Why can many lenses focus past the infinity mark?
Asked 8/12/2010
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On several lenses, the focus ring keeps turning a little past the infinity mark. This seems inconvenient when manually focusing at night or on distant subjects. Why is there extra travel beyond infinity instead of a hard stop exactly at the infinity position?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
16y ago
2 Answers
29
The most obvious reason is to be sure that you can reach the spot where it focuses at infinity. It would be hard to make the lens stop at exactly infinity, and any little change (temperature, humidity, filters, et.c.) might move that point slightly, making it impossible to focus exactly at infinity.
On a prime lens you would need only a small margin, so they can be adjusted to stop just slightly beyond infinity. On a zoom lens the focus varies somewhat depending on the focal length, so it needs a lot more margin.
Originally by user149. Source · Licensed CC BY-SA 4.0
user149
16y ago
0
Generated from our catalog & community — verify before relying on it.
Because the true infinity focus point is not fixed exactly the same in every situation. Lens makers leave some travel past the infinity mark so you can always reach precise infinity focus.
Reasons include:
- manufacturing tolerances in the lens and camera body
- temperature changes and thermal expansion/contraction
- differences across zoom settings
- filters or different wavelengths of light, including infrared
- autofocus operation, so the focus system can overshoot slightly without hitting a hard mechanical stop and causing wear
On some older manual-focus lenses, infinity may be set as a hard stop, but many modern AF lenses allow extra movement for reliability and flexibility. In practice, the infinity mark is a guide, not always the exact focus point under all conditions.
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