A well-engineered lens is a thing of beauty – a work of art from which we create art. Looking into a pool of highly polished glass, the depths of which reveal the beauty of multi coated elements, it is difficult not to marvel at each element’s seemingly perfectly refined properties. Some lenses, diminutive in their size, lend an almost jewel like quality to their design. Over the decades, many have acquired legendary status thanks to the unique way they draw an image. Camera bodies come and go, but good lenses become life long partners. Technological changes push back boundaries in their quest for perfection, but sometimes there is little to improve on and a lens manufactured forty years ago can have the same appeal as a new design.

Are new designs optically perfect? Simply put… no. Lens makers are challenged by a number of faults, all of which compromise perfection. In the quest to reduce bulk and weight, the challenges increase exponentially and require more and more correction. This can come in the form of exotic glass, specially moulded elements or coatings that combine to give us the best a lens maker can offer. Each of these corrections has a specific purpose, which is to address one or more optical faults.

Some faults are common to many lenses, manifesting themselves across a range of focal lengths. The purpose of this article is to explain a little about some of the more common ones and how they degrade image quality.

• Chromatic Aberration (CA)

  More commonly known as ‘colour or purple fringing’ is top of my list when it comes to lens unpleasantries. It is caused by a lenses inability to bring all colour wavelengths into focus at the same point. Typically, red, blue, green, magenta etc. wavelengths travel at different speeds along the optical path, causing them to focus at slightly different points on the image plane. This causes colour fringing along edges with high contrast, the most common of which is purple or green. As the different colours overlap, it can also result in softer than optimal images as high contrast edges are not reproduced as sharply as they could be.

  Chromatic aberration affects many types of lenses with prime telephoto or wide angle (particularly high-speed ones) being most affected. I once owned a fast 180mm lens that had little in the way of correction to combat the problem. Generally, results were acceptable, but this was because I had learned to omit high contrast areas from compositions. Where such areas were included, purple fringing ruined most of the shots (this was in the days when I shot slide film, so the result was WYSIWYG with no option for post processing corrections unless scanned). A particular image that springs to mind was a chrome motorcycle shot in full sun; nearly all reflective parts of the bike were tinged purple with additional green haloes toward the edges of the image. Stopping the lens down helped to reduce the effect a little but this took away the primary benefit of using a fast telephoto.

  The problem is usually addressed by the use of achromatic or apochromatic elements. Achromatic lenses are constructed in a way that allows two primary colour wavelengths (e.g blue & red) to focus at the same point. While this does not remove the problem completely, it does provide a very acceptable solution, particularly for the budget conscious photographer. True Apochromatic lenses (APO) ensures that three or more wavelengths are brought into focus at the same point, effectively removing the problem. Apochromats are usually expensive and the buyer should proceed with caution when presented with one that is not; the chances are the manufacturer is using the ‘APO’ designator as simple marketing hype which may equate to a well made achromatic design.

  Having used both designs, I can confidently say that apochromats are the best solution to the problem. I currently have two APO telephotos that stand head and shoulders above achromats used in the past.

• Spherical Aberration

  Spherical aberration is caused by rays of light coming into focus at different points. The problem tends to worsen on the periphery of the image circle with fast lenses. The results for the photographer is one of reduced clarity giving the impression that the lens is not correctly focussed. Wide angle lenses are particularly susceptible to this due to the light from their huge field of view being brought together on the image plane. A poorly constructed, uncorrected example will see light rays severely misaligned.

  Good quality lenses will contain an aspherical element, usually manufactured from high quality glass. The element’s design looks a little unorthodox when viewed in cross section, as it curves outward towards the centre to ensure that lights rays converge in the same place.

  Lenses with no aspherical design will always be problematic; while stopping down may improve the problem, it will never eradicate it. To be completely satisfied, look out for examples that contain aspherical elements.

• Vignetting

  Vignetting affects most lenses in varying degrees and can be particularly apparent when the aperture is wide open. It is also referred to as ‘light fall-off’ and manifests itself as darker image corners when compared to the centre. The simplest way to spot it is to point a lens at a blue sky or other area of light colour and shoot an image wide open. What you see is a shading that appears strongest at the lenses periphery which gradually reduces as it approaches the optical axis (centre). It is a result of light being partially blocked by the outer edge of the lens barrel.

  Wide angle and/or high-speed optics are particularly prone to vignetting and from my own experience, even expensive ‘exotic’ optics do not dispel the problem completely. Stopping the aperture down goes a long way to mitigating things to negligible levels.

  Telephoto lenses are not immune, as I found out after recently purchasing my 55-135 zoom. Even with a relatively sedentary maximum aperture (f4.5 at the 135mm end), vignetting is significant and very noticeable in some situations. Stopping down helps a lot, but this comes at the risk of camera shake due to the use of slower shutter speeds.

