Teleconverters RF vs EF

Shawn1519 wrote:

Asking the question a different way, take any lens/teleconverter combo (staying with the same format of EF or RF), put an extension tube between the lens and the teleconverter, and how does that effect image quality? That is in effect what Canon did with their RF 600 F4 and RF 400 F2.8 lens by incorporating the RF to EF adapter into the lens.

The answer is badly. By how much depends.

That is not what canon did with those. The distance from the lens to the sensor is still exactly the same in the RF version than it was for the EF version. It is the same whether you used "EF + DSLR", or "EF + RF2EF + R-body" or "RF + R-body". That dimension is the critical one.

If you add TC to the mix, things change. The rear of the TC must be the correct distance from the sensor. That is again a constant. The distance itself is a matter of design - in that respect EF and RF TCs differ ... as does the one embedded in RF 800 and RF 1200 as it seems to be even further away from sensor than EF TC.

Now when you put the lens in front of the TC, the lens distance is no longer the one it expects. That difference is, however, built into the TC design. It corrects the distance difference created by its own length. No more and no less than that.

So if you use "EF + EF TC + DSLR", everything is fine. If you use "EF + EF TC + RF2EF + R-body" instead, everything is still fine. It is also just as well to use "EF + RF2EF + RF TC + R-body" ... as long as you can find a fitting adapter of correct length. It is even optically OK to use "EF + EF TC + RF2EF + RF TC + R-body". The distances play still correctly as both TC:s correct the distance difference caused by their own length.

However, if you put extra extension somewhere in between, things are no longer going to play nice. The strength of the effect likely depends on how much you add compared to how much distance there is supposed to be from the last element to the sensor. That's why it is really bad idea to put extra extension on body side of TC:S ... and particularly RF TC:s.