[Leica] Re: Lens elements & groups

[Bert Otten <e.otten@med.rug.nl>]

The fact that more than one piece of glass is used has to do with a number of factors:

1)    Getting rid of chromatic aberration:

        any piece of glass has two important properties:
        n, the refractive index, which determines how much light rays are bent
        the dispersive power, a measure of how different colours of light have a different refractive index

        By combining different types of glass with different refractive indices and dispersive powers,
        it is possible to get rid of chromatic aberration, or the blue and red fringes in contrasty borders
        in the image

2)    Getting rid of spherical aberration

        All light rays coming from a pointsource need to be focussed in a single point. Thick lenses
        with spherical surfaces do not have that property. Partly this can be solved by combining
        lenses and using aspherical surfaces.

3)    Getting rid of geometric deformation

        Because single lenses have a thickness and are asymmetric, they do not behave perfectly in imaging
        straight lines in the object as straight lines in the image. By combining lenses this can be taken care of

4)    Placing principal planes in agreement with the space in the camera

        Sometimes lenses have a shorter focal length than space available between film and glass
        Then it is nice to place the back principal plane behind the lens.
        In telephoto lenses, you want to keep the physical length of the lens shorter than its focal length
        Again, you place the front principal plane in front of the frontlens.

This list is incomplete, but covers the main problems. So lenses with many pieces of glass can be better corrected
for the above imperfections. So usually more pieces means a better lens.

However, each group of lenses has two air-glass surfaces, producing reflections, which reflect on and on in
all groups. Thanks to multicoating, it is not as bad as it used to be.
Also, glass absorbs light, making it necessary to increase the aperture to reach the same effective
aperture, making the lens even heavier.

If you look at the data of top lenses in large format phototgraphy, they tend to have fewer elements than hightly
corrected lenses in medium and small format photography. This has to do with the principal planes: most
large format lenses are more symmetrical, having the planes closer to the middle of the lens. Top prime lenses in
small format photography can have up to 9 elements (for instance the 21/2.8 ASPH Elmarit). Zeiss produces
the 40mm Distagon FLE for Hasselblad with 11 elements. This is a highly corrected lens with extreme demands on
the principal planes.

So in short: many elements is usually a good sign, but only because of the multicoating and in the presence of
skilled lens designers using good ray-tracing software.

Reference: Born, M and E. Wolf, Principles of Optics, Pergamon Press, Oxford, New York.

Hope this sheds some light (pun intended),

    Bert Otten