Everything You Need to Know About Lens Quality
Understanding the physics of light puts you at a great advantage when trying to produce great photographs. Arguably, how light interacts with your lenses is the most important place to start. In this article, we’ll consider the physical construction of a photographic lens, allowing you to understand the elements that warrant the price tag.
Trying to decide which lenses to purchase can be a real headache, there are so many factors to consider, such as build quality, cost, aperture and image stabilization, but what actually makes one lens different to another?
Groups, Elements, and Why They Matter
Each lens is made up of a number of individual glass lenses known as elements. The idea behind using many elements is to try and reduce aberrations so that images are sharp and free from imperfections.
Differently sized and shaped lenses are grouped together in order to refract different wavelengths of light at varying angles to allow the light to converge and therefore reduce aberrations. Try and picture the way that light travels through a prism, entering at one angle, then being refracted by the glass, and therefore exiting in a different direction.
Each differently shaped glass element will refract the light coming into the lens in a different way, allowing lens designers to control the light. The grouping of the elements, like a stack of differently shaped magnifying glasses, allows for greater control of the light to restrict distortion.
Types of Elements
The majority of lens elements have a curved surface and are known as spherical, as they would fit within the surface of a sphere. Historically, these were relatively cheap and easy to manufacture using grinding, but the design allows for the distortion of light wavelengths and therefore results in imperfections within the image.
These distortions are reduced in higher quality lenses through the use of aspherical lenses, which I’ll explain more later.
Apochromatic (APO) elements are employed mainly in telephoto lenses. Long lenses are particularly susceptible to chromatic aberrations, resulting in reduced contrast and sharpness within the image. The APO element brings three wavelengths of coloured light, usually green, blue and red, into focus on the same plane, reducing the distortion.
High end lenses also feature ‘floating’ internal elements, which move depending upon the focal length, reducing the curvature of field that causes the edges of images to loose sharpness.
Moulded vs Ground Lenses
The physical manufacture of lens elements also has a bearing on the quality of image that they are able to produce. There are three main production methods, the first being ‘ground polished glass aspherical.’ Made of actual glass, the grinding and polishing process is time consuming and expensive and therefore these are only found on pro-level lenses. Canon use these large diameter elements for their ‘L’ range of lenses to offer superior definition as it aids the focus of the light from all angles.
The next level of elements are ‘molded glass aspherical’, or in Nikon terminology, Precision Glass Mold (PGM). The glass is heated to such an extent that it can be molded into an aspherical shape and set using a metal die or form. Nikon claims that the precision aspect of these lenses is evident through the fact that they measure each element in micons, or 1/1000 of a millimeter to you and me. This type of lens is less expensive to manufacture and will therefore be found in lenses aimed at advanced amateurs and enthusiasts.
The third of the most common element manufacturing techniques is a hybrid consisting of a glass lens with a plastic aspherical surface to form the shape. These lenses are susceptible to environmental changes such as humidity and temperature and are therefore not really suitable for professionals, instead being aimed at the consumer market.
You may be unaware of this, but ordinarily, lenses loose a lot of light due to surface reflection. In some cases, each element within the lens can loose around 5% of light, and as a result, the amount of light getting into a lens with 10 elements would be reduced by nearly 50%.
The lens coatings were developed in order to restrict light reflection and allow the light to pass through the lens, just like the coating on your sunglasses reflects certain light wavelengths, while allowing others to pass through to reach your eye.
Materials such as magnesium fluoride and silicon monoxide are used as coatings, with extremely thin layers applied across the surface, with each lens often requiring multiple layers in order to cut out reflections from the spectrum of different wavelengths of light.
The high-end Canon lenses, for example, feature over 10 layers of coatings which provide light transmission of 99.9% ranging from ultraviolet to near-infrared light.
Distortion and Aberration
In an ideal world, a lens would capture any straight line as perfectly straight. However, in reality, any lens that has a curved surface cannot converge parallel light rays at a single point and therefore they are distorted and curve. This curvature is a feature of any lens constructed with spherical elements, but will vary greatly depending on the lens and the focal length being used.
This distortion is most noticeable when working with parallel lines and subject matter at the edges of the frame, where the effect is maximised. Most zoom lenses will suffer from ‘barrel’ distortion at the wider end, in which there appears to be a bulge in the centre of the image.
They may also suffer from ‘pincushion’ distortion at the longer end, in which the opposite phenomena appears, with the centre of the image appearing to have been sucked in slightly. However, there’s usually a mid point in a zoom lens at which straight lines appear straight, which is certainly worth finding!
Distortion isn’t simply dependent upon the lens. It varies depending upon your proximity to the subject. For landscape and architectural photographers, lens distortion is a major issue, as they want their images to appear straight and in correct proportion, whereas portrait photographers aren’t often working with lines and therefore distortion isn’t as much of an issue.
Most major lens manufacturers now build lenses using aspherical lens elements, designed to restrict aberrations and distortion. In contrast to the spherical lens, the aspherical lens features a curved surface that is able to correct aberrations. It achieves this by allowing the light to pass through the element and meet at a single point, causing a single line of light to fall upon the sensor and therefore reducing the distortions caused by multiple beams traveling through the elements.
