auto focus speed

ericande · Jan 31, 2007

What makes a camera/lens focus quickly? I've noticed that some lenses focus faster then others and some cameras focus more quickly then others. It also seems that a lot of the f/2.8 lens focus more quickly then slower lenses. Is this just because they are generally more expensive lenses and built better or a direct correlation to wider aperture? Which is more likely to focus quicker, my D50 with a fantastic lens or a D200 with a cheap kit lens? Are there more variables in play here?

shingfan · Jan 31, 2007

i think it is the aperature on the lense........lager aperature = faster focus

(aside from mechanical construction and processing speed)

Big Mike · Jan 31, 2007

There are several factors.

The brain and the sensors for the AF are in the camera...so a better camera will help. Faster (large aperture) lenses let in more light when you are focusing...more light makes it easier for the camera to focus. Some cameras, like my 20D, have an AF sensor that is more sensitive when used with lenses that have a max aperture of F2.8 or faster.

The focus motor is in the lens...so a better/faster focus motor will help. The size and weight of the lens elements can also make a difference. Some motors are purely mechanical...faster lenses use ultra sonic technology. And yes, lenses with a max aperture of F2.8 are higher end lenses...and are often combined with better focus motors etc.

ericande · Jan 31, 2007

Ahhh, makes sense. I also was just reading that if your lens is smaller then f/5.4 your autofocus speed will start to suffer. I get it...

edit: Almost forgot. Nikon uses a screw from the body to lens to focus whereas Canon uses motors within the lens right? Is one system generally better then the other? I've heard between the D50 and XT, the XT focuses more quickly but the D50 more accurately. Is this a result of their systems or just the way they are programmed to work?

Big Mike · Jan 31, 2007

I would say that there are too many variables to make this an issue. Both are modern cameras with good AF...but neither are top of the line either.

If you are choosing between the two...I wouldn't make the AF system a big part of that decision.

JIP · Jan 31, 2007

On top of other things that come on upper level bodies along with high resolution sensors and durbility etc. is faster focusing. As far as the Nikon vs. Canon systems they are pretty much the same as far as whose is fastest Nikon has Silent wave Canon has USM.

fmw · Feb 1, 2007

I wonder why it matters. They all focus faster than manual focus did in the pre AF days.

In the Nikon world, the lenses with built-in focusing motors are faster. I think they are called AF-S. They are also quieter, hence the name Silent Wave. Some of the older ones were actually slower than manual focus. My old 300 f4 is that way. Sometimes it just hunts around and never settles on anything.