Unlike the shape of a regular spherical lens which maintains a constant curvature, an aspherical lens has a shape with a varying curvature.
If one tries to correct for aberrations such as distortion and spherical aberration only using spherical lenses, multiple types of spherical lenses need to be combined in multiple layers. An aspherical lens controls the refraction of the incident light rays by varying the curvature over the surface of the lens, allowing it to product corrective effects similar to multiple spherical lenses with just a single lens. Aspherical lenses have been actively introduced since the development of the 28-200mm F/3.8-5.6 (Model 71D), Tamron's first-generation all-in-one zoom lens. By employing aspherical lenses, Tamron has succeeded in removing aberrations and reducing the number of lens elements.
One type of aspherical lens known as a compound aspherical lens is formed by pressing a special resin onto the a base material glass lens to form it into an aspherical shape, but at one time technical issues made it difficult to mass produce such lenses. By the time development of the Model 71D had started, Tamron had already accumulated a wealth of expertise in the fabrication of compound aspherical surfaces and succeeded in mass producing aspherical lenses ahead of its competitors in the industry. Today, technologies for glass-molded aspherical lenses in which heat-softened glass materials are formed using a mold created with ultra-high precision machining technologies have also progressed, and Tamron seeks to optimize lens structures with the use of glass materials that utilize their respective characteristics.
XR (Extra Refractive Index) Glass & UXR (Ultra-Extra Refractive Index) Glass
XR glass use glass materials with a higher refractive index than regular optical lenses, while UXR glass use glass materials that boast an even higher refractive index than XR glass. By placing XR or UXR glass elements in the front group of a lens, overall optical length can be kept shorter.
The structure of a regular lens is designed to control aberrations by placing glass with a low refractive index in the front group, but for its all-in-one zoom lenses, Tamron employed a revolutionary lens structure, taking the bold step of placing high refraction lenses in the front group and correcting for aberrations in the rear group. In doing so, it succeeded in improved image quality while further reducing overall lens size.
In fact, Tamron succeeded in making the 28-200mm F/3.8-5.6 LD XR (Model A03), the first lens to use XR glass elements, significantly more compact in terms of length, weight and filter diameter compared with its predecessor the 28-200mm F/3.8-5.6 LD (Model 371D). With the use of UXR glass elements in the latest model, the 28-300mm F/3.5-6.3 Di VC PZD (Model A010), the design has been made even more compact. XR and UXR glass serve as a key technology giving Tamron's all-in-one zoom lenses smaller sizes and higher image quality.
LD (Low Dispersion) Lenses & XLD (Extra Low Dispersion) Lenses
An LD lens is made from specialized glass materials capable of effectively removing or correcting for chromatic aberration using properties that produce extremely low light dispersion due to refraction. An XLD lens is even less dispersive than an LD lens, exhibiting properties close to those of fluorite lenses.
Chromatic aberration that degrades image sharpness in the form of color fringing is caused by a phenomenon where dispersion occurs through the same action as a prism when incident light refracts in a lens (spectral phenomenon where white light splits into its seven constituent colors). There are two main types of chromatic aberration. Axial chromatic aberration tends to occur in lenses with a long focal length, while wide-angle lenses with a short focal length are susceptible to chromatic aberrations of magnification. In the design of an all-in-one lens that includes a wide range of focal lengths from wide angle to telephoto, the utilization of LD lenses to reduce chromatic aberrations are important for improving image quality.
Tamron was among the first to introduce LD glass elements starting with the 28-200mm F/3.8-5.6 LD (Model 171D), its second-generation all-in-one zoom lens, delivering sharp image quality with high contrast.
AD (Anomalous Dispersion) Lenses
As with an LD lens, an AD lens is made from a specialized glass material that is effective at removing or reducing chromatic aberrations. As the partial dispersion ratio (the percentage of a range with a specific dispersiveness) of wavelength ranges within the visible spectrum is abnormally large, in combination with other optical glass an AD lens is able to reduce chromatic aberrations by controlling dispersiveness at specific wavelength ranges.
BBAR (Broad-Band Anti-Reflection) Coating
BBAR coating is a proprietary multiple-layer coating technique developed by Tamron to prevent the reflection of incident light on the lens surface as well as flare and ghosting to attain the best possible color balance.
To produce anti-reflection effects across a wide range of wavelengths, Tamron applied BBAR coatings to many of its lenses from early on. Later during the 1990s, Tamron developed a new BBAR coating to respond to the improved image quality provided by film and advances in camera performance. To cater to digital cameras, which suffer from more severe ghost and flaring due to reflection of the image sensor itself, Tamron developed a further evolved version of the coating that increases light transmission across a wider range at both longer and shorter wavelengths. Even more stringent steps have been taken with regard to the latest BBAR coating and these advances have been employed in newly designed lenses.
High-Precision Multiple-Cam Mechanism
The high-precision multiple-cam mechanism extends the lens barrel from the lens chassis in two steps to enable reliable zooming from wide angles to the telephoto end. When the focal length is changed by zooming, on the outside the lens appears to be extended straight out in two stages, but inside the lens barrel the lens groups that move back and forth in a complex fashion in concert with the changing focal length are controlled by the multiple-cam mechanism.
While the high-precision multiple-cam mechanism was developed with proprietary technologies at the early stages of all-in-one zooms, as all-in-one zooms gained acceptance and the calls for expanded magnification ranges grew louder, the cam mechanism needed are more precise construction and greater precision to lengthen the amount of zoom extension. With the 28-300mm F/3.5-6.3 LD (Model 185D), which expanded the focal length at the telephoto end to 300mm, Tamron developed the previously used Triple-Cam Zoom with a three-layer construction into a Quad-Cam Zoom with a four-layer construction and succeeded in fitting a long lens barrel into a compact lens chassis.
