New drives for autofocus lenses promise to be more accurate, faster and quieter than previously used. However, the advantages and disadvantages of the procedures used by the different manufacturers as well as their similarities remain often unclear.
From rotary to linear motion
In this article we explain the current drive concepts as well as their advantages and disadvantages. The second half is about linear and stepping motors that are currently being introduced to mirrorless system cameras.
DC motor
With the exception of linear motors, the principle of focusing a lens is that the rotary movement of the focus ring or motor is converted by a focusing unit into a linear movement of the lens elements to be displaced
Ultrasonic motor
Usually, a tube with control curves is used, which moves the lens elements back and forth when the tube is rotated. In the meantime, a tube with a surface is threaded is rare.
Conclusion
In an autofocus lens, a motor must produce the rotary motion, usually using DC motors or ultrasonic motors.
Some manufacturers have long installed the autofocus motors directly into the camera. The disadvantage is that the motor can not be adapted to the specific requirements of the lens. In addition, the coupling between the lens and the motor is a source of clearly audible noise.
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As a result of these disadvantages, all manufacturers of digital SLR cameras are now integrating the autofocus motor into the lens, but Pentax and Sony still have many lenses without a motor in the program.
A still widespread solution is the integration of a DC motor into the lens. In most cases, DC motors with an ironless core are used, which have a high efficiency and can accelerate quickly.
The main drawback of DC motors is that only at high speeds is sufficient power available to move the lens elements. A gear box must therefore be used to reduce the speed and increase the torque.
DC motors have some disadvantages
An alternative drive concept is ultrasonic motors, which can deliver a comparatively high torque at low speeds. Ultrasonic motors use piezoelectric elements that expand when an electrical voltage is applied. This deformation and friction creates a movement. In addition to ring, micro-motors with ultrasonic technology are also used.
The ring motor consists of two rings, a non-moving stator and a rotor with a similar diameter to the lens. The stator is vibrated by piezoelectric elements, as a result of which the tips of the stator describe an elliptical movement, which is transmitted to the rotor by friction. The frequency at which the stator oscillates is in the ultrasonic range and is therefore not audible.
Autofocus motors on mirrorless camera systems
The ring motor has many advantages over DC motors. Even at low speeds (5 rpm) a sufficient torque can be generated, so no gear is necessary. In addition, the motor is precisely controllable, and the movement of the lens elements can be started and stopped quickly. All in all, a high autofocus speed as well as a good fine adjustment with very low noise can be achieved.
In addition, the ring motor provides a simple mechanism that can be manually focused at any time. However, the control of a ring motor is relatively complex and, among other things, strongly temperature-dependent. Disadvantages of the annular ultrasonic motor are the comparatively high costs; in addition, a new motor would have to be designed for each lens diameter. For these reasons, only high-quality lenses are usually equipped with ring motors.
Stepper motors with thread drive
A variation of this principle is ultrasonic micro-motors, which are used in less expensive lenses. They are often smaller than comparable direct current motors and are integrated into the rear part of the lens.
While the ring motors of the manufacturers are very similar, the micromotors differ in some cases considerably. Canon's micro-USM, like the ring motor, is based on a traveling wave. Tamron's PZD technology, on the other hand, is driven by a standing wave.
Well designed ultrasonic micro motors are quieter than DC motors and offer comparable or better performance. In individual cases, this does not have to be the case, for example, if an under-sized ultrasonic micro motor is used because it is smaller than a direct current motor or can be marketed better.
Linear motor as direct drive
Advantages and disadvantages
List of abbreviations
In comparison to ring motors, micro motors offer poorer performance; in addition, it is usually not possible to manually focus at any time. Exceptions are Pentax SDM lenses and the Canon 1.4 / 50mm, where this functionality is realized by means of a complex differential gear.
Who is doing what?
If one considers which motors the individual manufacturers use, the result is a mixed picture. For some manufacturers, DC motors are a discontinued model, Nikon does not have lenses with a DC motor (AF-I) in the program. With Canon and Sigma, ultrasonic motors are clearly superior, DC motors are found here primarily in older lenses.
Buying advice Nikon system: The full-format cameras D700, D800 / E, D3x and D4
Pentax, on the other hand, has unveiled the only lens with an integrated DC motor (DC) at the end of 2010, and DC motors are used in Olympus Four Thirds lenses and some Sony lenses (SAM). In addition, Sony and Pentax still have many lenses in the program that use a DC motor within the camera.
In the case of ultrasonic motors, manufacturers often do not allow a distinction between the ring and the micromotor. Nikon (AF-S), Canon (USM) and Sigma (HSM) sell both ring and micromotors under the same designation. Pentax uses an ultrasound micromotor for lenses with the designation SDM, but manual focusing is possible at any time.
Tamron distinguishes between ring (USD) and micromotors (PZD). Olympus (SWD) and Sony (SSM) seem to use this designation exclusively for ring motors. Panasonic is the abbreviation for ultrasonic motors on the 4/3-lens XSM, for example the Leica 14-150.
Ring motors are the best solution from the approaches presented here, especially for large teleobjectives, in which comparatively heavy lens groups have to be moved. For smaller lenses, micromotor solutions can achieve a good performance. DC motors, on the other hand, seem to be an end-of-run model due to their inherent drawbacks, since the trend is becoming ever more quiet and the autofocus is as low as possible.
