AF faster?

[mxracer32]

i only have experience with my two sigma lenses (24-70 2.8 and 70-300 f4) and my all manual 8mm peleng all used on my 20d. when i was in best buy looking around for a new TV i went over to the cameras and i played with a 40d they had with a canon 28-135 lens. the autofocus on this is incrediably fast when comapred to my sigma lenses. is it normal for canon brand lens to be much faster? i did a quick search of the site to see if anyone has asked this question before but couldnt come up with anything.

 

[Josh66]

I haven't used any non-Canon lenses to compare, but it really varies a lot from lens to lens.

Even one lens can vary in it's AF speed a lot. The Canon 100mm macro is lightning fast if it's already relatively close to the right focus, but if it's off by too much it hunts forever.

 

[musicaleCA]

The camera body's AF sensor also plays a role in how quickly it can autofocus. Do your Sigma lenses have a feature called "HSM"? If they don't, yes, Sigmas tend to be slower than their Canon equivalents (or so I've heard). But HSM can out-perform USM sometimes.

HSM and USM are high-speed motor thingamajigs for Sigma and Canon lenses, respectively.

 

[Big Mike]

As mentioned, it's the camera's AF abilities and the focus motor that are mainly responsible for the speed of the AF. The 40D is well renowned to have very fast AF.

Many older or cheaper lenses use a regular electric motor to turn gears which focus the lens. These can be slow and noisy compared to the Ultrasonic motors that Canon uses on their higher end lenses and some of their mid-grade lenses. So when you look for a Canon lens, look for USM in the title.

Note that Canon actually has two types of USM. The better type is 'Ring USM' although it's not always clear which type a lens has when you read the specs.

Sigma has their own upgraded focus motor, designated as HSM.