DX on FX, a success, with a TC!

ProfHankD wrote:

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.

With your advice from older messages in mind and some positive reviews, thread reports and images collected over time I decided to adapt a secondhand Canon EF-S 55-250mm 3.5 IS STM to my A7RII. David Kennard used a Sigma MC-11 for that but in my case that combination did not work. https://www.davidkennardphotography.com/blog/1075-canon-55-250mm-stm-on-a7r-ii.xhtml So I bought a secondhand Metabones IV with some doubt that it would work with my other AF EF and chipped manual lenses. That turned out to be more than alright: https://www.dpreview.com/forums/post/64394061

The EF-S lens I had already shaved at the rear for the MC-11 and given a 3D printed cover there for a neater look. Not needed for the Metabones IV as that one allows EF-S lenses without modifications. However the FF compatible Kenko C-AF 1.4x Teleplus MC4 DGX teleconverter can now be used too and it looks like it does a perfect job, the max focal length is now 350mm and with little tests so far (weather) at least the center is tack sharp on long distances. Vignetting with the teleconverter on is hardly a problem throughout the zoom range. Without teleconverter a square image of approx 20-22mm is usable within the FF sensor. I added a 3D printed hood with a square mask that does not add more vignetting with and without the teleconverter. Lens probably covers more than APS-C or you could be right for the converter factors being more than indicated.

I measured the difference in length between the Sigma MC-11 and Metabones IV for what theoretically should be 26mm, Canon EF register distance minus Sony FE register distance. The Sigma is 0,05/6 mm less than that, the Metabones IV 0,12/13mm less. With zoom lenses, floating element lenses and I think the tele converters/speed boosters one should not underestimate that depth difference (On the Metabones Speedbooster there is a user calibration method integrated). For me there is another reason as I calibrated all my manual lenses to infinity for the MC-11 + A&RII combination. So I tweaked the Metabones IV to the Sigma MC-11 depth. Making them both 26mm might not be a good idea, getting a completely snug fit on one extra mount connection will not happen and a small piece of dirt already throws it over 26mm.

Shaved rear with 3D printed cap reversed to show how it will fit with some glue.

It is a long total with the 3D printed hood + Kenko + Metabones IV adapter. Minolta MD I 200mm 4.0 converted to EF mount next to it. Weight of the Minolta is however more than the 55-250 + hood + Kenko.

Met vriendelijke groet, Ernst
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