The DC and ultrasonic motors, which have been treated so far, are primarily used in the autofocus of mirror reflex systems. In the case of mirrorless camera systems, however, the manufacturers mainly use linear drives, which are implemented either by means of stepping motors or linear motors - this sounds strange at first, but is associated with the completely differently configured autofocus of mirrorless cameras.
Buying advice Sony system: The mirrorless NEX models
The autofocus in SLR cameras is based on the principle of phase detection and already records the degree of defocusing with the first measurement. It gives the AF engine a clear signal, which in the best case leads directly to the correct focusing.
Mirrorless system cameras, on the other hand, use the contrast autofocus system, which gradually adapts to the optimal setting: the system changes the focusing of the lens until the measured contrast is maximal. This means that the focus has to be adjusted quickly and precisely in small, precise steps.
Another aspect is the increasing importance of video capabilities. While the lens needs to focus as fast as possible, it is important for videos that the autofocus is as quiet as possible and that the focus is soft. In compact lenses for mirrorless systems, thirdly, the space for the autofocus motor is often limited.
The above-mentioned DC motors are hardly suitable for all three conditions. Ring-shaped ultrasonic motors should, in principle, meet all requirements but are not used in their SSA (Supersonic Actuator) lenses, with the exception of Samsung.
The main reason for this is probably the comparatively high costs. The majority of the manufacturers rely on linear drives and pursue two approaches: stepping motors that produce a linear movement by means of a thread drive and electromagnetic linear motors which directly produce a linear movement.
Stepper motors can deliver high torque even at low speeds, solving one of the main problems of DC motors. As before, however, a rotary motion is generated, which must be converted into a linear motion.
For this purpose, a threading drive is used. The principle: The motor drives a threaded rod on which a nut is moved. Stepping motors thus make it possible to operate at a low speed and consequently quietly. In addition, step motors do not require a position sensor for positioning tasks at low speeds and small loads. The engine responds to an input pulse with a defined step, so it is easy to count the steps of the motor.
However, there is the possibility that steps will be lost at higher speeds and strong accelerations. This is relevant for lenses with large travel distances (e.g., Panasonic 4.0-5.6 / 45-200 mm OIS), since the lens elements must be displaced at a higher speed in order to enable fast focusing. In order to prevent errors, an angle sensor is then installed on the motor.
The latest trend in autofocus motors are so-called voice coil motors. These linear motors produce a linear movement directly and can be easily integrated into a lens. For this purpose, a coil is attached to the lens group to be moved and guided through a magnetic field, and the magnetic field is generated by a permanent magnet. If current flows through the coil, the coil or the lens element is moved through the magnetic field by the Lorentz force.
This type of motor therefore does not require either a thread drive or a gearing, which almost eliminates all noise sources. Voice coil motors are characterized by a very good response and good controllability with respect to force and position. On the other hand, the travel distance is limited by the length of the magnet. In addition, the position control must always be active and hold the lens group in the desired position, while, for example, an ultrasound motor is holding its position when switched off.
Stepping motors represent a significant advance compared to conventional DC motors. They are considerably quieter and better suited for new applications. Linear motors are, together with ultrasonic ring motors, the quietest and fastest of the solutions presented.
The fields of application differ, at least at the moment, but very strongly. Ultrasonic ring motors play their advantages, especially in large, heavy telephoto lenses, which is still a domain of SLR cameras.
On the other hand, linear motors are designed for small travel distances and masses - this in turn fits many lenses of system cameras. The manual focusing is implemented electronically in both the stepping motor and the linear motor. For this purpose, the focus ring is provided with a sensor which detects rotations of the focus ring and transmits it to the control of the motor.
In principle, linear motors seem to be the more elegant and better approach compared to stepping motors. Whether the advantages can be proven in practice, however, can only be judged conclusively when more lenses with linear motors are available. In particular, the question arises as to how good linear motors are suitable for lenses with larger travel ranges.
At the moment stepping motors are more widespread than linear motors, which will certainly be used more frequently in the future. Almost all manufacturers have at least a lens with linear motor on offer. Olympus and Panasonic use both step and linear motors with Micro Four Thirds lenses.
The exact data on which motor is used can be taken from the data sheets. In the Nikon 1 system, step motors with the abbreviation STM (silent stepping motor) and linear motors with VCM (voice coil motor) are called. Sigma uses linear motors for its DN lenses. Samsung uses both linear motors (VCM) and ultrasonic motors (SSA). Sony and Tamron lenses for the NEX series use partial stepping motors. In the Sony SEL 1.8 / 50 mm, however, a linear motor adopts the focus and a stepping motor controls the aperture control.
S = stepping motor, L = linear motor, U = ultrasonic motor, D = direct current motor
Nikon STM: Silent Stepping Motor (S) Nikon VCM: Voice Coil Motor (L) Canon USM: Ultrasonic Motor (U) Olympus SWD: Supersonic Wave Drive (U) Olympus MSC: Movie & Still compatible (S, L) Panasonic XSM: Extra Silent Motor (U) Pentax SDM: Supersonic Direct Drive Motor (U) Pentax DC: Direct Current Motor (D) Samsung SSA: Supersonic Actuator (U) Sigma HSM: Hypersonic Motor (U) Sony SSM: Super Sonic Motor (U) Sony SAM: Smooth Autofocus Motor (D) Tamron PZD: Piezo Drive (U)
The high engine speed in combination with a gearbox leads to a clear noise development, especially focusing on lenses with heavy lens elements and large travel distances, the focus is rather slow, the autofocus has to be switched off before you can manually focus
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