Why do adapted DSLR lenses autofocus more slowly on a Sony A7 than native lenses?
Asked 4/11/2014
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I can adapt many Canon, Nikon, and other autofocus lenses to a Sony A7, but autofocus is usually much slower than with native Sony E-mount lenses of similar focal lengths. Why does this happen, even though the A7 has on-sensor phase-detect autofocus points? Is the slowdown caused by contrast-detect AF, the adapter, lens/body communication, or differences between on-sensor PDAF and dedicated DSLR PDAF systems?
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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With phase detect autofocus, it is possible for the camera to know more about how far out of focus an image is, so it can make a better guess of how far it has to move. If it knows how to get all the lens details, then it can quickly give instructions to move close to the target point. If it doesn't, then it has to more iteratively move the focus in to position until the proper spot is reached.
Additionally, PDAF sensors built in to normal imaging sensors tend to be a little bit slower and less precise than dedicated PDAF sensors due to their smaller size and more design compromises.
Both of these factors combine to make third party lenses much slower on a mirrorless body than on a first party DSLR with dedicated PDAF sensor.
Originally by user11392. Source · Licensed CC BY-SA 4.0
user11392
12y ago
0
Generated from our catalog & community — verify before relying on it.
Adapted lenses are usually slower because the camera, adapter, and lens are not designed as one native AF system.
Phase-detect AF can estimate both how far out of focus the lens is and which direction to move, so it can make a large, efficient focus move. Contrast-detect AF is more iterative: it often has to move the lens, measure, and try again, so it is inherently slower.
Even when the A7 uses on-sensor PDAF, it may still not perform like a native DSLR or native E-mount setup. On-sensor PDAF points are generally less capable than a dedicated DSLR PDAF module, and the camera may not know all the lens characteristics well enough to drive it optimally.
The adapter also adds a major limitation: it must translate electronic communication between different lens and camera protocols. That translation can add delay and may prevent the body from sending the most efficient focus commands. Many adapters don’t support AF well for exactly this reason.
So the slowdown is typically a combination of less-direct lens control, protocol translation in the adapter, and the difference between native dedicated PDAF systems and adapted/on-sensor AF operation.
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