The patent application (2024-140521) for the Canon RF 28mm f/2.8 IS STM has shown up in Japan. This is a little curious as we get to see the design criteria that Canon used, and how the theoretical lens should perform.

Here is the optical formula in the patent application compared to the actual lens. As you can see, it's the same lens.

Canon RF 28-70mm F2.8 IS STM

Here's the data on the lens, and I'm actually surprised that there is no image stretching at the wide angle with this lens as the image height is 21.64 which is the full frame image circle radius. So this is a good thing, and better than I actually expected from a consumer grade lens.

 Wide-angleMid-rangeTelephoto
Focal Length28.8049.0067.90
F-Number2.882.882.92
Half Angle of View34.9323.8217.67
Image Height21.6421.6421.64
Lens Length132.38144.25158.83
Back Focus15.3926.3134.87

There are other 28-70mm F2.8 designs in the patent application, but no other lenses with different focal lengths. It was a very specific patent application made for one lens and one lens only.

Canon describes the patent application as;

The present invention relates to a zoom lens, and is used in digital video cameras, digital still cameras, and broadcast cameras. The zoom lens is suitable for imaging devices such as cameras for photographic use and cameras for photographic use using silver halide film.

In recent years, there has been a demand for a zoom lens with high optical performance, high zoom ratio and small size for imaging optical systems used in imaging devices.

In order to obtain an imaging optical system with a high zoom ratio and small size, Patent Document 1 discloses an optical system having six lens groups. However, since the refractive power of the second lens group is strong, the chromatic aberration of magnification at the wide-angle end tends to become large, and the fluctuation of the curvature of field occurring during zooming tends to become large.

Therefore, an object of the present invention is to provide a zoom lens with high optical performance, high zoom ratio and small size. 

Japan Patent Application 2024-140521

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3 comments

  1. It did. Where were you?
    Must have been a sleep and miss read, I was thinking of something else. :unsure: This is what happens when I'm told no coffee for 24 hours before my stress test
    • 0

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