Canon claims that the use of two or more lens groups with anomalous dispersion glass and a negative refracting power reduce the size and weight compared to such a lens without the elements.
Analomous dispersion glass delays light passing thru it, but delays long wavelengths more than short wavelengths, so it works to correct chromatic abberations when the lenses are formulated properly.
Selections from the patent translation:
"The present invention relates to optical length's miniaturization and the zoom lens which can reduce especially a magnification color blot, and is suitable to especially a digital camera and a video camera.
The example which uses two anomalous dispersion glass for a negative lens with a short focal distance relatively in the second lens group of negative refracting power as an example which corrects optical performance, especially a chromatic aberration satisfactorily is disclosed.
Then, the present invention finds out the conditions for reducing a chromatic aberration, though it is related with a suitable zoom lens, the path of the second lens group of negative refracting power is made small and size and weight reduction of the second lens group of negative refracting power is attained, when mounting on a digital camera etc. "
Example 1.
Various data
Zoom ratio 19.42
Focal distance 41.09 92.80 152.20 225.00 797.99 323.00 575.00 781.02
F number 4.67 4.66 4.65 4.67 5.71 4.93 5.22 5.59 Field angle 27.79 13.13 8.10 5.50 1.55 3.83 2.16 1.59
Image height 21.65 21.65 21.65 21.65 21.65 21.65 21.65 21.65
Whole length of the lens 489.32 489.32 489.32 489.32 489.32 489.32 489.32 489.32
BF 39.28 39.28 39.28 39.28 39.28 39.28 39.28 39.28
Example 2.
Various data
Zoom ratio 19.70
Focal distance 40.50 92.80 152.20 225.00 798.00 323.00 575.00 783.52
F number 4.67 4.61 4.57 4.67 5.32 4.87 5.06 5.22
Field angle 28.13 13.13 8.10 5.50 1.55 3.83 2.16 1.58
Image height 21.65 21.65 21.65 21.65 21.65 21.65 21.65 21.65
Whole length of the lens 490.00 492.69 493.84 494.58 497.89 495.22 496.32 497.32
BF 40.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00
Example 3
Various data
Zoom ratio 18.18
Wide angle Middle Looking far
Focal distance 6.60 45.00 120.00
F number 2.80 4.01 4.00
Field angle 30.42 4.92 1.85
Image height 3.88 3.88 3.88
Whole length of the lens 135.94 153.33 155.71
BF 12.70 10.22 13.73
Example 4
Various data
Zoom ratio 16.67
Wide angle Middle Looking far
Focal distance 6.60 56.14 110.00
F number 2.80 4.10 4.00
Field angle 30.42 3.95 2.02
Image height 3.88 3.88 3.88
Whole length of the lens 137.20 156.46 153.20
BF 16.00 13.46 16.26
