Original double gauss vs sonnar optical design

Using focal length of 100mm f/2 as example

Double gauss:

Double gauss performance:

To solve a double gauss using zemax YouTube

Sonnar performance:

Datasheet of sonnar f/4 13.5 cm

Another source: 100mm f/2

Obviously a copycat:

Zeissikonveb:

If you look at the data sheet above, you will be amazed: no heavy or even very heavy crown is needed, no barite flint or similar "exotic" glasses. The two front convex lenses are made of simple boron crown BK7 - one of the cheapest crown glasses that can be produced in large quantities. However, it has decisive advantages if you do not need particularly high refractive indices: it only splits the light into its primary colors to a small extent, which makes chromatic correction easier. Since the 1890s, it had become the standard material for prisms due to its high transparency and, due to its low thermal expansion, it was also often used as a base for mirrors. Only the subsequent flint glass F9 was relatively new in the glass catalog.

The result is barely perceptible spherical zones and a practically completely flat image field. Distortion is also very low due to the limited angle of view of one degree. It is simply astonishing how the simplest version of the Sonnar type has achieved maximum image performance with very moderate use of materials. The Sonnar 4/13.5 turned out to be compact and slim - it was only heavy because of the chrome-plated brass frame that was common at the time. But there is no need to rave about this lens in more detail at this point: There is a reason why the 13.5 Sonnar is one of the longest-produced lenses in history .

Marco Cavina on sonnar 50mm f/1.5:

One of the major advantages of the classic "Sonnar type" is the fact that it corrects coma flare at full aperture, allowing you to obtain sufficiently contrasted images even at f/1.5 or f/2, even without the use (prevented at the time) of very high refraction glasses, an unavoidable requirement to obtain the same effect with the Gauss type, even if this technical limitation poses difficulties in correcting astigmatism up to the edges; furthermore, the markedly symmetrical design of the Sonnar makes it very difficult to correct pincushion distortion, which is very noticeable and by its nature has a backfocal space that is too short to adapt normal focal length lenses to a reflex... This last characteristic, however, turns into a strong point because, together with the aforementioned asymmetrical structure, it allows you to create Sonnar lenses with a brightness of f/1.5 characterized by an admirable compactness,

it presents a superior resolution with the lens fully diaphragmed, because the Sonnar intrinsically suffers from a marked focus-shift that prevents it from significantly increasing its performance compared to the full aperture values: for this reason, the Contax Sonnar 50mm f/1.5 closed only up to f/8 (beyond this value the focus displacement was too noticeable), and even the modern Zeiss Sonnar ZM 50mm f/1.5 CT*, despite a revised scheme by air-spacing the front group and eliminating the Krown fluor glass, presents a similar drawback, to the point that two versions were made, chronologically consecutive: the first with the telemetric focus plane calibrated at f/2.8 aperture (but at f/1.5 the blur was evident) and the second optimised at f/1.5, with focus shift becoming increasingly evident as the aperture progresses( when stopping down the aperture).

Btw, "focus shift" is not an issue anymore in modern rangefinders with lcd and liveview.

Comparisons:

Double gauss:

Pros:

Double Gauss is closer in symmetry, so better in coma, distortion, and transverse chromatic aberration

back focal length is long. Good for SLR

Example: Nikkor -P 85mm f/2, 105mm f/2.5,

-P.C. 105mm f/2 xenotar/biometar . Opticallimits

Sonnar

Pros:

Larger aperture / fast optics

Better in spherical aberration

Better in sagittal abberation

Better in higher order coma

Cons:

Pincushion

Focus shift. ( Makes almost impossible to accurate focus at middle apertures , or close shooting distance s)- a non issue in modern Leicas with liveview feature

Skyllaney UK resurrected the Bertele sonnar 50mm f/2 and selling at 710 GBP( sold out). 35mmc review.

OmnarBertele, 50mm f/2, introduce floating lens block design

Matt Osborne review 50mm f1.5

Cong Soft and glowy look

Reference

Pencil of rays

Rangefinder forum

Marcocavina on sonnar 50mm. Very good review

MSU on aberrations

Sonnar 50mm with MTF

Modern days sonnar with MTF

Camera wiki