There was a period some time ago, that a high speed lens, that is a lens with an aperture of at least f/1.4 was regarded as the true hall mark of optical design for 35mm cameras. All major manufacturers had some of these lenses in their program, and some even dared to go where no one else had been and produced lenses with speeds of f/1.2 and even f/1.
Canon had an f/1 lens in their program, but in the latest catalogue there is no mention of this lens anymore and it seems to have phased out quietly. Leica now is the only company that offers a lens with an aperture of f/1. On the other hand, Canon recently introduced a new version of its well-known 1.2/85mm lens.
In this article I will review a number of these lenses in a loose historical order. It is impossible to give attention to every high-speed lens that has been made by a multitude of companies. The examples presented here, are however quite representative of what can be accomplished with current technology. The focus on the MTF curves implies that many important issues will not be discussed, but this reduction of topics may be helpful in illuminating the important issues.
It is not easy to design a really good high-speed lens and many compromises can be made: spherical aberration and coma can only be combated by contrast reduction, evenness of definition over the image area, focus shift, vignetting and a host of other optical parameters.
NOTE: read the diagrams with some care: I have used diagrams from Zeiss (giving 10,20 and 40 lp/mm) and Leica (giving 5, 10, 20 and 40 lp/mm). On superficial inspection of the diagrams one may see more differences than is available!)
Leica Summilux-M 1.4/50, first version f/1.4 and f/5.6

One of the first attempts to design a true 1.4 design was this Summilux lens. The step from the classical 1.5 design to a true 1.4 design seems to be simple, but is not. The designers chose to retain the classical Double-Gauss layout, but added split one element in two parts, thus creating a seven-element design. Most companies followed this principle, as it is indeed the most sensible method. But even within the same construction parameters, many versions are possible as we will seen. Wide open the lens has quite low contrast and the definition of fine detail is very soft, creating blurred details and a generally soft image. Stopped down to 5.6 the lens has excellent performance, but a strong curvature of field can be noticed at the 40 lp/mm lines.

Leica Summilux-M 1.4/50, second version f/1.4 and f/5.6

The second version of the Summilux (around 1962) offered a much higher contrast wide open, but only for the main structural outlines of the subjects. In the centre of the image the definition of fine detail was improved too, but this wide-open improvement was bought by a slight increase in curvature of field. The logic of this option was of course dictated by the use of the lens on a range finder camera in relation to documentary photography.

Leica Summilux-R 1.4/50, first version f1.4 and f/5.6

The R-version of the Summilux design hardly differed from the M-version. You can note that the R-version has a more even performance over the image field, but with a lower contrast in the centre of the image field.
Canon FD 1.4/50mm at f1.4 and f/5.6

The Canon lens designers used a seven-element design too, but the balancing of aberrations was different from the Leica approach. The famous FD design offered a quite low contrast wide open, but the quality over the image area from centre to edge differed less than in the Leica case.
Note that the Canon design shows hardly any astigmatism in comparison to the Leitz version.Stopped down the Canon lens gave outstandingly good imagery.
Nikon 1.4/50mm at f/1.4 and f/5.6

The Nikon lens offers almost identical performance wide-open as the Canon lens, but is slightly behind the competition stopped down. The differences are small and for all practical purposes we may say that both lenses perform equally well. Both Canon and Nikon follow the philosophy that the stopped down quality must be as good as what can be delivered by the normal f/2 lenses. Leica assumes that good wide-open performance is the primary goal of high-speed lens design.

Leica Noctilux 1.2/50mm at f/1.2 and f/5.6

Leitz was well aware of the limitations of classical designs when the maximum speed had to be increased. The use of aspherical surfaces was an option and Leitz introduced the 1.2 Noctilux as the lens that showed no compromise at maximum aperture. The lens indeed performed at f/1.2 as the best f/1.4 designs and offered a much higher overall contrast. Definition of fine detail you would not find in the Noctilux pictures wide open, but that was not the design goal of the lens. Stopped down the lens was on a par with contemporary designs, but showed the typical Leica fingerprint with a strong curvature of field (see the Nikon and Canon examples)
The Leitz design also has a fair amount of focus difference.
Canon FD 1.2/55 Aspherical at f/1.2 and f/5.6

The Canon design departs from the classical seven-element construction, by offering an eight element that is stationary to improve close-up performance. Generally it is quite close to the image quality offered by the Leitz design. Stopped down the Leitz design seems to have the edge in overall contrast, which is contrary to the Leica myths.
Nikon Nikkor-Noct 1.2/58mm at f/1.2 and f/5.6

The Nikon design of those days was not as good as the Canon or the Leitz. Especially the wide-open performance was quite weak as can be seen from the graphs. This does underscore the opinion that Canon is the best optical company in Japan. Nikon and Canon both cheat a bit by offering focal lengths of 55mm and 58mm, where Leitz used a true 50mm design.
Canon 1.2/50mm L at f/1.2 and f/5.6

