DX on FX, a success, with a TC!

Tord S Eriksson wrote:

Over in another discussion forum, Cosmicnode asked if anyone had tried using a Nikon AF-P 70-300 VR DX lens with a Kenko TC on an FX camera, would it work, and if so, how well?!

I've been recommending this for a while. At Electronic Imaging 2016, I published Mixing and matching sensor format with lens coverage , which showed that the APS-C-lens-on-FF -body-via-1.5X-teleconverter trick works very well indeed.

Surprising results for me, do you have any other examples?

Here is a sample from that paper using a cheap APS-C fisheye -- arguably the type of lens one would least expect to produce good results on a teleconverter:

Opteka 6.5mm APS-C rectangular fisheye with 1.5X teleconverter on FF

Crop from the white rectangle above showing quite good quality

Aside from the fact that a 1.5X teleconverter will reduce the light by about 1 stop (sqrt(2) magnification would be precisely 1 stop), here are a few key issues:

1. The teleconverters are not very accurately labeled. Some 1.4X converters are actually magnifying more than some 1.5X (and even one marked 1.6X). However, lens coverage is usually not too tight on the native format, so there's usually enough extra coverage to be ok. The possible exception would be lenses designed for Canon's 1.6X crop not-really-APS-C cameras... but most of those still have coverage of 1.5X APS-C.

2. Many teleconverters do not properly adjust lens info electronically passed through the teleconverter. Some are simple pass-thrus that don't adjust focal length and aperture to reflect the presence of the teleconverter, which can cause various issues, the worst being incorrect IBIS correction of shake and flash exposure issues. Some teleconverters don't even implement pass thru.

3. Optical quality of teleconverters varies a lot. A lot of teleconverters are optically lousy, but some are quite good. Happily, the good ones aren't necessarily more expensive than the bad ones.

The bad news is that it can be hard to figure-out which are the good ones by #2 and #3 above without actually trying them. I've had teleconverters that were the same brand and apparently very similar model numbers, but completely different in terms of electronics and optical quality.