Canon has published a pretty interesting optical design patent for large aperture zoom lenses. The two designs in this patent are pretty interesting.
Canon’s goal in this patent
To provide a zoom lens having a short total length and a small size but having high optical performance, and an image pickup apparatus having the same.
As you can see, these lens designs use the “reverse” zoom design like the original EF 24-70mm f/2.8L USM. This probably helps in reducing the length and size of the lens. As you can see, the image height changes when zooming in and out.
Overall length is quite impressive at 105.42mm (about 4″) at the wide end. Keep in mind that those measurements are from the image sensor to the front element of the lens design.
Focal length: 28.84mm – 67.90mm
Half angle of view: 32.68° – 17.67°
Image height: 18.50mm – 21.64mm
Overall length: 138.40mm – 105.42mm
Back focus: 10.97mm – 21.12mm
Focal length: 28.84mm – 60.00mm...