- VCM typically draws around 250mW continuously during focusing. Although new designs claim to reduce this drain to around 80mW, this is still too high to permit the VCM to be used for anything other than momentary operation during photography. Attempting to capture a video clip with auto-focus operating the entire time would rapidly drain the handset battery. Having such high power dissipation in close proximity to the image sensor can also degrade image quality.
- The power requirements of a VCM actually increase as the mechanism gets smaller. This effectively places a limit on the minimum size of an auto-focus camera module. The problem is that as the electro- and permanent magnets get smaller, the available magnetic force decreases. For the electromagnet, the field is dictated by the Ampere-turns, while for the permanent magnet it is the volume. Higher power magnets cannot be used due to their cost, so the only method to compensate for a reduction in dimensions is to increase the amperes through the coil, which is highly undesirable due to battery power drain and heating of the image sensor.
- Another problem with auto-focus mechanisms based on VCMs is speed of actuation. More accurately, the problem is time of settling. A half-stroke jump of 125ìm may only take the actuator a few milliseconds to achieve, but it then bounces about the new set position as the oscillatory magnetic and spring forces diminish to balance. It can easily take 50ms before the oscillations reduce to less than 5ìm and the lens position is considered stable enough for image capture.
- Lens tilt:
Tilt of VCMs can be controlled. Some of the options are a longer barrel, more springs, larger springs and more complex springs. As might be expected, all of these in some way impact the power consumption, size or cost so the scope for their implementation is limited.
