2x Teleconverter = 2 stops always?

[sepia]

Good Evening. Just got a Teleplus 2x teleconverter to use with a EF85 f1.8. I was expecting a 2 stop loss. To verify, I use spot metering + AV mode + fixed ISO + autofocus to shoot a high contrast, flat surface with and without the teleconverter. Surprisingly, I was consistently getting 4/3 stop shutter speed difference, at all apertures.

Suspecting, it was not exactly a 2x teleconverter, I tried shooting a ruler with and without the teleconverter to estimate the magnification. It was 1.88x.

Shouldn't I be getting 1.5 to 2 stop loss? Or was my method flawed? Would really appreciate if anyone could share their experiences on verifying stop losses when using teleconverters.

Thank you in advance.

 

[calebk]

Try testing via this method. Without the converter, shoot a subject at a certain exposure setting at Manual mode. Remembering that setting and put the converter on now. Then, staying on Manual mode, shoot at two stops overexposed from the original settings and see if you get the same exposure.

Do this instead of relying on AE and you will be more certain as to whether you are getting a two stop difference.

 

[sepia]

Just tried this method. Turns out it is 2 stops after all.

Also, rechecked the magnification. It was 2x.

Thanks for your help, calebk.