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Thread started 08 Mar 2012 (Thursday) 18:13
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Is Fast Focusing Dependent on the Body or the Lens?

 
blackzarg
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Mar 08, 2012 18:13 |  #1

Or both?

Thanks! :cool:


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pulsar123
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Mar 08, 2012 18:46 |  #2

In principle it should be both (camera does need to do some processing before sending AF signals to the lens). In practice, the AF speed is mostly determined by the lens. The AF accuracy, on the other hand, is clearly both camera and the lens.


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nate42nd
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Mar 08, 2012 19:32 |  #3

I would say the body and lens. There is a difference between fast and accurate. Say you put my 7D against a 20D or 40D. I know what would be more accurate averaged in all situations. The 7D, but would the 20D or 40D be just as fast? Don't know. It also depends on the system. Some systems have image stabilization and focus drive in the body.


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shmoogy
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Mar 08, 2012 20:52 |  #4

It's both-- My 1d2 can drive/rack focus from near to infinity slightly more quickly than my 5d2 with the same lens-- I don't think it's a large difference though.

e: It is noticeable, but I don't think it's more than 30% faster with any of the lenses I have.


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Mark1
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Mar 08, 2012 20:55 |  #5

The "hunting" is a shortcoming of the AF system that is in the body. If the subject is within the focus range of the lens, then the body missed it some how. So it tries again. And takes longer.

As far as speed, if it can lock on, in one shot. The speed is determined by the motors in the lens that moves the focusing lens. They can only turn one speed. They are off or on. There is nothing else. A given lens will focus at the exact same speed no matter what body it is on. As stated above if focus is missed it may physicaly take longer but that is bacause the body messed up.

Different lenses will focus at different speeds. They may have the same motor. But are moving a different lens with a different weight. Or different motor altogether.


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blackzarg
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Mar 09, 2012 01:15 as a reply to  @ Mark1's post |  #6

Thanks a lot for the responses! I guess i was talking about the "hunting" that Mark1 mentioned. I never thought about the camera motor moving different weights. Good point!


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Xcelx
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Mar 09, 2012 02:00 |  #7

Mark1 wrote in post #14053302 (external link)
The speed is determined by the motors in the lens that moves the focusing lens. They can only turn one speed. They are off or on.

I don't think that statement is true, with the lens cover on (24-105 & 200L 2.8) and in AI servo my 5Dc moves the focus ring quicker from infinity to closest and back, than if it's in one shot mode, which would indicate the focusing speed can be selected by the body.

I've also read about the 1D bodies focusing faster and moving the elements quicker with the 85L than other camera series do.




  
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jonneymendoza
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Mar 09, 2012 03:21 |  #8

What about the AF outside the centre point? is that lean towards the camera or the lens?


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JeffreyG
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Mar 09, 2012 05:33 |  #9

Mark1 wrote in post #14053302 (external link)
As far as speed, if it can lock on, in one shot. The speed is determined by the motors in the lens that moves the focusing lens. They can only turn one speed. They are off or on. There is nothing else. A given lens will focus at the exact same speed no matter what body it is on.

This part is not correct. My 1D4 (and the 1D3 I used to own) will drive the 85L from end to end in about 60% of the time it takes my 5D to do it.

The speed of the focus motor is clearly different between the two bodies, and I think the slowness of the 85L makes this difference apparent.

I suspect the difference carries over to other lenses, but since they are all fairly quick anyway it is harder to note the change.


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Mark1
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Mar 10, 2012 13:39 |  #10

It may be a pro body is sending a higher voltage to the motors in the lens giving it a bit more power. But there is no "selectable" speed range that the body can use when it wants to focus faster or slower. Focus is a on or off operation.


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The ­ One ­ Pixel ­ Wonder
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Mar 11, 2012 11:39 |  #11

jonneymendoza wrote in post #14054719 (external link)
What about the AF outside the centre point? is that lean towards the camera or the lens?

AF points are a factor of the camera - that's why the same L lens will focus differently (faster/slower) on a 5D series with it's 9 points, vs on a 7D series with it's 19 points, or a 1D series with it's 40+ points.


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pwm2
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Mar 11, 2012 11:56 |  #12

One thing is that the lens has a number of parameters stored. They contain info about acceleration, overshoot etc.

So the camera can compute a value and then tell the lens what the intended position is - then off goes the lens.

But a lens with lots of wear will no longer behave as the parameters stored all the way from factory production. So it may overshoot more.

The camera performs a second AF evaluation to verify if the subject is in focus. For a "perfect" lens, the answer would be true. But with war and tear, the lens may have failed to move at full speed and then breaked to the exact correct location. So the camera may need to perform another cycle of evaluating focus and ordering the lens to move.

The hunting is what happens when the camera can't even predict where it is. Maybe the focus is too far off, or there are no good contrasty details. So the camera just takes a bit and have the lens move, in the hope that the new position will show any details that the AF system can compute a focus match on.

I wouldn't be surprised if high-end bodies can supply more power to the lens, and that the lens can adapt to that. After all, the really big lenses have lots of moving mass so it takes quite a lot of power to make them focus quickly. At the same time, some of the film bodies have puny batteries.


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Xcelx
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Mar 11, 2012 12:15 |  #13

Mark1 wrote in post #14062757 (external link)
But there is no "selectable" speed range that the body can use when it wants to focus faster or slower. Focus is a on or off operation.

That is simply not true, I just tested it with my 5d and 200L II 2.8. Filming the AF ring at 30fps (don't have faster), it takes the lens 17 frames to go from infinity to closest once it starts moving in One Shot.

In AI servo it only takes 12 frames for the AF ring to reach the closest focusing distance. This means the camera can indeed change the speed of focusing.

This is without considering the shorter AF pause at the closest end while in servo mode

You can download the jpg sequence here (external link)




  
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totoyo
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Mar 11, 2012 13:28 |  #14

The answer is both, the AF of my 7D is very fast but if i coupled the tamron 17-50 the AF speed becomes very slow....but if i coupled the magic drainpipe the AF seek time is fast...
conclusion: your system will be as fast as the slowest component (camera or lens).
cheers.


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Foggiest
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Mar 11, 2012 15:17 |  #15

The wiki about the USM motors is interesting , especially the use of a travelling wave (like a walking caterpillar)and resonance within the device .
As for big lenses , the motor is a ring around the outside diameter of the lens , bigger lens = bigger motor = more torque .

Edit , just thinking as I get a Ruby Murray ....
If the frequency of the travelling wave is driving a fixed resonant frequency wave of device wave , then increasing amplitude may only increase the torque ... not sure .

Nope , still not got it !
If the frequency is increased the travelling wave is increased and whilst the "legs" are still moving at the same period(resonant) there are more cycles travelling ?
If the amplitude is increased the throw of the "legs" is increased , (material has max throw though) ?

Tricky little things indeed ! Halp ?




  
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Is Fast Focusing Dependent on the Body or the Lens?
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