Canon Patent Application: Canon RF lens convertor

Canon has applied for a patent for an RF mount teleconvertor of a strange design.  This isn't your normal teleconvertor.  It will take APS-C lenses, and enlarge the image circle for full frame as well, as increase the focal length.  Basically with the 1.6x teleconvetor, you'd get your 1.6x image crop benefit back.

It could have a dramatic impact to those people using APS-C cameras to get more "reach".  Effectively you can move up to full frame and still have your reach using this teleconvertor with your existing APS-C EF-S lenses.  Lenses like the 55-250mm EF-S which is quite good, all of a sudden look that much better on say, and EOS RP camera.

We are going to coin this as a focal enhancer.

AThere is known a rear converter lens (denoted as converter lens below) capable of increasing the focal length of an entire system when being arranged between an interchangeable lens and an image pickup apparatus.

What's surprising is that this teleconvertor doesn't seem to be limited to L lens telephotos, but Canon demonstrates it on a 24mm F2.8 lens and comes up with the two nominal Canon teleconvertors; 1.6x and 2.0x. The standard lens design used with the teleconverter in the examples appears to be the Canon EF-S 24mm F2.8 pancake lens.

Various items of data of master lens

Focal length 24.50
F-number 2.88
Half angle of view (degrees) 29.14
Image height 13.66
Lens full length 64.83
BF 35.68

Two the embodiments that show the effect of the teleconvertor. Of key note is the increase in the image height, it's now full frame, with an expansion of the image height from 13.66 to 21.64.  Of course, your F-number increases by a little more than 1 stop.  The back focus of the combination is the RF mount, with a back focus distance of 13.08mm.

Focal length 39.21
F-number 4.61
Half angle of view (degrees) 28.89
Image height 21.64
Lens full length 88.80 
BF 13.08
Various items of data of converter lens
Focal length -83.09
Lens structure length 42.40
Front principal point position 0.34
Rear principal point position -36.78
Enlarging magnification 1.60

The next embodiment we are showing also increases the image height, and the back focus is certainly RF mount, it has a 1.5 EV stop increase to the F-number and a doubling of the focal length.

Focal length 49.27
F-number 5.79
Half angle of view (degrees) 23.71
Image height 21.64
Lens full length 97.18 
BF 11.00
Various items of data of converter lens
Focal length -81.18
Lens structure length 52.85
Front principal point position -9.30
Rear principal point position -71.02
Enlarging magnification 2.01

US Patent Application 20190391364