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TeddTucker
02-11-2009, 02:02 PM
I read a lot of reviews of lenses and bodies and it leaves me wonder who handles the Auto Focus for a picture (lens or body or both)? Could someone explain this to me?

Thanks

adam75south
02-11-2009, 04:06 PM
the answer is they work together.

tedd, i'm no expert, but then again, we're on a forum and this is what happens when you ask general public a question haha.

here goes. the lens has it's own autofocus motor, the camera tells the motor where to go and when to stop. some lens' motors are slower than others...some cameras take longer to decide where the motor needs to go and when to stop.

Rasidel Slika
02-11-2009, 05:16 PM
I'll give my 2 cents, I was shooting indoors with a friend's 40D once with the 17-85 and it SUCKED, it was hunting around like mad, pretty much unusable. I put his 85mm 1.8 on and it was gold. Awesome performance. This is why I was asking about lenses in the other thread - I think it's definitely important.

cdifoto
02-11-2009, 05:25 PM
Lens lets light hit AF sensors, AF sensors tell lens how much to move. Lens does the moving.

Mark_48
02-11-2009, 05:29 PM
Compare it to MF. With manual focus you are provided with information through the viewfinder and your hand turns the focus ring accordingly to achieve the clearest image you can. Your brain does the decision making and your hand makes it happen.

AF is not much different. The processor brains in the camera takes information from contrast sensors in the camera and sends commands to the lens AF motor to make focus adjustments until the camera sensor thinks it's correctly focused.

Rooz
02-11-2009, 05:37 PM
I'll give my 2 cents, I was shooting indoors with a friend's 40D once with the 17-85 and it SUCKED, it was hunting around like mad, pretty much unusable. I put his 85mm 1.8 on and it was gold. Awesome performance. This is why I was asking about lenses in the other thread - I think it's definitely important.

someone correct me if i'm wrong here but thats cos its using 1.8 for AF, even if you're shooting it at f4. thats a shitload of light getting to the sensor as opposed to the woeful 17-85 at 85mm. same amount of ambient light...vastly different performance.

i think most lens always use their max aperture for AF. this is another reason why alot of indoor sports guys use 2.8 lens'. its not just cos they can use it at 2.8, its also cos they can acheive better af accuracy using the 2.8 max aperture when it;s AF'ing.

Rasidel Slika
02-11-2009, 05:44 PM
^^ so if that's the case, it shows that the lens is VERY important in terms of AF!

Rooz
02-11-2009, 05:53 PM
coldy told me that a couple of years ago and he was rarely wrong on the techno stuff.

another thing of note is that the power generated in turning the lens for AF by the camera body itself, (for nikon anyway), also make a diffference. the d3 and d2x etc drive the lens' faster. maybe not alot. but its noticable in continuous drive modes. not sure if this is the case with canon aswell with the 1D bodies.

Rasidel Slika
02-11-2009, 05:56 PM
I just feel like generally, people attribute slow AF more to the camera than the lens - whereas from experience I feel that the lens is just as important as the camera.

Rooz
02-11-2009, 06:13 PM
whereas from experience I feel that the lens is just as important as the camera.

agree 100%. in fact i would go so far as to say the lens is more important. 3 excellent real world examples for me recently:

first example: 50/1.4G VS 50/1.4D. same max aperture, same body, same focal length. the G lens outperfromed the D in AF accuracy. not only accuracy, but once focus had been achieved, small incremental adjustments on the G lens are fast and true. the D lens was slower to make those adjustments.

second example: 24-70/2.8 VS 16-85VR. the 24-70 at the same focal lengths and same aperture, (f5.6-f8), the 24-70 absolutely smoked the 16-85. its more accurate and focusses faster on the exact same body.

third example: sigma 150/2.8 macro vs nikon 105VR. lets leave speed aside for this one cos the afs is obviously much faster on the nikon lens than on the sigma. but i could rarely get the sigma to hit AF on the d300's extreme AF points, even of STATIONARY objects it just wouldnt lock on reliabley. the 105VR locks on every time.

nism
02-11-2009, 09:00 PM
i think most lens always use their max aperture for AF. this is another reason why alot of indoor sports guys use 2.8 lens'. its not just cos they can use it at 2.8, its also cos they can acheive better af accuracy using the 2.8 max aperture when it;s AF'ing.

yea true, the aperture only stops down to what you've set it to, when you take the photo.
and also some bodies have special points on the AF sensors that work only with 2.8 lenses or faster, which obviously helps AF.

as for how AF knows what you want to focus on, i think it works by way of contrasting points, which is why shooting a plain wall is difficult.

Csae
02-11-2009, 10:51 PM
someone correct me if i'm wrong here but thats cos its using 1.8 for AF, even if you're shooting it at f4. thats a shitload of light getting to the sensor as opposed to the woeful 17-85 at 85mm. same amount of ambient light...vastly different performance.

If that were true wouldn't my 17-40F4L take nearly twice as long to focus then my 50 1.8 II ? On the same Camera ? Lets factor in the different focusing motors, and the 50 should still focus faster then the 17-40, maybe not as much as 2x because of its slower focusing motor, but still faster even if just slightly. It does in fact focus slower.

Im not saying i know, im just saying that specific theory doesn't sound plausible to me.

Rasidel Slika
02-11-2009, 10:54 PM
^^ here's my thinking.... it also has to with the motors in the lens... I think the L lenses have better/faster ones in em.

Rooz
02-11-2009, 11:00 PM
^^ here's my thinking.... it also has to with the motors in the lens... I think the L lenses have better/faster ones in em.

correct. the 50/1.8 does not even have a USM motor so its not the same. in fact i'm pretty sure that there is a "newer" version of USM which is faster than the older type USM aswell which makes it even more confusing.