Why lenses with built-in-motor focus slower?


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szeping

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#1
Tried to google around on the saying that "built-in-motor lens focus slower than non-BIM" but couldn't find any valid evidence/explaination.

Actually I have tested some lenses (17-50, 28-75), typically Tamron which has built-in-motor with them. However, I didn't really notice the slow focusing speed (as compared with those without) as claimed by many people.

From my logical point of view, 2 motors (body + lens) will drive something faster than 1 motor, so I really don't have any idea that a BIM lens will focus slower than the non-BIM. Can anyone share the mechanism difference that can truly explain the saying?

Thanks.

P/S: Please don't advise me to just get those lenses that have BIM since I can live with the "slow" focusing, I just want to know the mechanism difference instead of following what people are "circulating".
 

j.funkie

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#2
urm, just wondering where you've gotten the idea that "built-in-motor lens focus slower than non-BIM".

It's my understanding that it's more the other way around. :confused:
 

szeping

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urm, just wondering where you've gotten the idea that "built-in-motor lens focus slower than non-BIM".

It's my understanding that it's more the other way around. :confused:
Maybe it's for Tamron specifically. You can see many CS-ers here keep recommending people to get Tammy 17-50 without BIM as it focuses faster.

Or perhaps, with BIM focusing is always > non-BIM?
 

huggable

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#4
Lenses with built-in motor does not require the camera body to drive the AF gears. For lenses without built-in motor, the AF speed depends on the camera body. Generally pro bodies focus faster than the entry-level bodies.
 

szeping

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#5
Lenses with built-in motor does not require the camera body to drive the AF gears. For lenses without built-in motor, the AF speed depends on the camera body. Generally pro bodies focus faster than the entry-level bodies.
OK, my understandings from your statement are:
- Only 1 motor will be driving the AF, if the lens is with BIM... then BIM will dominate the AF, which is slower than pro camera body motor.

So a user with pro body (or mid level and above?) is recommended not to have BIM lens as it will be the bottleneck?

Correct me if I am wrong.
 

Luval

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#6
i think it should something lie this .
lens w/o BIM intially will be focusing from the body. Like the Nikon AFD series and alot of other like tamron and tokina.

Generally, Focus speed depend on amount of light entering onto sensor.With the body doing the focusing.

now that some lens (recent addition from tamron) come in BIM that take over the focusing .. the focus motor on the lens isnt as powerful as the one in the body.

but yet Nikon with the AFS series.. they share focus in both body and lens. which is why is so fast.
 

Rashkae

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Nov 28, 2005
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#7
It depends greatly on the actual type of motor in the lens. Some have a faster one, others, especially the older ones, a slower one. But it's true that in many cases, the in-body AF motor can be brutally fast. But in-lens can be faster too.

On my A900, the in-body AF motor can beat the tar out of most in-lens motors with a lot of my lenses. But the CZ 24-70 SSM focuses faster than most lenses I've ever tried (with some exceptions). So there are lots of factors. There is no clear winner, and anyone who tells you that in-lens will always be faster than in-body has no idea what they are talking about or believes everything a salesman tells them.
 

szeping

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#8
i think it should something lie this .
now that some lens (recent addition from tamron) come in BIM that take over the focusing .. the focus motor on the lens isnt as powerful as the one in the body.

but yet Nikon with the AFS series.. they share focus in both body and lens. which is why is so fast.
It depends greatly on the actual type of motor in the lens. Some have a faster one, others, especially the older ones, a slower one. But it's true that in many cases, the in-body AF motor can be brutally fast. But in-lens can be faster too.

On my A900, the in-body AF motor can beat the tar out of most in-lens motors with a lot of my lenses. But the CZ 24-70 SSM focuses faster than most lenses I've ever tried (with some exceptions). So there are lots of factors. There is no clear winner, and anyone who tells you that in-lens will always be faster than in-body has no idea what they are talking about or believes everything a salesman tells them.
Hi Luval and Rashkae, thanks for explaining. By reading your experience, I should say that:

- there will be only 1 motor driving the AF, body and in-lens motor won't be driving AF concurrently
- in-lens motor will be triggered to AF at the first place

In that sense, we really need to know how fast our camera body can drive the AF before letting the job to in-lens motor.