Canon introduced the label L for lenses that offer the best of optical quality. The high-speed lens 1.2/50mm was one of these. Compared to the previous 55mm design the improvement is marginal at best. The wider angle of view (from 55m to 50mm) added design difficulties and the overall performance is close to what the predecessor offered. Flare reduction seems to have been improved.
And we should not forget that the physical dimensions of the lens have been reduced. It is very dfficult to provide the same performance while reducing the size. In this respect the new Canon lens deserves praise.
Leica Noctilux-M 1/50mm at f/1 and f/5.6

Leica was not happy with the high production cost of the Noctilux with two grinded aspherical surfaces and introduced a new version, now with the familiar seven-element design, but with very high refraction glass. With this construction, Leitz was able to stretch the maximum aperture to f/1, once described as 'faster than the human eye'. This neglects the simple facts of the pupil of eye, which has a maximum aperture of about f/3.5, relying on the sensitivity of the receptors to see in low light conditions. (This is also the approach of the modern solid-state capture).
The Noctilux became a sensation on the market as a lens with quite peculiar pictorial characteristics. Purely optical however, the design is not that spectacular, as the MTF curves indicate. It is generally accepted that the performance can not be improved without substantially changing the design, giving the lens a price tag that is beyond belief.
Zeiss Planar 1.2/55mm at f/1.2 and f/5.6

For many years Zeiss was not involved in the high speed race and settled for the more sedate 1.4 designs. But for once, Zeiss did offer a special design on the occasion of the Planar anniversary. The Planar 1.2/55mm design is based on the seven-element construction, but expanded to become an eight element seven group design. As such its potential for improvement might be increased. The design is good, but wide open one might feel a slight disappointment at the quality of the image. This lens design indicates that just adding lens elements is not the only key to improvement. One sees this as example in the Pentax Takumar 1.4 design that had seven elements, but wide open had a very soft definition compared to the standard six element 2.0 design.
Zeiss Planar 1.4/50mm at f/1.4 and f/5.6

The 1.2 lens has not been offered to the general public. Zeiss considers the standard 1.4 design as best suited for general 35mm photography. The seven-element version of the Planar has seen many lives. At first offered for the Contax RTS system, it is now available in Nikon mount. This design has been hailed by Popular Photography as the best high speed standard lens available and the marketing department of Zeiss claimed recently that this lens is the resolution champion with a resolution of 400 lp/mm. In fact the MTF analysis gives a maximum resolution of about 160 lp/mm, where the contrast goes to zero. This kind of promotion does the lens a disservice, as it is on itself a quite good lens, although not of the highest order.
Leica Summilux-R 1.4/50mm (2) at f/1.4 and f/5.6

Leica has also stayed on the safe side with its 1.4 designs for the R-system. Some studies in the past indicated that a 1.2 design would not be up to the Leica expectations. On the other hand, the company had to accept that the original version was no longer up to date and a new eight element design was introduced in 1998. It was not a major jump forward but stopped down the performance increased substantially, making it now a fully univeral standard lens.
Voigtlander Nokton 1.5/50mm at f/1.5 and f/5.6

The rangefinder concept offers more room for improvement. It was Cosina that picked up the gauntlet and introduced the Nokton 1.5/50mm, a quite large design to help suppress aberrations. It is definitely an improvement over the Summilux-M designs wide open, with a larger area of good definition. The price you pay is a less than satisfactory performance stopped down where contrast is low and images are a bit soft.
Leica Summilux-M 1.4/50mm ASPH. at f/1.4 and f/5.6

Leica, faced with the Cosina challenge in the rangefinder area, could not rest on its laurels and created the best high-speed standard lens to date. A new design with eight elements, a floating element and exotic glass types were thrown into the battle. The result is a quite compact design with outstandingly good performance. It is unlikely that we will see a lens with even better image quality. The combination of very high quality and compact dimensions is quite unusual as we have seen in the course of this review
This review of the development of the high speed lens has indicated that a truly satisfactory design is very difficult to create. With the exception of the current Summilux-M lens, most designs have been compromises of one kind or another. It is dangerous to make global assertions, but there are a few differences of character or fingerprint. Most Japanese designs go for an overall balance at the expense of the best wide-open performance, making their designs useful as general-purpose lenses. The Leica designs were often biased to deliver good wide-open performance, but with some lowering of the stopped down image quality. A general trend to be observed is the fact that Leica allows a higher level of curvature of field, where the Japanese and Zeiss designs are more trimmed to lower astigmatism. But one should be careful here and not over-emphasize subtle differences. But they are here for the discerning photographer.
There is some discussion in the Leica community whether the MTF graphs, provided by Leica and based on computation are as trustworthy as the Zeiss MTF graphs, based on actual measurement. Find below two graphs that give the measured MTF of the current Summilux-M ASPH and compare these graphs with the calculated ones above.