Friday, 12 October 2012

Aspherical lenses

There has been a lot of improvement to camera technology lately. The fact that we have affordable consumer digital cameras is one thing, another is the improvement to image quality. Lately, one could argue that the image quality has reached a plateau level, and we are now mostly seeing improvements to features, like video and live view, and to high ISO capabilities.

However, it is easy to forget that we have also had significant development in the lens technology.

Most modern lenses involve some kind of optical compromises. This can be, for example, a large zoom range, a bright (fast) lens, a compact pancake design, very wide angle lenses. These lens types require clever and complicated optical designs.


Now, I don't claim to be an expert in optical design, but from what I understand, using aspherical lens elements is a key to avoiding poor image quality when there are significant optical compromises that must be met.

Today, it is not easy to find lenses that do not complicated like this. Most lenses involve a zoom, a large aperture, a compact design, a wide angle, or even some combination of these. Exceptions from this is the Sigma 30mm f/2.8 EX DN. It is not a compact lens, it is not very fast, at f/2.8. It is not a zoom lens, and finally, at 30mm, which is significantly more than the register distance, it does not require a retrofocal design.

For this reason, the Sigma Sigma 30mm f/2.8 EX DN has a very simple optical design, with only seven spherical lens elements in five groups, and still achieves an impressive image quality. In fact, I would argue, for the ultimate in image quality, one should look to simple lens designs like this: Not too fast, not too wide, and no zoom. These lenses are relatively easy to design, and in theory, perform the best. In practice, though, some lens manufacturers would invest the most prestige in designing fast prime lenses, and for that reason, the may be among the best performers as well.

Traditionally, most lens elements have been spherical, meaning that the surfaces, convex or concave, shaped like parts of a sphere surface. These are most easy to produce, and has been used for centuries, ever since Galileo Galilei produced spherical concave lenses by grinding glass sheets with cannon balls.

Aspherical lens elements have one or two surfaces which are not spherical. They can have some more complicated geometrical shape to optimize the sharpness across the image frame or other optical properties.

To produce large aspherical lens elements cheaply is a key factor in making affordable consumer lenses. So let's see what has happened with Panasonic's lenses over the time span of Micro Four Thirds. Here is the ratio of aspherical lenses used per lens, plot against the announcement date:


Some lenses are not in the list, for example the Lumix G 45-200mm f/4-5.6, Lumix G 100-300mm f/4-5.6, and Lumix G 8mm f/3.5 fisheye, as they have no apsherical lens elements.

In the diagram, we see for example that the Lumix G 14mm f/2.5 pancake lens is rated at 50% since it has three aspherical lens elements, out of a total of six.

In general, we can see that there is a trend towards a higher ratio of aspherical lens elements used.

The next diagram shows the diameter in mm of the largest aspherical lens element per lens:


In the diagram, we see that apart from the premium HD 14-140mm and wide angle 7-14mm lenses launched early in the timeline, there is a tendency towards using larger aspherical lens elements.

Finally, this diagram shows the average total aspherical lens area per lens price in dollars. So a high value means more aspherical lens elements for the price:


This picture shows most clearly that aspherical lenses have become less expensive to produce. The ultra compact Lumix X 14-42mm f/3.5-5.6 is on the top of the list, and achieves an impressive set of features in a compact form factor.

The new value tele zoom lens, the Panasonic Lumix G 45-150mm f/4-5.6 is also high on the list, indicating a high value for money in terms of lens design.

In the context of industrial design, the Lumix G 14-42mm f/3.5-5.6 basic kit zoom is one of my favourites. Considering the low price, and the low weight, I think it performs very well for its class. I have accidentally dropped it on a hard floor a couple of times, with no perceptible impact on image quality or function. I think this illustrates the superb design of this lens.

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