Erm... exception case for Nikon's AF-S which allow the body and lens to share AF load?
 

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boyboy

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#9
i would like to add that focus mechanism gearing in the lens also matters.

u can have a damn fast body motor, but if your lens is geared slowly, it will still AF slower. i guess the AF might be more precise though. or maybe better for MF.
 

kisim

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#10
Recently I tried both the 50mm AF-D, and the new 50mm AF-S in the shop, on D700 body.
The old AF-D focus significantly faster. Image quality wise, I cannot really tell the difference (according to other threads, the bokeh of the new AF-S is better), so i bought the AF-D version which cost $350 less. :thumbsup:
 

groupwest

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#11
this discussion brings me an idea. maybe i can sell my idea to Nikon or Canon for $1mil.
what if the camera has the capability to combine the speed of both body motor and lens motor into one turbo charged motor :)
 

karnage

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#12
Recently I tried both the 50mm AF-D, and the new 50mm AF-S in the shop, on D700 body.
The old AF-D focus significantly faster. Image quality wise, I cannot really tell the difference (according to other threads, the bokeh of the new AF-S is better), so i bought the AF-D version which cost $350 less. :thumbsup:
It has been discussed that even though the new Nikkor 50/1.4 is AFS, its focus speed is slower due to the longer turn required to focus from minimum to maximum. This was done to aid those who manual focus to be more precise in their focus.

If you have a Nikon body with an in-built motor, I'd say go for the lens without. But that's just me. =)
 

huggable

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#13
If you'd compare the AFS 18-55mm DX with the AFS 17-55mm DX, you'll realise that even tho' both are AFS lenses (with built-in motor), the latter focus much faster.

So it still boils down to lens design.
 

szeping

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#14
Hi Luval and Rashkae, thanks for explaining. By reading your experience, I should say that:

- there will be only 1 motor driving the AF, body and in-lens motor won't be driving AF concurrently
- in-lens motor will be triggered to AF at the first place

In that sense, we really need to know how fast our camera body can drive the AF before letting the job to in-lens motor.

Erm... exception case for Nikon's AF-S which allow the body and lens to share AF load?
Just to know whether my 2 assumptions stated above are correct or not.

And regardless of which AF-S or AF-D models focus faster, I am just curious on the mechanism of AF-S: Is the silent wave motor the sole contributor to the fast AF speed or because AF-S does work together with camera body motor?

Thanks to all replies.
 

Rashkae

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Nov 28, 2005
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#15
Just to know whether my 2 assumptions stated above are correct or not.

And regardless of which AF-S or AF-D models focus faster, I am just curious on the mechanism of AF-S: Is the silent wave motor the sole contributor to the fast AF speed or because AF-S does work together with camera body motor?

Thanks to all replies.
No, it doesn't work together with the camera body motor. There are 2 factors:

1. Focus motor speed (can be in-lens OR on the body)
2. Accuracy of AF sensors on the camera body (to tell the lens when to stop focusing)
 

Jamesf

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Apr 1, 2008
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#16
Just to know whether my 2 assumptions stated above are correct or not.

And regardless of which AF-S or AF-D models focus faster, I am just curious on the mechanism of AF-S: Is the silent wave motor the sole contributor to the fast AF speed or because AF-S does work together with camera body motor?

Thanks to all replies.
If you look at the mount of a AFS lens, you will find that there isn't any slot for lens drive shaft coupling like those AF/AFD lenses. Hence, the focusing is driven sole by the lens motor. Hope this helps.;)
 

boyboy

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Oct 15, 2007
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#17
Just to know whether my 2 assumptions stated above are correct or not.

And regardless of which AF-S or AF-D models focus faster, I am just curious on the mechanism of AF-S: Is the silent wave motor the sole contributor to the fast AF speed or because AF-S does work together with camera body motor?

Thanks to all replies.
the SWM motor rate of operation is fixed. but the camera sensor will tell it where to turn, by how much, when to stop

the motor rate of operation also varies between different models of lenses. kit lenses probably get a smaller and lighter (of course, slower) motor. pro lenses get bigger and faster motors.

AFD will depend on camera body's motor and sensors
 

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