Kenko TC Question

I have a Kenko TC 1.4 Pro 300 that I have used with several lenses (on a Rebel XSi).

With the 70-200 2.8 IS, it behaves as expected: AF works normally, aperture reduction of 1 stop, and reporting of the modified focus length, correctly recorded in the EXIF.
However, with the 85 1.8 and the 28-105 USM II it behaves differently: the AF works (may chase sometimes), but the focus length displayed is the native one, not taking the TC into account. Moreover, while the 28-105 loses 1 stop as expected, the 85 can still open at 1.8!
From what I read elsewhere, the Kenko Pro 300 is supposed to report correctly?
Besides, the image quality is OK, with a very slight degradation as can be expected.

Thanks to anybody who can explain the differences in behavior between these lenses.

Gerard

GMHY wrote in post #7715479
I have a Kenko TC 1.4 Pro 300 that I have used with several lenses (on a Rebel XSi).

With the 70-200 2.8 IS, it behaves as expected: AF works normally, aperture reduction of 1 stop, and reporting of the modified focus length, correctly recorded in the EXIF.
However, with the 85 1.8 and the 28-105 USM II it behaves differently: the AF works (may chase sometimes), but the focus length displayed is the native one, not taking the TC into account. Moreover, while the 28-105 loses 1 stop as expected, the 85 can still open at 1.8!
From what I read elsewhere, the Kenko Pro 300 is supposed to report correctly?
Besides, the image quality is OK, with a very slight degradation as can be expected.

Thanks to anybody who can explain the differences in behavior between these lenses.

Gerard

I can't answer the issue about why the 1.4 x Tcon reports differently from lens to lens, but it doesn't prevent any lens from opening to its full aperture. it just reduces the amount of light that is transmitted through the Tcon/lens combination by one stop. So, even though the 85 opens fully and it may report as f/1.8, the effective aperture, due to loss of light transmission, is reduced by one stop.

runninmann wrote in post #7715829
I can't answer the issue about why the 1.4 x Tcon reports differently from lens to lens, but it doesn't prevent any lens from opening to its full aperture. it just reduces the amount of light that is transmitted through the Tcon/lens combination by one stop. So, even though the 85 opens fully and it may report as f/1.8, the effective aperture, due to loss of light transmission, is reduced by one stop.

But, when using it with the 70-200, it's impossible to open at 2.8 - you're limited to 3.5 - which is not he case with the two other lenses (yes, I checked with the 28-105 and it goes all the way down to 3.5) ?

I believe what you're describing is that the tcon is reporting with the 70-200, but not with the other 2 lenses. However, 1 stop down from f/2.8 would be f/4, not f/3.5. I can't explain the fraction of a stop difference (between 1/4 and 1/3 stop), but it's probably due to "tolerances".

runninmann wrote in post #7715944
I believe what you're describing is that the tcon is reporting with the 70-200, but not with the other 2 lenses. However, 1 stop down from f/2.8 would be f/4, not f/3.5. I can't explain the fraction of a stop difference (between 1/4 and 1/3 stop), but it's probably due to "tolerances".

OK, f/4, my bad - Point is, why with the 70-200 and not with the two others?

GMHY wrote in post #7715962
OK, f/4, my bad - Point is, why with the 70-200 and not with the two others?

look at the back of your 70-200 and compare that with your lenses where the tcon is not reporting. There should be 3 extra contacts on the 70-200 and all other lenses that support tcons. This is required for the lens to detect the presence of the tcon and is also why only the tcon closest to the lens is detected when you stack them.

XsiBrian wrote in post #7716002
look at the back of your 70-200 and compare that with your lenses where the tcon is not reporting. There should be 3 extra contacts on the 70-200 and all other lenses that support tcons. This is required for the lens to detect the presence of the tcon and is also why only the tcon closest to the lens is detected when you stack them.

Makes sense! Thank you.

Gerard