Af speed

[Nostromo]

Not sure if this is the right section but here goes.
This may seem a dumb question, but what determines the speed of af, is it the lens motor or the camera af abilities?
I understand about the light levels and contrast having an effect, but my question is more to do with the physical speed at which af is achieved.

 

[HoppyUK]

The lens only responds to instructions from the camera, but that's usually pretty much instantaneous with phase-detect AF on a DSLR. Phase-detect knows where the sharpest point is from the start, so it's the lens really.

Contrast-detect AF is different, eg mirrorless cameras, where there's constant communication between the camera and lens, checking and double-checking progress towards the sharpest point. Then it has to go past optimum focus to know exactly where it is, and doubles-back. So it's both camera and lens there, but different systems vary a lot.

 

[Nostromo]

Thanks for your reply.
It was just a question i was asking myself out of curiosity.

 

[PaulButler]

Further to Richards response, it also depends on which system when considering a DSLR lens. With Nikon for example some older lenses do not have an AF motor so rely on the in camera body af motor. A pro spec body will af faster than a consumer spec body using the in body motor.

 

[mpe]

It also greatly depends on design of the lens. Many older lenses simply move rear group of elements during focusing which is usually smaller and lighter.

Modern lens design use internal focus which often involves moving of heavier elements. That's why some modern lenses, despite having AF-S motor, actually focus slower than their predecessors.