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07-01-2012, 07:21 PM   #1
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What determines AF speed? body or lens

Just wondering because I see people saying the K5 is slow but other times people are saying a lens is slow to focus can it be both?

07-01-2012, 07:26 PM   #2
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Things on lens that affects AF speed:
1) Aperture - the faster the lens wide open, the more light reaches the sensor to focus.
2) Lens focus throw - the shorter the focus throw, the faster it is for the lens to focus.
3) Lens focal element weight - the heavier the focus element, the slower it is to focus.
4) Lens focusing mechanism - Some silent motor focus slowly, while others focus quickly. Screw drive focuses pretty quickly.

Things on camera that affects focusing:
1) Camera focus lock - If the camera is not good at locking focus, it can take a long time to achieve focus.

That's about it I think - in general, it's the lens. The focus accuracy of a camera body is more important than focus speed in general. Speed is usually within a fraction of a second between different bodies/brands.
07-01-2012, 11:10 PM   #3
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QuoteOriginally posted by JinDesu Quote
Things on lens that affects AF speed:
1) Aperture - the faster the lens wide open, the more light reaches the sensor to focus.
2) Lens focus throw - the shorter the focus throw, the faster it is for the lens to focus.
3) Lens focal element weight - the heavier the focus element, the slower it is to focus.
4) Lens focusing mechanism - Some silent motor focus slowly, while others focus quickly. Screw drive focuses pretty quickly.

Things on camera that affects focusing:
1) Camera focus lock - If the camera is not good at locking focus, it can take a long time to achieve focus.

That's about it I think - in general, it's the lens. The focus accuracy of a camera body is more important than focus speed in general. Speed is usually within a fraction of a second between different bodies/brands.
Camera's Focus motor for screw drive?
07-02-2012, 12:16 AM   #4
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QuoteOriginally posted by Chaos_Realm Quote
Camera's Focus motor for screw drive?
Indeed. Weak or Damaged AF Motors will drastically affect AF Speeds.

07-02-2012, 12:31 AM   #5
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K5 is fast when you use Sigma HSM or some screw drive lenses. Pentax SDM sucks,.
07-02-2012, 01:48 AM   #6
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Speed depends on both, camera's algorithm is probably the most important single factor, then there are physical factors, focus motor torque and the mass it moves. The latter depends on the lens, the former on the body for lenses that don't have their own.
07-08-2012, 03:10 AM   #7
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Last edited by beholder3; 08-11-2013 at 07:23 AM. Reason: [deleted]
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