you a close-up of a far-away target. The attraction of zooms is that — while there are currently no ultra-zooms covering the ultra-wide (12 mm in FFE, full-frame equivalent) to long-tele (>400 mm FFE) – a photographer can seamlessly cover a wide range of focal lengths using only 2–3 zooms, instead of having to fill a Pelican case with primes and still find that they don’t have a lens offering a 65 mm (FFE) field-of-view.
2) Brightness. In some situations (most typically when you are shooting in the dark or want to maximize subject isolation), you want the brightest possible (affordable) lens, whereas in other situations (e.g. when you’re shooting architecture in broad daylight) you’d rather have a small, sharp, and distortion-free lens. It is not untypical for a photographer to have their favorite focal length covered by a multitude of lenses, each of which offer a suitable combination of brightness, size and other relative strengths.
3) Speaking of strengths, lenses can have additional weaknesses, such as distortion, various chromatic and non-chromatic aberrations etc. Often these lenses have such weaknesses, because the manufacturer either was unable to design a lens that would correct these weaknesses perfectly, because the manufacturer found it more important to correct for another weakness, or because the manufacturer (often: rightly) figured few photographers would want to pay for that level of perfection (nor schlepp such a lens around). Lens design is a question of trade-offs.

Interestingly, in several cases, these very weaknesses may make such lenses especially wanted by some photographers — namely, by photographers who know that they’d be able to turn such weaknesses into strengths (or mitigate them). Whether the Helios-44 (Biotar) or Trioplan (and many more I could mention), these are lenses which today are so sought-after specifically because their imperfections offer a signature look. And even when the Helios swirl or Trioplan soap bubbles are absent, many of these weaknesses collaborate to set legacy lenses apart and – in concert with cost – make legacy lenses a value proposition.

Therefore, a lens is more than a combination of focal length and maximum aperture, as each lens design imbues a specific character — a character based on the combination of ways in which that specific lens design is imperfect. So when we’re talking about field curvature, over-corrected spherical aberration, longitudinal chromatic aberrations or mustache distortion we might as well be making a sales pitch or listing caveats — and as long as you don’t know what these mean and how to use them to your advantage (or mitigate them), you’re liable to throw money away, or pass on a good deal. This is why JAPB wants to offer you a definitive2 guide to optical flaws and aberrations, so that you can make informed decisions.

Oh! And one more thing: Lenses produce pictures based combining the properties of numerous lens elements used to refract (bend) light. This is physics (not magic). Therefore, unavoidably, the explaining of these phenomena, will involve some physics. Do not worry, however. Even if you hated physics in school and flunked more exams than you’d care be reminded of, you’ve grown, and learned since then. This is optics, not rocket science. Moreover, we’re going to try to explain stuff without resorting to formulas, so not having an understanding of physics or math is not problem, assuming you’re willing to try to bear with us.

Footnotes

1 Note the phrasing: “affect many lens designs“. By this JAPB wants to make clear that we’re not addressing here (in this series) the kinds of flaws and aberrations that fall entirely within the realm of sample variation, wear, or damage. For instance decentering – while certainly serious – is not something that any lens has by design. Instead decentering may be caused by sample variation, inexpert repairs, or one-too-many hard knocks. Similarly loss of contrast due to a scratched front element is also not a design issue (even though design may increase a front element’ propensity to get scratched), but is a sample-related issue. If you’re interested in sample-related issues, maladies, and flaws, JAPB already has a series of articles on lens [sample] defects.

2 Well, to be honest, this guide goes against a lot that has been written online. It is, however, my contention, that the representation this article series offers is better than the ones I’ve yet found. It is more thorough than most, it is more complete than most, it is more context-aware3 than most, and that it is more correct than some. Furthermore, I hope I’ve managed to make it more easily understandable to laypersons than some of the more technical (and complete and correct) explanations found out there. But even if I’ve failed in one of these aspects, this guide is definitive in that it offers the definitions of optical flaws and aberrations used throughout JAPB.

3 By saying context-aware, JAPB is again reminding the reader that lens design is the business of trade-offs. Not only is that true in that aiming for an optically perfect lens typically sacrifices other aspects (price, size, weight, optical complexity), but also in that in many cases the aim to eradicate one optical flaw makes another optical flaw all the more strong. Therefore, not only is there no such thing as the perfect lens (optical performance vs. size, weight and cost), there also is no optically (screw price and ergonomics) perfect lens (and we hope that some day we’ll be proved wrong).

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