BACK TO BASICS
MORE ABOUT LENSES
Following our introductory article back in the September/October 2017 issue, we’re delving a bit deeper into the design of accessory lenses to look at the latest technologies which are helping enhance optical performance and extend capabilities.
There has never been a more exciting time in lens design, with a greater choice of brands and models than ever before. Much of this is being driven by the mirrorless camera which, due to the shorter flange back distance compared to an SLR, allows for greater flexibility in optical designs. Meanwhile the development of new technologies is enabling specifications and capabilities that could only be dreamt about a decade or so ago.
Developments in design processes, more advanced materials (particularly engineering plastics of various types) and manufacturing techniques have enabled many of the technical challenges which were restrictive in the past to be overcome. These have allowed for more compact and lightweight lenses without compromising performance, either optically or mechanically. Additionally, more ‘exotic’ designs – in terms of the focal length, zooming range, lens speed or close-up capabilities – can now be achieved… and, importantly, at affordable prices.
So let’s examine some key design components in a modern camera lens which contribute to the ever improving capabilities, handling and performance. When light hits the surface of a lens element some of it is transmitted and some of it is reflected… or it would be if it wasn’t for special multi-layered coatings. Not only would the reflected light – between four to ten percent at each lens surface – be lost to the exposure, it would also bounce around inside the lens, creating ghosting and flare which compromises both colour and contrast. Multi-layer coatings – better known simply as multicoatings – are designed to ensure the widest possible spectrum of light wavelengths are passed through the lens elements.
As the name suggests, these are multiple coatings – applied via a vacuum deposition process so they’re incredibly thin – comprising
various layers each with a different refractive index, optimised to complement the element’s refractive index. The refractive index of an optical material refers to its efficiency at bending – or refracting – light. The higher the refractive index, the more efficient it is at transmitting light.
Internal reflections within a lens became more of an issue with digital cameras as sensor surfaces are highly reflective and this has demanded the development of more effective coatings. Likewise modern zooms which use a large number of elements. A lot of work has gone into devising ultrathin coatings – at the nanometer level – which are also formulated to the design of a particular lens. The number and type of coatings applied to each element’s surfaces is individually calculated to match the lens type and the glass used.
Optical Image Stabilisation
Blur caused by camera shake will ruin what might have otherwise been a great photograph. Sometimes it’s not feasible or desirable to use a tripod to steady the camera, and this is where optical image stabilisation in a lens proves its worth. The lens makers all have different names for their optical image stabilisation systems, but all essentially work along the same lines. (For the record, Canon introduced the first IS-equipped accessory lens in a 75300mm telezoom, in 1995.)
Tiny gyros within the lens, known as angular velocity sensors, detect the small movements associated with camera shake. This information is fed to a microprocessor which translates it into drive commands for the image stabiliser itself. This comprises a small group of lens elements that can be moved in any direction around the lens’s optical axis to counter camera shake. It all happens very rapidly and, with steadily more powerful microprocessors and micromotors, can now provide up to four or five stops of correction for camera shake.
The rule-of-thumb for the ‘safe’ hand-holding of a lens is that the slowest shutter speed usable equates to 1/focal length… so, for example, this would be 1/200 second with a 200mm lens. Image stabilisation enables you to use slower speeds and, with four stops of correction, you could feasibly go as slow as 1/15 second. In reality, it’s advisable to experiment as there are other factors involved, such as the physical challenges of holding a bigger and heavier lens absolutely steady. But image stabilisation undoubtedly provides extra leeway. It’s worth noting here that if you’re using a camera with an ‘APS-C’ or Micro Four Thirds size sensor, the 1/focal length rule is still based on the effective (i.e. 35mm equivalent) focal length.
A growing number of mirrorless camera systems are combining lens-based optical stabilisation with camera-based sensor-shift stabilisation for even more effective correction of camera shake. The basic principle of the latter is the same, but instead the sensor is shifted around the centre of the image to provide the correction. Panasonic claims its latest ‘Dual I.S.’ combined stabilisation extends the correction for camera shake up to 6.5 stops. It employs a combination of a gyro, accelerometer and data from the image sensor to more accurately determine the direction of movement. The extended correction range effectively enables hand-held shooting with shutter speeds as slow as one second, and effective focal lengths up to 280mm.
