AF speed difference between 1.8 and 2.8 lenses

Started Jul 6, 2012 | Discussions thread
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mg428
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AF speed difference between 1.8 and 2.8 lenses
Jul 6, 2012

Please correct me if I am wrong:

1) One one hand, perhaps the biggest factor affecting AF speed and accuracy is the amount of light that can fill into the lens, meaning that the larger the aperture, i.e. the lower the f number, the more light will get in, assisting the AF system to focus faster and more accurately. This is true for regardless of whether you stop down or not, because the camera will always gather most light per the largest aperture for focusing purposes.

2) One the other hand, for very fast lenses such as f/1.4, even though lots of light passes through the lens, due to the extreme shallow depth of field, these lenses generally focus slower than 2.8 lenses.

Therefore, when making a decision for buying a fast prime, this is one of the factors I consider between 1.4 and 1.8. (You may comment seperately for this statement if you find it frivilous)

Anyhow, I am trying to evaluate the above pieces of information along with 2 statements of Ken Rockwell (PLEASE DO NOT LEAVE THE THREAD JUST AS SOON AS READING HIS NAME. IF I HAD RELIED UPON HIM, I WOULD NOT BE ASKING FOR YOUR HELP!) which are as follows:

  • "As with all AF lenses, it won't focus in dimmer light or any faster than slower (f/2.8 or f/4) AF lenses; Nikon's AF system doesn't care about lens speed." (taken from his 28mm 1.8G review)

  • "SLR AF systems have never used the full speed of lenses; they look through anulii equivalent to about f/8 regardless of lens speed." (taken from his D4 review, and I don't have an idea about what "anulii" means)

I will appreciate if you can educate me on the matter.

Nikon D4
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