Canon extender with Tamron lens question

[Pauly_6_String]

Hi all.  I just picked up a used Canon EF 1.4x II extender and attached it to a Tamron G2 70-200 f/2.8 lens.  

The camera displays f/5.6 to f/45????   Exactly two stops high on each end.  Is that a compatibility/communication issue between the two brands?

Also was disappointed to find that it won't fit my EF 17-40 or my EF 24-105 lenses.  

thanks

[kvbarkley]

Canon extenders are only designed to work with Canon lenses.

You should only lose 1 stop.

[Pauly_6_String]

Thanks. Is the stop loss supposed to be from a reduction of light entering the lens or physically not being able to select the largest aperture?

[Tronhard]

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@Pauly_6_String wrote:
Thanks. Is the stop loss supposed to be from a reduction of light entering the lens or physically not being able to select the largest aperture?

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There is a fair amount of glass in an extender and the optical algorithms required to render the resultant extension will lose about 1 stop on the 1.4x and 2 stops on the 2x extender.   There are tables available from Canon defining what lenses extenders will work with what lenses and the resultant performance of those combinations. Furthermore different camera models give different performance results when used with extenders, especially in terms of autofocus and f-stop values for shooting.

If you want to invest in extenders, some serious research is in order to avoid disappointment.

[kvbarkley]

It is an optics thing, and has nothing to do with the physical aperture. Since you are effectively increasing the focal length, the ratio of the max Aperture to focal length goes down.

[RobertTheFat]

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@kvbarkley wrote:

It is an optics thing, and has nothing to do with the physical aperture. Since you are effectively increasing the focal length, the ratio of the max Aperture to focal length goes down.

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And to make it go down by one stop, you have to increase the focal length by the square root of 2. And what is the square root of 2? Almost exactly 1.4, which is why they make "1.4x" extenders.

EDIT: For maximum clarity, I should have said "multiply" rather than "increase"..

[ebiggs1]

Keep in mind, using a tele-con does not change the physical aperture. You are magnifying an image that is already projected by the lens. The physical aperture is unchanged so the depth of field is unchanged.

[kvbarkley]

Since the "circIe of confusion" is changed, I think that the DOF is changed too, just like the fact that the DOF changes when you use the same lens on FF vs APS-C.

ETA:

This is wrong, the change in depth of field is entirely due to the effective increase of the focal length.

[ebiggs1]

You are talking sensor. I am talking lens.

[ebiggs1]

" I just picked up a used Canon EF 1.4x II extender and attached it to a Tamron G2 70-200 f/2.8 lens."

It makes more sense to get a Sigma Tamron tel-con for a Sigma Tamron lens, doesn't it?  The Canon 1.4x tel-con only works with prime lenses except for a very few zooms. The 17-40mm and 24-105mm are not one of those few.

Question, why would anyone want to put a tel-con on a WA lens like the 17-40mm? It seems that concept is contrary.

Edited sorry for the mustake!