Why does my Canon 500D kit lens struggle to autofocus at longer zoom settings?
Asked 5/19/2011
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2 answers
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I’m using a Canon 500D with the 18-55mm kit lens. Lately it autofocuses normally at the wide end, but at longer focal lengths it often won’t lock focus unless I zoom back out or recompose. What could cause autofocus to work at short focal lengths but struggle at higher zoom settings?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
4
If this is the f/3.5-5.6 lens, then as you zoom out the max aperture is reduced ( f/3.5 at 18mm, but f/5.6 at 55mm). Autofocus will struggle in lower light at the f/5.6 end.
You can try to find something high contrast to focus on, or use manual focus if you have to.
If you focus at 18mm then lock focus and zoom in, that won't necessarily still be in perfect focus at 55mm.
Originally by user4191. Source · Licensed CC BY-SA 4.0
user4191
15y ago
0
Generated from our catalog & community — verify before relying on it.
Two likely causes were mentioned.
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Normal AF limitation at the long end: On the 18-55mm kit lens, the maximum aperture gets smaller as you zoom in (about f/3.5 wide to f/5.6 at 55mm). That means less light reaches the AF system at longer focal lengths, so autofocus can struggle more in dim light or on low-contrast subjects. Try focusing on a high-contrast edge, using better light, or switching to manual focus if needed.
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Possible lens hardware fault: A common failure on some kit lenses is an internal autofocus ribbon cable beginning to break. Early symptoms can be autofocus failing only at certain zoom positions, then later failing more completely.
Also, if you focus at 18mm and then zoom to 55mm, focus may not remain exact, so that isn’t a reliable workaround.
If the problem happens mostly in low light, it may just be the lens’s AF limits. If it happens consistently at specific zoom settings even in good light, the lens may need repair or replacement.
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UniqueBot
AI15y ago
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