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UnclePat (unregistered) wrote on
12/06/2010 - 10:33.

lynn Amaral wrote:

But do we know which camera bodies the autofocus will work with??

Is there a similar list for which bodies they work with? i.e. 1DMii or 70d?

thanks

It's all pretty simple. The reason the AF ceases to work with an extender attached is not that the extender isn't capable of AF function or that it disrupts any AF signal or something like that. The reason is that the camera's AF sensors need a maximum f-stop number to function. This is f/5.6 for all EOS except the 1D series, and f/8 for all 1D cameras (but only the center AF point will work as a line sensor in that case).

So what you have to do to figure out if the AF will work with an extender is simple: You take the lens's f-number at the maximum focal length and multiply it with the extender factor (1.4x or 2x). If the resulting number is <= 5.6, then AF will work, if it's larger, it won't (except on 1 series, see above).

Matt wrote:

how many stops of light will you lose by having one of these attached - I shoot primarily with 1.2's and 1.4's so am concerned about losing too much by attaching this, but would love the extra reach on an 85 1.2 per say. Any idea what the difference would be in terms of loss?

The extenders won't work on fast primes like the 85/1.2. It can't be connected physically. The shortest prime it can be used with is the 135/2L.

Besides that, using an extender on a short prime or even zoom lens (which is possible using 3rd party extenders) normally doesn't yield good image quality. In most cases even just cropping your image gives you better results.

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[quote=UnclePat]&nbsp;[quote=lynn Amaral]
But do we know which camera bodies the autofocus will work with??
Is there a similar list for which bodies they work with? i.e. 1DMii or 70d?
thanks
[/quote]
It's all pretty simple. The reason the AF ceases to work with an extender attached is not that the extender isn't capable of AF function or that it disrupts any AF signal or something like that. The reason is that the camera's AF sensors need a maximum f-stop number to function. This is f/5.6 for all EOS except the 1D series, and f/8 for all 1D cameras (but only the center AF point will work as a line sensor in that case).
So what you have to do to figure out if the AF will work with an extender is simple: You take the lens's f-number at the maximum focal length and multiply it with the extender factor (1.4x or 2x). If the resulting number is &lt;= 5.6, then AF will work, if it's larger, it won't (except on 1 series, see above).
[quote=Matt]
how many stops of light will you lose by having one of these attached - I shoot primarily with 1.2's and 1.4's so am concerned about losing too much by attaching this, but would love the extra reach on an 85 1.2 per say.&nbsp; Any idea what the difference would be in terms of loss?
[/quote]
The extenders won't work on fast primes like the 85/1.2. It can't be connected physically. The shortest prime it can be used with is the 135/2L.
Besides that, using an extender on a short prime or even zoom lens (which is possible using 3rd party extenders) normally doesn't yield good image quality. In most cases even just cropping your image gives you better results.[/quote]

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