  An additional manifestation of the issue can be found hidden within a lens’ bokeh (or the out of focus part of an image) – particularly where points of light are captured with high speed lenses. These points of light are rendered circular in the central part of the frame but take on a more elliptical shape towards the periphery. How distracting this is depends on factors such as optical construction, lens speed and of course, the photographer’s acceptance. Having owned f1.2 lenses and witnessing the problem first hand, I never found it distracting as bokeh attributes usually consist of other properties that contribute to satisfactory results.

  It should not be forgotten that vignetting can contribute to the final image. Many software packages provide algorithms that apply it in varying strengths, shifting its status from aberration to effect. It seems odd that we choose to purchase lenses that are engineered as best as possible to remove aberrations, only to reintroduce them during post processing. If I was to be honest, I find myself applying the effect to an increasingly significant amount of my work, especially when wishing to add a little drama to mono images, or those where I wish to draw the viewers attention to a central part of the image.

• Coma

  Coma or Comatic Aberration is one of the more transient faults that makes itself apparent in specific situations. If you enjoy photographing star fields or other subjects that contain smaller points of light, this is likely to be one of the problems encountered. Coma tends to affect the periphery of an image, usually leaving the centre unaffected. Using a starry sky as an example, the tiny points of light in the central part of the image look fine, but closer inspection toward the edges reveal that the points become ‘stretched’ and reproduced as small teardrop shapes. The closer to the edges they appear, the more pronounced and stretched they become. What we are seeing here is the result of poorly rendered off-axis light points. A badly corrected lens may also exhibit chromatic aberration around the light points due to high contrast.

  Coma is not specific to a certain lens type, but I have noticed it more in super wide-angle lenses and more so in budget brands. A generally accepted workaround is to close the aperture a couple of stops, which is not really a problem when photographing night sky as the camera will probably be fitted onto a tripod. Additionally, a lens containing aspherical elements also helps reduce the effect but will likely increase the cost.

• Diffraction

  Diffraction is a problem affecting all lenses and is a consequence of the physical aspect of lens design – more specifically the aperture mechanism. As a lens aperture is closed, the area that light is permitted to travel through is reduced, causing the light waves to ‘bend’ or diffract. This is amplified the more the lens is stopped down.

  The impact this has on a camera sensor or film is that the light waves diffuse due to them bending, causing what is termed as an ‘Airy Disk’ (a central pinpoint of light surrounded by dimmer concentric circles) - imagine a stone dropped into still water. When a lens is wide open, there is no light restriction placed on it by the aperture blades, therefore the problem is not apparent, but once aperture comes into play, diffraction is introduced in growing values as the f-stop increases.

  What this means for the photographer is a blurring of detail that is dependant on what value the aperture is set to. Lower f-stops (e.g. 2.8, 4.0, 5.6) result in negligible impact as the ‘hole’ through which the light waves pass is only marginally affected. But as the aperture is shut down (11, 16, 22, 32 or even 45) light waves pass through a much-reduced hole, resulting in the Airy Disk appearing larger, which causes more blurriness.

  The simple workaround for this is to avoid using smaller apertures wherever possible. Most lenses have a ‘sweet spot’ or setting that can be used for optimum results. This is usually around the f5 mark, but some are optimised for use wide open, which means that performance may start to fall off sooner than this, although not really noticeable until around f8 or f11.

  Some wide-angle lenses I used a few years ago seem to be diffraction limited by f8, but given their massive depth of field, I could use them confidently at f5.6 while ensuring that foreground and background detail were both in focus.

  A 200mm lens I once owned was also diffraction limited by f8, which was odd given that it could be stopped down to f32. Using it at such a small value gave a noticeable softness to the image, to the point where I questioned why it was included.

• Field Curvature

  Field curvature is a problem I have had a lot of dealings with in the past but to a lesser degree today. When the lens is focussed, light rays fall on the sensor or film producing a sharp image. But this degrades further away from the central area, particularly at the edges. If the lens were to be refocussed to bring the edges into perfect sharpness, the central area would be out of focus.

  Wide-angle lenses are particularly affected and it is not hard to see why when we take a closer look at the problem. When viewed from the edge, many optical elements are usually curved. Therefore, light waves travel further to the image plane at the centre than they would at the edges. Typically, the larger the angle of view a lens has, the more curved the front elements have to be (think of some super wides that have huge protruding front elements). So, it seems that whichever part of the image is perfectly focussed, other areas will not be.

  Fortunately, lens design has come along way in the last few decades. The first time I noticed curvature was many years ago when using optics that were designed in the late 1960’s and early 70’s. Their optical construction contained few elements and very little in the way of correction for curvature. If used wide open (particularly at close distances) the aberration was very visible. New designs I used around a decade later utilised close focus correction mechanisms (basically a lens group that was realigned at different distances to the rest of the optics, as it was moved between short and long focus), which helped a lot.

