Why can two Nikon AF-D lenses focus at different speeds if they both use the camera’s screw-drive motor?
Asked 9/6/2013
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I’m comparing Nikon AF-D lenses, especially the 80-200mm AF-D push-pull version versus the later two-ring version. Since AF-D lenses use the camera body’s screw-drive motor, I’m confused about how one lens can focus faster than another when the focal length and minimum focus distance are similar. Is the difference mainly due to the lens design, and does the camera body also affect it? I’m using a Nikon D7000.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Yes, different lenses can definitely have different focus speeds. When focusing, a specific group of elements are typically moved.
The size and weight of a group of elements varies with a given lens formula. When focusing a small prime with only a few elements/groups, the AF motor isn't going to have to move much weight. Zoom lenses almost always have more complex groupings of elements, which undoubtedly makes them heavier and therefore takes more work to move. Additionally, the distance the elements need to move will effect how fast focus seems to be achieved.
Additionally, the focus motor's strength and speed varies with camera body. Simply, better cameras have better focus motors. Smaller and lighter lenses will gain no advantage from this, but larger and heavier lenses will.
Originally by user8473. Source · Licensed CC BY-SA 4.0
user8473
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. Even though Nikon AF-D lenses are driven by the camera body’s screw-drive motor, different lenses can still focus at different speeds.
The main reason is the lens’s internal optical/mechanical design. Autofocus works by moving a particular focusing group inside the lens, and that group can differ in:
- weight
- size
- distance it has to travel
- overall mechanical resistance
So two lenses with the same focal length and similar minimum focus distance can still have noticeably different AF speed if one has a heavier or more complex focusing group.
Camera body matters too. Different Nikon bodies have different AF motor strength and speed. A stronger body motor can drive heavier screw-drive lenses more effectively, while small/light lenses may show less difference.
So the claim that one 80-200 AF-D version focuses faster than another is completely plausible, even though both use the same body motor.
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