The image below contrasts two images that I recently took at a wedding reception, with the image on the left evidently struggling with lens flare and light distortion, whereas the image on the right produces a warm glow.
One of the main features that photographers look for in a lens is its maximum aperture, as this will dictate its potential for depth of field and ability to work in low light. The aperture, measured in f/stops, of a lens is set by the size of the pupil (aperture opening) of the lens, which is proportional to the square of the focal length of the lens.
For example, a 50mm lens may be able to reach an f-stop of f/1.2, but for a lens with a focal length of 100mm to reach f/1.2, it would require a pupil 4 times the size of the 50mm lens. So an f-stop isn’t dictated by a set pupil diameter, it’s relative to focal length.
You also need to take into account that the 50mm lens has a wider field of view and therefore may well let in more light. Large telephoto lenses compensate for this by having very large front elements, however, this also allows for more spherical aberrations, which requires more groups of elements to overcome and ensure the sharpest of images, and more glass means more expense.
In photographic terms, bokeh is the way the lens renders out-of-focus points of light. It is most visible in the small background highlights that often appear as circles of light. Each lens will offer a different bokeh depending upon its design. Bokeh is often misused as term to describe shallow depth of field, where a subject is in focus and the background is out of focus. Bokeh actually describes how out of focus areas look.
The ability of the lens to correct spherical aberration contributes to bokeh, as it will allow highlights to increase in size as they fall further out of focus with an even distribution of light across the disc. Professional standard lenses will have greater capacity to deal with light distortions through their combination of grouped elements.
However, it’s the technical construction of the aperture pupil that will most affect the bokeh of the lens. The largest contributing factor is the number of aperture blades, as this will allow for a smoother more rounded aperture, which produces bokeh more appealing to the eye.
Professional standard lenses typically have more blades and therefore produce a better bokeh, as portrayed in the image below, showing the bokeh produced by a Canon EF 50mm lens on the left, with the far smoother bokeh of a Canon L 24-105mm lens on the right.
Common Lens Designs
There are a number of lenses that originating from the Zeiss optical company at the turn of the century that shaped lens design for many years. You can still find these lenses being used today, the original designs have been modernized, but the optical construction of grouped elements remains largely the same.
The Planar lens was conceived by Paul Rudolph while working at Carl Zeiss in 1896. Its six element symmetrical design had an original aperture of f/4.5 and produced extremely sharp images, although it suffered with flare as a result of many air-to-glass surfaces, a problem which lens coatings now counter. The most highly regarded Planar lens is probably the f/2.0 110mm model. It was a popular choice for owners of the medium format Hasselblad 2000 and 200 series cameras.
The Tessar is another lens designed Paul Rudolph during his time at Zeiss optics. First revealed in 1902, the Tessar took its name from the Greek ‘téssera,’ meaning four, thanks to its design featuring four elements. With an original aperture of f/6.3, the Tessar was a compact lens that provided high optical performance at an affordable level. Many 50mm lenses are based off the Tessar design.
The Sonnar came slightly later, patented by Zeiss Ikon in 1929 having originally been designed by Dr. Ludwig Bertele. The first Sonnar was a 50mm lens featuring five elements designed for the Zeiss Contax I rangefinder. Its name was taken from the German word ‘Sonne,’ which means ‘sun,’ thanks to its wide aperture of f/1.5.
The Sonnar was able to counter the design flaws of the previous lenses, featuring much better contrast and less flare than the Planar and a much faster aperture and lower chormatic aberrations than the Tessar.
When it comes to the quality of the images that a lens can provide, the image stabilization (IS) or vibration reduction (VR) systems play an extremely important role, allowing you to take sharp photographs up to four shutter speeds slower than your standard handheld shooting speed.
Both Canon and Nikon lenses utilise extremely clever technology that uses motion sensors to detect unwanted movement that could cause blur in the image. That signal is then sent to a microcomputer that relays the information to a motor, which adjusts the IS or VR lens group accordingly, all of which happens in fractions of a second.
Rotating Front Elements
There are some lenses which have a front element that rotates, which isn’t an issue until you try and use certain filters such as a circular polarizer. The issue here being that once you’ve set the filter to its desired position, as soon as you change the focus, the polarizer setting is shifted, which can become extremely frustrating. There are solutions, such as purchasing a square filter mount. However, this is something worth considering when buying lenses if you're a filter user.
I’ve also found that certain lenses, even in the Canon ‘L’ range, have a tendency for the zoom to slip when held facing downwards. It’s just gravity having its way, but it can be annoying when shooting. If you rest the camera by your side and then bring it up to your eye only to find the lens is fully zoomed in.
Some lenses have a built in lock that restricts the movement of the zoom and holds it at the desired focal length, which solves the problem, but can also be a hindrance if you need to work quickly and alter your zoom without wanting to have to flick a switch every time.
It’s certainly true that the lens makes the most difference when it comes to image quality. For those looking to purchase a lens, aspects such as cost, maximum aperture and build quality will be the most important factors. However, it’s important to understand the physical construction of a lens and the materials within it to appreciate what you’re actually paying for.
Next time you’re using your camera, you’ll be able to take a minute to appreciate the physics of the light and incredible technical achievements that allow you to take the photographs that you capture.