optics – Calculating f/stop chance for teleconverter that increases lens size

With a rear-mount converter, it’s always the factor of 2 (unless there’s a gross mismatch between lens and converter), but with your front-mount one, it’s not that simple.

The relevant question is where the light path is effectively limited.

Or, to put it another way: Does all the light collected in your converter’s front lens reach the sensor, or is some part of it blocked by the front opening of your base lens? If part is blocked, then the large converter front lens doesn’t help and is just a waste of material.

As a quick check, you can detach the combo from the camera body and look into it from the rear side, a few centimeters behind the lens, roughly where you’d expect the sensor. Do that with aperture full open. If you can fully see the circular edge of the converter’s front lens, then the 40mm-based calculation is indeed valid (all the light collected on that 40mm circle reaches the sensor). If you don’t, that means that the 30mm opening of the base lens still is the limiting factor, and the 30mm calculation will probably give better results.

And, if you want to do some experiments, compare the exposure times for full-open shots with and without the converter (of course, in a constant-lighting situation). If there’s a factor of 4 between the two times, the classical calculation applies, if it’s a factor of about 2, your 40mm-based calculation is correct.

Having said all that, I bet that you won’t be able to see the converter front lens edge, and that the exposure-time experiment will result in a factor of 4.