As promised, I tried again today, using a slightly different approach. Rather than taking all the images at the same distances and then cropping the shorter focal lengths to equal the subject size of the 2.0x teleconverter image, I equalized the distance in relation to focal length. Specifically, I took the bare 400mm lens image at 4', the 1.4x teleconverter image at 5' 7" and the 2.0x teleconverter image at 8", using a construction tape measure. The subject sizes in the images were still a bit different, but, at least, the depth of field was more comparable shot to shot vs. keeping the distance constant. I used the 42.4mp Sony a7R III for all three shots. These are handheld, so I took several safety shots of each and made sure to use the sharpest of the groups. Of course, stabilization was engaged. All are at ISO 100, f/11, 1/200-sec.
Bare Lens
Bare Lens by
David Stephens, on Flickr
1.4x Teleconverter
1.4x Teleconverter by
David Stephens, on Flickr
2x Teleconverter
2.0x Teleconverter by
David Stephens, on Flickr
I'm very happy with the bare lens and find the 1.4x teleconverter not far behind. I have no reservations about using the 1.4x for critical subjects like birds and furry mammals.
The 2.0x teleconverter has serious problems in my testing. First, I think that the varying images sizes has to do with the focal lengths being "nominal" readings. For instance, I've read that actual focal lengths for some 100-400mm lens may actually be closer to 105-385mm. I've never read the same things about teleconverters, but wouldn't be tremendously surprised if the Sony isn't actually closer to 2.1x. That said, there are sharpness issues with the 2.0x. Contrast and color are acceptable. The lens seemed to be back-focusing, being in focus behind the plane where the focus was taken. Despite taking the 800mm image from twice the distance of the 400mm image, the depth of field seems quite shortened. Look at the very fine bristles at the top of the bloom to see sharpness that makes me think that the lens is back focusing.