Focussing Speed

[Macky]

Hi everyone! I wanted to ask about focussing speed of the Canon setup. I was playing around with the 1Ds III and the 24-70 f2.8L and the 50mm f1.4. I noticed that the focussing speed of the 24-70 was noticeably faster than the 50mm!! I would have thought that the f1.4 would result in a faster focussing speed compared to the f2.8 since a larger aperture would let in more light, hence easier for the camera to focus. Am I wrong?

Anyone else with the same experience?

[coolsigg]

24-70 uses full ring type USM whereas 50F1.4 uses micro USM only...

Source

[grantyale]

Originally Posted by Macky Hi everyone! I wanted to ask about focussing speed of the Canon setup. I was playing around with the 1Ds III and the 24-70 f2.8L and the 50mm f1.4. I noticed that the focussing speed of the 24-70 was noticeably faster than the 50mm!! I would have thought that the f1.4 would result in a faster focussing speed compared to the f2.8 since a larger aperture would let in more light, hence easier for the camera to focus. Am I wrong? Anyone else with the same experience?

No camera sees beyond F/2.8 for AF, AFAIK, and the micro-USM isn't really fast.

[Macky]

Thanks for the enlightening info people!!

Pardon my ignorance but how do you tell which lens is micro USM or "normal" USM?? All the lenses just say USM. So regular USM is faster than micro USM?? Are there any other types of USM that I should know about? The Canon website didn't have much info.

[coolsigg]

Originally Posted by Macky Thanks for the enlightening info people!! Pardon my ignorance but how do you tell which lens is micro USM or "normal" USM?? All the lenses just say USM. So regular USM is faster than micro USM?? Are there any other types of USM that I should know about? The Canon website didn't have much info.

the best way to find out is to do research on those lens/camera gear review site like the link I posted in my previous post...

[zj2000]

Originally Posted by grantyale No camera sees beyond F/2.8 for AF, AFAIK, and the micro-USM isn't really fast.

not true, most cameras do not see beyond f2.8 for metering because the focus screen in dslrs nowadays are designed to be usable with slow lenses ie f5.6 and you won't see any difference through the viewfinder between a f2.8 and a faster lens.

When it comes to AF, all dslrs see light as it comes through the viewfinder. Otherwise all your shots shot with a faster than f2.8 aperture will be OOF due to difference in dof between f2.8 and f1.0 say...

[IsenGrim]

i tot no camera sees beyond 2.8 because the opening for the reflected light (off the mirror) to enter the focusing device is only f2.8 large. so if your aperture is any larger than that, still only f2.8 amount of light can enter the focusing device.

well i guess why is it only f2.8 large, is coz of physical limitations? space is quite limited inside a camera body...

read this somewhere on Dan Heller's tutorial i think.

[grantyale]

Originally Posted by IsenGrim i tot no camera sees beyond 2.8 because the opening for the reflected light (off the mirror) to enter the focusing device is only f2.8 large. so if your aperture is any larger than that, still only f2.8 amount of light can enter the focusing device. well i guess why is it only f2.8 large, is coz of physical limitations? space is quite limited inside a camera body... read this somewhere on Dan Heller's tutorial i think.

More or less.

[grantyale]

Originally Posted by zj2000 not true, most cameras do not see beyond f2.8 for metering because the focus screen in dslrs nowadays are designed to be usable with slow lenses ie f5.6 and you won't see any difference through the viewfinder between a f2.8 and a faster lens. When it comes to AF, all dslrs see light as it comes through the viewfinder. Otherwise all your shots shot with a faster than f2.8 aperture will be OOF due to difference in dof between f2.8 and f1.0 say...

Why does AF has anything to do with viewfinder?

[zj2000]

Originally Posted by IsenGrim i tot no camera sees beyond 2.8 because the opening for the reflected light (off the mirror) to enter the focusing device is only f2.8 large. so if your aperture is any larger than that, still only f2.8 amount of light can enter the focusing device. well i guess why is it only f2.8 large, is coz of physical limitations? space is quite limited inside a camera body... read this somewhere on Dan Heller's tutorial i think.

Maybe you can share the link to that tutorial. The light from the lens is reflected off the submirror on to the AF module. The submirror is behind and smaller than the main mirror that reflects light upwards into the prism. Now when you use a larger aperture the image circle does not become bigger but rather the light in the image circle becomes more intense so in theory more intense light hitting the AF module will mean better sensitivity and faster AF no?

[zj2000]

Originally Posted by grantyale Why does AF has anything to do with viewfinder?

No, I'm just saying that if there is an f2.8 limitation it will be on the metering and finder. Not the AF.

[Macky]

Thanks for your input everyone!

I was wondering if there would be any appreciable difference in AF speed between, say a 24-70 f2.8 and a 24-105 f4? Would the AF speed on the f4 be slower by much? Given that the USM mechanism is the same on both lenses.

Sorry about this infatuation about AF speed but the sluggishness of the 50 f1.4 especially in low light conditions really really bothers me!! Plus I just found out today that the 50 f1.4 doesn't come with the garden variety "ring type USM" but rather a knocked down version of it. Just wondering what other surprises these lenses had in store for me!

[IsenGrim]

hmm i tried ti find it again, but to no avail. maybe it isn't dan heller's site. i have nothing to do at work for my IA so sometimes i read too much...

eh but.. doesnt smaller sub mirror reflect lesser light? coz (its gonna get technical...) the mirror size and the sensor size is similar right? so as much light is falling on the mirror and on the sensor (when the mirror flips), so a smaller sub mirror behind the main one will capture (and reflect) less light? the rest is juz lost? (like the analogy of the FF lens on a APS-C sensor, the sensor wont get a more intense picture, right? the rest of the light is simply lost)

but i could be wrong.. imma eletrical engineer, not mech LOL