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Author Topic: Lenses, Focusing Speeds and Teleconverters  (Read 1741 times)

danski0224

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Lenses, Focusing Speeds and Teleconverters
« on: March 30, 2012, 11:02:02 AM »
Expanding on a couple of threads concerning long lenses and teleconverters...

Do lenses focus at the same speed on any given camera body?

Does a 500 f4 focus any slower or faster than a 400 f2.8? If there are differences, could a 400 f2.8 + 2xIII equal a 500 f4 + 1.4x III as far as focus speed is concerned?

It seems that a 300 f4 does focus slower than a 70-200 II f2.8. Unscientific, seat of the pants test.

Does a 7D drive the lens any differenty than a 5DII or III?

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Lenses, Focusing Speeds and Teleconverters
« on: March 30, 2012, 11:02:02 AM »

neuroanatomist

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Re: Lenses, Focusing Speeds and Teleconverters
« Reply #1 on: March 30, 2012, 11:24:36 AM »
Do lenses focus at the same speed on any given camera body?

Does a 500 f4 focus any slower or faster than a 400 f2.8? If there are differences, could a 400 f2.8 + 2xIII equal a 500 f4 + 1.4x III as far as focus speed is concerned?

It seems that a 300 f4 does focus slower than a 70-200 II f2.8. Unscientific, seat of the pants test.

Does a 7D drive the lens any differenty than a 5DII or III?

Focus speed is difficult to measure, because there are many variables.  But yes, there are differences among bodies, between lenses, and with teleconverters. 

The 7D will not be significantly different from the 5DII/III in terms of AF motor speed.  The main difference there is with the 1-series bodies, which have a higher voltage power supply and are able to drive the AF motors of all lenses faster.

Different lenses use different designs and have different size focusing groups to move different distances - that accounts for differences even among the same type of motor (e.g. ring USM).

The 70-200 II is, indeed, faster than the 300/4 IS.  The 400/2.8 is only very slightly faster than the 500/4, likely not enough to compensate for the difference between the effect of 1.4x vs 2x TC (the former slows AF speed by 50%, the latter by 75%).
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Re: Lenses, Focusing Speeds and Teleconverters
« Reply #2 on: March 30, 2012, 11:27:26 AM »
good questions! i know some lenses indeed focus faster than others, so that is a factor. and extenders always decrease focussing speed. but i don't know if a camera body can 'boost' the focussing speed of a lens. i sure would like to know.

update: neuro already replied faster. thanks for that info!
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danski0224

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Re: Lenses, Focusing Speeds and Teleconverters
« Reply #3 on: March 30, 2012, 11:53:27 AM »
Focus speed is difficult to measure, because there are many variables.  But yes, there are differences among bodies, between lenses, and with teleconverters. 

The 7D will not be significantly different from the 5DII/III in terms of AF motor speed.  The main difference there is with the 1-series bodies, which have a higher voltage power supply and are able to drive the AF motors of all lenses faster.

Different lenses use different designs and have different size focusing groups to move different distances - that accounts for differences even among the same type of motor (e.g. ring USM).

The 70-200 II is, indeed, faster than the 300/4 IS.  The 400/2.8 is only very slightly faster than the 500/4, likely not enough to compensate for the difference between the effect of 1.4x vs 2x TC (the former slows AF speed by 50%, the latter by 75%).

Interesting.

Is there any way to put a numerical value to how much faster a 1 series body drives the AF motor in comparison to everything else? (twice as fast, 1.5x as fast, etc)

In another thread it was pointed out that the 400 2.8 does rather well with the III series TC's, and that lens would autofocus on less than a 1 series body with f8 capabilities compared to the 500 f4 and a 2xIII (disregarding the 200 mm native differences between the lenses + TC).

In effect, the 400 f2.8 plus the two III converters could yield 3 useful combinations on something that does not have f8 AF, where the 500 f4 would require a 1 series body for AF if the 2x III converter is used.

But, I have no idea if the focus speed penalty would wash out any apparent advantages... or if the combination would fare worse IQ wise on a 7D.

Are the IQ penalties noticeable in the real world, or is it pixel peeper stuff? I do not own a 2x converter.
« Last Edit: March 30, 2012, 11:57:30 AM by danski0224 »
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Re: Lenses, Focusing Speeds and Teleconverters
« Reply #4 on: March 30, 2012, 12:14:37 PM »
i just wondered, does a batterygrip on for example a 7D, add more juice to the system? and therefore let you have your lens focus faster?
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danski0224

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Re: Lenses, Focusing Speeds and Teleconverters
« Reply #5 on: March 30, 2012, 12:20:44 PM »
If I had a grip, I'd try it.

My guess is no.
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neuroanatomist

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Re: Lenses, Focusing Speeds and Teleconverters
« Reply #6 on: March 30, 2012, 12:27:50 PM »
The way I'd formally test would be to pick a few lenses and measure the time taken to rack the focus from MFD to infinity (with the lens cap on) on various bodies.

Obviously, though, that's only raw AF motor speed.  In the real world, type and quality of the AF point, the AF system's algorithms, and the subject will play a big role - probably more than the actual AF drive speed.

The IQ penalty with the 2x is real, but minimized on top end lenses like the 70-200 II and the supertele lenses.  I'd still advocate getting the focal length needed, if possible (budget and other factors notwithstanding), rather than planning on constantly using a TC with a shorter lens.

i just wondered, does a batterygrip on for example a 7D, add more juice to the system? and therefore let you have your lens focus faster?

The grip has no effect on AF speed.  The two batteries are used alternately, not simultaneously, so there's no voltage boost with the grip.  The LP-E6 battery used by the 5DII/5DIII/7D/60D puts out 7.2 V, whereas the LP-E4 used by the 1-series bodies delivers 11.1 V.
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Re: Lenses, Focusing Speeds and Teleconverters
« Reply #6 on: March 30, 2012, 12:27:50 PM »