Lens Autofocus Speed

Started Apr 10, 2012 | Discussions
peterthepooh
New MemberPosts: 8
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Lens Autofocus Speed
Apr 10, 2012

I was wondering how to determine the autofocus speed of a lens? Is there a way other than testing it before I buy?

For example I have a Canon 100mm f/2.8 macro lens and it focuses very quickly but my Canon 50mm f/1.8 focuses very slowly. Yes I know there is a huge price gap between these lens but just wondering if there was a way to tell in the specs.

Thanks,

Peter

Steve Balcombe
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Re: Lens Autofocus Speed
In reply to peterthepooh, Apr 10, 2012

peterthepooh wrote:

I was wondering how to determine the autofocus speed of a lens? Is there a way other than testing it before I buy?

For example I have a Canon 100mm f/2.8 macro lens and it focuses very quickly but my Canon 50mm f/1.8 focuses very slowly. Yes I know there is a huge price gap between these lens but just wondering if there was a way to tell in the specs.

There is no way to get an actual measure of speed. However, it boils down to there being three types of focusing motor/mechanism:

Micro motor - slowest
Micro USM - slightly better
Ring USM - best by far

This is not the whole story, as there's some variation within those 'bands', but it's a good guide. As you probably know, your 100/2.8 macro has Ring USM and the 50/1.8 (assuming it's a MkII) has micro motor AF so they fit the pattern.

There is also an older type of motor, AFD or Arc-Form Drive which is the predecessor to micro motor and survives in a small number of old designs, notably the 35/2. It is as slow as micro motor, and noisier, but does have one advantage which is that the focusing ring doesn't rotate in AF mode.

Canon has a tendency to gloss over which USM lenses use the inferior micro USM mechanism, but it's the 50/1.4, the 70-300 IS (the non-L), and all lenses such as the 75-300 which are/were available in both USM and non-USM versions. This document lists them, although it's getting rather out of date now so the newest lenses are not included:

http://software.canon-europe.com/files/documents/EF_Lens_Work_Book_11_EN.pdf

For a technical (but easy to read) description of the various AF mechanisms:

http://software.canon-europe.com/files/documents/EF_Lens_Work_Book_9_EN.pdf

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BAK
BAK
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What's the test based on?
In reply to peterthepooh, Apr 10, 2012

Are you taking a lens that is focussed at infinity to start with, and seeing how long it takes to get to three feet?

Or are you pointing a lens in the general direction of, say, six feet, pre-focusing, and then seeing how long it takes to focus precisely to five feet eight inches, where the face you want sharp has now placed itself?

BAK

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jitteringjr
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Re: Lens Autofocus Speed
In reply to peterthepooh, Apr 10, 2012

You can search youtube for video's of lens AF speed, but you need to be careful when comparing results from different uploaders. For instance any lens will focus faster in AI Servo mode than one shot under the same conditions. Also the same lens for instance, on my 7D will focus faster than on my 350D/Rebel XT. If you are compairing lens video's its best to find one showing a bunch of lenses on the same video.

If you are looking at specific lenses, you can always ask the question here too as many of us may have personal experience with each and can tell you which gives snappier responses.

 jitteringjr's gear list:jitteringjr's gear list
Canon EOS 7D Canon EF 50mm f/1.4 USM Canon EF 85mm f/1.8 USM Canon EF 135mm f/2.0L USM Canon EF-S 60mm f/2.8 Macro USM +5 more
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WilbaW
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Re: Lens Autofocus Speed
In reply to jitteringjr, Apr 11, 2012

jitteringjr wrote:

any lens will focus faster in AI Servo mode than one shot under the same conditions.

That sounds interesting. Personal observation? Documented? Please tell me more.

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