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Does anybody know of a reasonably priced teleconverter for the 70-300 mounted on an A7Rii? We are traveling to Europe and could use extra reach, but don't want to break the bank on the 70-200 with it's tele.

Tested the new GM FE100400 lens on my A7RM2 today with a quality printed color and B&W target. The GM100400 is an excellent sharp lens at f6.7 and outstanding at f8 and f11. Wide open (f5.6) at 400mm it is quite good but some blurring of 2 & 3 point type is seen. At f8 2 point type is SHARP at 400 mm and almost equally sharp at 560 mm and f8-11 with the Sony 1.4X TC. The 100-400mm lens even operates reasonably well at 800mm with the Sony 2X TC, where 3 point type is fairly good and 6 point type is sharp. Of course contrast is lowered with the TC’s but not significantly for the 1.4X TC. No chromatic aberration was noted with this lens. I compared the GM 100400 with my (generation 1, 2014) Tamron 150-600mm lens. With a Sigma 2X TC at 720mm, 3 point type was good and 5 point type reasonably sharp. Without TC, at 600mm the Tamron is known to be much less than optimal and in this comparison test that proved to be the case: at 600mm very good type had to be 5-6 point type and even larger type could not be said to be truly sharp. Plus all tests required chromatic correction with the TAMRON lens. I did not test tonight at 500mm with the TAMRON, which should give better results than the 720mm using the 2X TC. In each test I moved the tripod so as to fill the frame with the target, so there is no correction needed for distance.

On Dec 24th I shot the Full Moon from Marco Island using my Sony A7R M2 and a new Tamron 150-600 mm lens with Sigma EX 2X teleconverter mounted using Sony LAE3 adapter. The RAW shot was taken on a good tripod using ir remote trigger, ISO 64, 1/13 second, f13 (essentially the most open possible), and manual focus, stabilization turned off and on for subsequent shots. At 1200 effective mm the moon filled something less than half of the short dimension of the frame. I processed in Lightroom and Photoshop with limited optimization, mostly increased contrast and clarity, and moderate smart sharpen, although I did use lens correction which eliminated most of the chromatic aberration. I cropped the photo so that limited black surrounded the moon, final jpg size was 2.4 MB; the uploaded photo is 800kb, JPG level 10). I set the camera to electronic front curtain shutter but did not use full electronic shutter option. I did not see a significant difference when I shot with and without in-camera image stabilization. I had separately tested the TAMRON lens when bought and it was not nearly as fine detailed or contrasty as my SONY 70-400 mm G lens but it was still pretty good, particularly at f11-14. Use of the teleconverter did not degrade the TAMRON as much as for the better Sony lens, which might be expected.