Optical image stabilisation first appeared in longer focal length lenses, which made sense given their higher magnification power also amplifies any movements. It’s now becoming increasingly common in wider-angle lenses, extending the hand-held shooting possibilities in low-light conditions. The latest stabilisers can also automatically detect when the camera is mounting on a tripod, recognising the action of panning and disabling the correction for movement in that direction. This eliminates the need to manually switch between IS modes.
The benefits of image stabilisation aren’t limited to enabling the use of slower shutter speeds when shooting hand-held. Alternatively, it allows for the selection of a smaller aperture – for example, f8.0 rather than f2.0 – which, in turn, provides greater depth-of-field. There is also the possibility of shooting at lower ISO settings to optimise image quality.
Extra Low Dispersion (ED) Optical Glass
Visible light is made up of the different colours which all have their own wavelength within the visible spectrum. All these wavelengths bend at slightly different angles when they pass through a lens element, which means they become dispersed… in other words, they do not converge at the same point. You can see dispersion at work when a beam of white light is passed through a prism and subsequently splits into a rainbow effect.
In photography dispersion is a problem as it creates chromatic aberrations which manifest themselves as colour fringing along high contrast edges.
Chromatic aberrations become harder to correct as the lens focal length increases.
Historically, Nikon was the first lens manufacturer to devise special formulations which created low-dispersion characteristics for its optical glass. Oxides of various rare earth elements are used in the glass to minimise the differences in the refraction of the colour wavelengths. Subsequent refinements have produced extralow dispersion (ED) glass which is also called ultra-low dispersion (UD) glass by some lens makers (Canon, for example) and superlow dispersion by others. These formulations have to be very precisely calculated so ED/UD glass is quite expensive to make.
It is, however, nowhere near as costly as creating fluorite elements. These have the lowest inherent dispersion characteristics of any optical material, but they have to be painstakingly grown from synthetic fluorite crystals. A number of very high performance telephoto lenses employ fluorite elements – they also have a very low refractive index – but these are very expensive indeed. Many lens makers have tried to come up with alternatives, such as Sigma’s ‘F Low Dispersion’ (FLD) glass – which was co-developed with Hoya – claimed to have very similar transmission and dispersion characteristics to fluorite, but much cheaper to manufacture.
It’s common to see the term APO in a lens’s model number and this is short for apochromatic (or apochromat) which refers to any optical design incorporating special elements to correct for chromatic aberrations by focusing the red, green and blue wavelengths at the same point. APO lenses also correct for spherical aberrations – where the light rays passing through the edges of an element converge at a different focal point to those passing through the centre, causing a loss of sharpness particularly towards the edges of the frame – primarily by employing aspheric (or aspherical) elements. Aspherical Lens Elements As already explained, a lens element works by refracting – or bending – the light rays that pass through it, thanks to its spherical surfaces. Put a number of elements together and they will bring these rays together at a point… which focuses the subject. However, the refraction process isn’t perfect and various lens aberrations – such as distortion – are introduced along the way.
Originally, this was corrected by adding more elements, but obviously this made for big, bulky and expensive lenses… and adding more elements created the potential for more performance issues. The breakthrough came with non-spherical – or aspherical – surfaces, which essentially could be shaped to provide ‘built-in’ correction for spherical aberrations in large-aperture lenses, and distortion in wide-angle lenses.
An aspherical lens element corrects for distortion and aberrations by continually changing the refractive index from the centre of the element (i.e. its optical axis) to the edges via nonspherical shaping of its surfaces.
