Curious result with teleconverters

[Shortsighted]

I noticed an unexpected result when I tried to compare a shot taken with a prime lens and a 1.4x TC vs the same lens with a 2x TC.  I mounted the lens on a tripod via the lens shoe.  I took a shot of a large knot in a tree across the street with the 1.4x TC attached and then took the same shot with the 2xTC attached.  Just for fun I left the 2x TC on the camera and added the 1.4xTC on the end of that.

CAMERA:  Canon 7D Mk II
Shot #1:  300mm f4 + 1.4x TC = 420mm f5.6
Shot #2:  300mm f4 + 2x    TC = 600mm f 8
Shot #3:  300mm f4 + 2x + 1.4x TC = 840mm f11  (manually focused)


Then I repeated the same thing using LIVE VIEW instead of looking through the viewfinder.

In the first experiment the 600mm  equivalent was softer than the 420mm and that outcome was expected.  The 840mm with manual focus was about equal to 600mm and I didn't expect that.  The ISO was at 1200.

In the second experiment using Live View I found that all the shots had better IQ (image quality).  The 600mm equivalent was almost identical to the 420mm.  What's more amazing is that the manually focused 840mm, while slow to focus, was almost as sharp.  So, why are the images sharper when using the slow live view AF than with the fast viewfinder AF?  Remember a time when all lenses were governed by manual focus?

[Shortsighted]

Example:  1/800th sec at f8 @ ISO 640

300mm f4 lens with 2X teleconverter = 600mm f8 lens.  With central focus point the Canon 7D Mk II can AF at f8.  While not is sharp as this lens is with the 1.4X TC attached, showing signs of softening at edges, it remains pretty good when using LIVE VIEW to focus.  Shortfalls in sharpness can be mitigated with post-processing.  I would like to try it adapted to an R-series camera using electronic shutter to reduce shake.  I'm looking into getting an R7 with adapter but it might not happen.

[Dr. John]

This goes beyond my rudimentary knowledge and comprehension. The photo looks lovely nonetheless.

[Shortsighted]

Just an additional note regarding the Canon 2X teleconverter ...  I've found that the camera needed an adjustment when coupled to my prime lens with the 2X TC attached and that no adjustment was needed when the 1.4X TC was substituted.  I used a long metal scale, about a meter long, and slanted it against a wall and then took a shot of a section of the scale face-on using the 2X TC plus prime.  Center focus was set, as it needs to be when used with the Canon 7D, Mk 2 when using AF at f8.  I made sure to focus on a selected spot.  When viewing the shot on the LCD screen it looked sharp, but then I magnified the image and found the 'selected spot' was soft-focus, but above this spot the image was sharper, which implies that the kit was focusing behind the 'selected spot' (higher up on the slanted scale).  I opened the camera's menu and selected microadjustments.  I then slid the microadjustment scale + 20% (slide to the left of center) and locked that adjustment into "300mm + 2X TC" so that the camera would automatically make correction when it sensed the presence of the prime and the 2X, but not making adjustment with any other lens combo fitted to the camera.  I've yet to test out this new setting on a bird because I have an upcoming date with a snow shovel.  I'm still not expecting good IQ with this 2X + prime combo but if I can manage an improvement, well then, my work is done.