Damaged Focusing Helicoid

In some old lenses or lenses which have taken a good knocking, the focusing helicoid may be worn or damaged. The problem may manifest as focus problems or noises when the lens is trying to focus.

To check, we must first start with a camera that is not known to have any auto-focus issues. Yes, the camera itself can be at fault. If your camera receives a hard enough knock to misalign the mirror, it may not focus properly with some lenses. If in doubt, try with an alternate camera.

First, try auto-focusing on various objects at different zoom lengths (if it's a zoom lens). This is important because a damaged focusing helicoid may be fully functional for most of the zoom range but will refuse to focus from a certain point onwards, resulting in a restricted auto-focus range. You should also try focusing on something far away, like the sky or a distant building.

If the lens cannot lock focus or back focuses at any point during your test, then there is a problem with the focusing helicoid. It could merely require some lubrication, or it could be worn out or damaged enough that you have to replace it.

You can consider buying the lens if the seller is willing to offer it to you at a firesale price. Replacing the focusing helicoid will set you back quite a bit, as it is a precision-machined part. But if all you needed to do was to get it lubricated, then you got lucky. Still, it's best to avoid such a lens if you can help it.

Damaged Lens Motor

This is a problem only for lenses that have an integrated electric motor to drive the focusing mechanism. This is less of a problem with newer lenses that use ultrasonic motors but they are more expensive to fix.

To test the lens motor, cover the lens with its lens cap. Then half-press the shutter release button. This will cause the auto-focus action to seek from the closest range to infinity. Make sure the auto-focus action is smooth and not "sticky" - it should not "jerk" or "hesitate" while it seeks from the closest focal point to infinity, and any whirring noise it makes should be smooth and even. Ideally, the manual focus ring should not move at all during the test.

If the lens does not appear to be focusing smoothly, or if the manual focus ring actually moves, then the lens motor may be suspect. Similarly, if the lens appears to focus normally but the noise generated by the motor sounds iffy (rough or uneven), you should think twice. Do note that older lenses use non-ultrasonic motors which are much louder, so don't avoid them just because they sound louder than newer lenses you are used to.

Fixing the lens motor is expensive. The repair job for the ultrasonic motor of a Nikon AF-S lens, for example, can cost anywhere from US$200-300, depending on what parts they need to change.

Malfunctioning Image Stabilizer

This obviously only applies to lenses that have built-in optical image stabilizer, like Nikon VR (Vibration Reduction) and Canon IS (Image Stabilization). Such a feature involves a complex of electromagnets and gyroscopic sensors to detect vibration and adjust the floating lens elements to eliminate or reduce camera shake.

To find out if the len's built-in image stabilizer is working, listen closely when you half-press the shutter release button and release it. You should hear a 'tick' during the press, and a 'tock' during the release. This is more obvious with the first generation image stabilizers but you can still feel the image stabilizer working if you pay attention.

The lens will work just fine, even if its image stabilizer fails. Optically and electrically, there's nothing to impede you from using the lens to take great photos. You will only lose the image stabilization assistance while holding the lens with your hands, which may result in less than ideal photos if lighting conditions are not favourable.

Repairing the image stabilizer isn't cheap because of the components it involves. Nikon, for example, charges US$200 to replace a malfunctioning image stabilizer.

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