The first aspherical elements were created using the timeconsuming process of selective grinding and polishing to create the more complex surface shapes. More recently, the refining of glass moulding techniques (as well as computer-aided design) has made it easier – and much less expensive – to produce aspherical elements. Another production technique creates what’s called a hybrid aspherical element; this involves coating a spherical core with an optical resin to shape the surface. Lenses which incorporate aspherical elements sometimes have the designation ASPH in their model numbers. Ultrasonic Autofocusing Drives Autofocusing needs to be responsive and fast, which places considerable demands on the mechanical side of the process… i.e. actually moving the group of elements that do the focusing within the lens. Conventional electric micromotors simply don’t get started quickly enough, so a commonly-used alternative is a drive system which employs ultrasonic pulses.
This concept was pioneered by Canon with its USM (Ultra Sonic Motor) ring-type drive, but now everybody uses the same arrangement, albeit under a myriad of different names – Silent Wave Motor (SWM, Nikon), Super Sonic wave Motor (SSM, Sony), Hyper Sonic Motor (HSM, Sigma), Supersonic Wave Drive (SWD, Olympus), Ultrasonic Silent Drive (USD, Tamron), and Supersonic Drive Motor (SDM, Pentax).
Ultrasonic AF drives enable a near-instant response followed by high-speed operation to enhance accuracy. Additionally, because these systems use ultrasonic sound pulses that are beyond our hearing range, autofocusing operation is also extremely quiet.
Modern lens designs employ internal focusing (IF) which means the focusing group (or groups) is located in the middle of the lens in front of the diaphragm and move independently of all the other groups. The key benefits are that the lens’s length doesn’t change during focusing, the front element doesn’t rotate (important when using orientation-sensitive filters such as polarisers or grads), the focusing group is lighter, thereby enabling faster autofocusing and, in most cases, the whole optical design is more compact. Some lenses, particularly zooms, employ a rear focusing arrangement where the focusing group is located behind the diaphragm, but the benefits are the same as for inner focusing.
Telephoto lenses often have provisions for limiting the focusing range for faster AF operation when the subject distance doesn’t change significantly, and for memorising a focusing point which can be instantly reset at the push of a button. Weather Protection Curiously, while weather-proofed camera bodies have been around for a while, similarly protected lenses have only started appearing comparatively recently. These designs employ seals at the various barrel tube junctions and a rubber gasket around the lens mount. In some cases, a special fluorine coating is used on the exposed surface of the front element (and sometimes the rear element as well) to help repel moisture and also to allow for easier cleaning.
The degree of weather sealing provided can vary from brand to brand and across models, with the precise degree of protection often hard to pin down. It ranges from being essentially only splashproof through to the capacity to withstand longer-term exposure to constant rain or heavy sea spray. Additionally, not all so-called
“the degree of weather sealing provided can vary from brand to brand and across models with the precise degree of protection often hard to pin down.”
weather-protected lenses allow for operation in sub-zero temperatures (which, amongst other things, affects the lubricants) so there could be issues when shooting, particularly for prolonged periods, in the snow or in icy conditions. Microprocessors Today’s lenses are as much electronic devices as they are optical ones, given so much of what they do is handled by one or more high-speed microprocessors.
As a basic level, the microprocessor delivers information about the lens (focal length/range, aperture range and the minimum focusing distance) back to the camera body. This determines AF operation, exposure control and, increasingly, in-camera corrections for lens aberrations, including distortion and vignetting. Most digital cameras, mirrorless or D-SLR, are performing some level of lens correction on-the-fly at the point of capture, in addition to allowing for manual selection of certain functions.
In-lens processors also control the AF drive and handle an image stabiliser’s operation.
More recently, some lens makers – most notably Sigma and Tamron – are offering the facility to customise these operations via a USB interface or dock and dedicated software. Sigma’s USB Dock is essentially a lens mount adapter which enables connection to a computer via a USB cable. Once this is done, the Sigma Optimisation Pro software allows for firmware upgrades as well as adjustments to the autofocusing speed and distance limiter range, corrections for front/back focusing, changes to the image stabilisation as monitored in the viewfinder, and the ability to set the custom operating modes for the lenses which have this capability (and which can be tailored to specific subjects or situations).