Combining the sophisticated process of accurately maintaining the lens barrel while containing the high-power zoom mechanism within this extremely complex mechanism and the ability to perform smooth and highly-stable zoom control are also features of the design. As a pioneer of all-in-one zoom lenses, Tamron takes pride in this sophisticated zoom control mechanism.
Internal Focusing System
With an internal focusing system, the focusing lenses are positioned from the middle to the rear of the overall lens structure.
In conventional zoom lenses, it is standard for the front lens group to extend when focusing, but this method has the drawback of producing a longer working distance. Through the revolutionary technique of employing an internal focusing system offering a favorable working distance in the 28-200mm F/3.8-5.6 LD (Model 171D), its second-generation all-in-one zoom, Tamron succeeded in shortening the working distance of 2.1m in the first-generation 28-200mm F/3.8-5.6 (Model 71D) down to 52cm (at a focal length of 135mm). Tamron has continued to make improvements thereafter, reducing the working distance down to 49cm across the entire zoom range with the release of the 28-200mm F/3.8-5.6 LD (Model 371D) in 2000, and attaining a working distance of 39cm across the entire zoom range with the latest 16-300mm F/3.5-6.3 Di II VC PZD MACRO (Model B016).
By combining the internal focus system with the aforementioned high-precision multiple-cam mechanism through the Integrated Focus Cam technology, Tamron was first able to incorporate an all-in-one zoom with a complex zoom mechanism. As well as featuring shorter working distances, the internal focusing system also speeds up autofocusing and is effective at controlling the image quality degradation associated with focusing, making it one of the indispensable technologies underlying improved image quality in all-in-one zooms.
VC: Vibration Compensation
VC is Tamron's proprietary optical vibration compensation mechanism, developed to reduce image disturbances caused by camera shake.
VC employs a three-coil system where three drive coils electromagnetically drive the optical system that performs correction (VC lens) through three ceramic or steel balls. Correction performance features excellent stability and enables smooth movement with minimal frictional resistance. Moreover, algorithms are optimized to a high level for each lens, ensuring the maximum corrective performance is produced at all times and attaining stable viewfinder images with the ability to track subjects with accuracy.
When VC was first equipped in the 28-300mm F/3.8-6.3 XR Di VC (Model A20), a moving magnet system that involved placing a heavy magnetic on the side of the corrective optical system was employed, but this system made it difficult to reduce the size of VC structurally, which unavoidably resulted in larger lens bodies. Given this, Tamron made improvements by switching the positions of the magnet and coil and placing a light-weight coil on the side of the corrective optical system, successfully making the system much smaller and lighter. VC with the moving coil system was employed in the 18-270mm F/3.5-6.3 Di II VC PZD (Model B008), making it the world's smallest and lightest 15x all-in-one zoom lens. This contributed significantly to more compact VC models of all-in-one zoom lenses.
PZD (Piezo Drive): Standing Wave Ultrasonic Motor
Tamron was the first in the world to successfully apply this type of ultrasonic motor with its characteristically compact mechanism as a lens drive system.
By applying high-frequency voltage to the rectangular piezo (piezoelectric ceramic) element, the entire element repeatedly expands, contracts and flexes to generate a swiveling vibration (standing wave motion). This causes the metal tip set at the end of the element to rotate elliptically, causing the rotor at the tip's contact point to rotate and provide the AF driving force.
PZD (Piezo Drive): Standing Wave Ultrasonic Motor
PZD was first equipped in the 18-270mm F/3.5-6.3 Di II VC PZD (Model B008), which successfully achieved a 24% size reduction in terms of volume over its predecessor, the 18-270mm F/3.5-6.3 Di II VC LD (Model B003).PZD provides excellent speed and quietness compared with the DC motor used in conventional all-in-one zooms and enables smoother and more comfortable focusing performance. Moreover, in the 16-300mm F/3.5-6.3 Di II VC PZD MACRO (Model B016) and 28-300mm F/3.5-6.3 Di VC PZD (Model A010), the latest models to incorporate PZD, Tamron has enabled full-time manual focus, which allows fine focus adjustments during autofocus without having to switch to manual focus. In this way, Tamron continues to deliver even greater focusing convenience in its all-in-one zoom lenses.
The stepping motor is used to drive autofocus in comparatively smaller lenses and allows extremely precise control. The motor performs one step of rotational movement for each electrical signal (pulse), moving the focusing lens into the designated position quietly and accurately in a single operation. As its simple construction makes it effective for making lens sizes more compact and AF drive sounds are not easily picked up as noise during video recording, the stepping motor is employed in the 18-200mm F/3.5-6.3 Di III VC (Model B011) and 14-150mm F/3.5-5.8 Di III VC (Model C001) all-in-one zoom lenses for mirrorless interchangeable-lens cameras.
The DC motor is a technology that enjoys widespread use in everyday life outside its application in AF drives, and utilizes the drive principle of moving a rotor using the forces of magnetic attraction and repulsion. While the newly developed PZD and stepping motor systems enjoy an advantage in terms of drive speed and quietness, as DC motors have practical benefits backed up by a long track record, such as simple rotation control, their ability to easily attain the torque necessary to drive autofocus and their superb cost performance, they have long been employed in many all-in-one zoom lenses.