  Like many other optical aberrations, particularly chromatic, stopping down the lens helps considerably. Usually reducing the aperture by one or two stops is sufficient, but lenses where the problem is heavy will require stopping down much more – possibly f8 and beyond. This puts the lens at risk of suffering from diffraction, so the photographer will need to consider the trade off in loss of overall image sharpness caused by this.

  Modern lenses also suffer from field curvature, but in my experience it has been significantly reduced, no doubt assisted by advanced element designs and coatings. Bear in mind also that it is a problem which reveals itself only under certain conditions. For example, if you shoot subject matter that does not require the entire frame to be in sharp focus, it will not be troublesome. However if you intend to shoot postage stamps, posters, graffiti on walls etc, study lens MFT charts before purchase and consider flat field designs where macro and short telephotos are concerned.

• Distortion

  Distortion presents itself in different forms depending on focal length. It falls into two main categories:

  • Pincushion which is prevalent in telephoto lenses of any focal length with zooms being particularly affected. It is termed pincushion as the effect can be compared to a real pincushion, that is to say vertical and horizontal lines at the image periphery have a tendency to bend inwards instead of being rendered straight. The problem is more apparent when photographing subjects that contain straight lines, such as a wall or building.
    Most, if not all telephoto prime and zoom lenses I have owned have suffered from the problem to some degree, although it would be fair to say that the zooms were the worst. Having just purchased a new telephoto zoom, which was a new design around three years ago, I was pleased to see that the problem has been greatly reduced, although not eliminated. Of the telephoto primes I have owned, three in particular were affected – 100, 135 and 180mm focal lengths. Closer inspection of the optical configuration revealed that they were all of the same design, each being an upscaled version of the other. This would explain why the problem was similar on all three.
    Fortunately, in today’s digital age, the problem can be quickly removed in post processing and some conversion software will have built in lens profiles that automatically remove distortion. If shooting jpg format only, the camera’s processing engine will likely remove it before it is apparent to the viewer. So, while it can prove to be irritable, it is one of the easier problems to fix. This is not the case when shooting film however as beyond closing the aperture down, options are limited.
  • Barrel; as the name suggests, creates images that render straight lines on the outer periphery to bend outward, similar to the shape of a barrel. Where pincushion distortion is endemic in telephoto lenses, barrel affects wide-angles. Its most exaggerated form is exploited in fisheye lenses where the problem goes untreated, thus creating those whacky images you either love or loathe.
    Even though I have owned many wide-angle lenses through the decades few, with the exception of zooms, have presented me with the problem. I used a 21mm f2 for many years and do not recall it being visible in my results, although my subject matter made little demands in this area. A 24mm lens behaved similarly. Even the less costly zooms (I’m thinking of a 28-105 here) I owned never gave me cause for concern. Maybe the dreaded ‘brick wall’ test would have revealed issues I was blissfully unaware of.
    Like pincushion, barrel distortion is easy to remove in post processing via the same means. And over in the world of film, eradication remains as challenging as for pincushion.
• Astigmatism

  Astigmatism manifests itself as areas of unequal sharpness across the image circle and is usually more pronounced toward the edges of the image (the centre being unaffected). It appears very similar to spherical aberration but affects the path of specific light waves. For example, light rays coming in parallel to the lens’ vertical aspect may be subject to spherical aberration (therefore appearing unsharp due to focussing at a different point), whereby rays entering the lens along the horizontal aspect may not be affected (appearing sharp). The result is an overall unsharpness of an image that is worse in certain areas.

  To best demonstrate its effect, a subject that has lines radiating out from a central point should be photographed at open aperture. If the lens is affected, areas of unsharpness will be seen at varying places along the lines.

  Optical correction is possible by means of an additional group of elements (usually two convex and one concave) being included in the lens design. If a lens does not contain optical correction, the problem can be controlled by stopping the aperture down one or two stops.

  Decentred lens elements can also cause astigmatism, so this possibility needs to be considered and will likely need the attention of a repair centre if it is found to be the cause.

The above is by no means an exhaustive list of optical faults and is simply a list of those that are more common. I have experienced all of them in varying degrees over the years, and some bother me more than others. A lens free of all aberrations does not exist. But there are truly excellent designs available that reduce the problems to a minimum. This is usually the amalgamation of exotic glass, multi coatings, floating elements and/or dedicated element groups.

When searching for the perfect optical companion, do your research; study test data, MFT charts, user reviews and, most importantly, results. Arguably it is the latter that is the most significant; optical perfection alone may not give you the look you are searching for and I know this from experience. A 50mm f1.2 lens I used to own suffered from a few of the above defects, but I loved the way it drew images, particularly wide open. Current lenses I own are not immune to optical defects and, assisted occasionally by a little post processing, give me exactly the results I have been searching for.