Busting the Lens dictates Focus Speed Myth

Started Mar 22, 2013 | Discussions thread
(unknown member) Veteran Member • Posts: 5,590
Re: Busting the Lens dictates Focus Speed Myth

Sovern wrote:

Why is that I hear so much on this forum and other forums that more expensive lenses especially primes AF faster than cheaper lenses and the kit lens a long with other cheap but more than capable lenses are such a bad lens for AF speed?

I have one of Canon's fastest focusing lenses (Canon 85 1.8 USM) and the kit lens and they both AF at the speed speed no manner the lighting conditions.

I have also had the Tamon 17-50 2.8 and 50 1.8 in the past and have never felt a difference in AF speed between any of these lenses they all lock on focus very fast.

Is this "This lens has faster AF" a myth sort of like how people claim that their $2,000 prime is "sharper" than a $350 primes or zoom (myth in my opinion)?

From what I have read the AF focusing system built on the camera determines the AF speed and not the lens itself (besides cycling between MFD and infinity being a minor difference obviously).

Whats your take on it?

Personally I'm not convinced.

I believe from my own personal experience that the body determines AF speed as my 40D AF's faster than my xsi but all of my lenses AF just as fast/precisely no manner the lighting conditions from my own experience using them myself.

There are various factors that affect AF speed in a DSLR using phase detection:

  1. The focal length of the lens - long focus lenses generally focus slower than wide angles (see 5)
  2. The maximum aperture of the lens - a wider maximum aperture will let more light through for the AF system to do it's measurement. However a wider aperture also means that the depth of focus is narrower and this often means that precise focus takes longer to achieve.
  3. The type of AF motor used in the lens - you get what you pay for. The latest USMs are usually faster than cheaper micro motors.
  4. The mass of the lens elements that have to be moved for AF - this is the main reason why the 85mm f/1.2L is not as quick as the 85mm f/1.8 for example.
  5. The distance the lens elements have to be moved for any given focus point change. A lens like a 100mm macro can have several rotations of the focus ring to go from its closest focus distance to infinity. Focus limiters can help keep the focussing within a prescribed range and thus speed things up.
  6. When the lens / AF system was designed - later designs are often faster.
  7. The AF processor in the camera - later designs are often faster.
  8. The AF sensors themselves - later design are often more sensitive.

In many cases it can be hard to tell the difference between AF speeds under 'normal' conditions. The fact is that AF speeds are pretty quick and for users it's not easy to measure AF speeds accurately in any case.

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