This is a performance analysis and review article of the SONY Sonnar T* FE 55mm F1.8 ZA.
You hardly understand the specific differences in how the lenses work and how their performance differs from each other, do you?
Even if you look it up in magazines or on the Internet, all you will find are similar "word-of-mouth recommendations" and articles like that.
In this blog, while researching the history of lenses and their historical background, we estimate lens design performance based on patent information and actual shooting examples, and analyze lens performance in detail from a technical viewpoint through simulations.
Professional lens designer Jin Takayama will carefully unravel optical characteristics such as optical path diagrams and aberrations, which are generally not visible, and explain the taste and descriptive performance of lenses in a deep and gentle manner.
Now, please enjoy the special information that you can read only on this blog in the world.
As of this writing (2021), there are 3 types of SONY E mount lens products with focal length around 50 mm.
- FE 50mm F1.8 (SEL50F18F)
- Sonnar T* FE 55mm F1.8 ZA (SEL55F18Z)This page
- Planar T* FE 50mm F1.4 ZA (SEL50F14Z)
The single-item FE 50 mm F1.8 has a so-called double Gaussian lens configuration, which is not particularly interesting as a lens for mirrorless mounts.
The three item FE 50 mm F1.4 has been analyzed in the past, and the result was an amazing result that seems to be the best performance for the 50 mm F1.4 that is worthy of the name of Zeiss brand.
Related article: SONY Planar T * FE 50 mm F1.4 za
The FE 55 mm F1.8, which is the 2 items mentioned in this section, is a lens that has been given the Zeiss name, which is said to be a high brand among SONY lenses.
With a focal length of 55 mm, which seems to respect the old lens, and a modest Fno of 1.8, the imaging performance is expected to be sufficiently high.
Now, this lens has the traditional name of a Zeiss lens.
The classic name is Sonnar, an optical system for rangefinders originally invented in the 1930s.
It was named after the optical system that was superior to the Gauss type in the era before the spread of SLR cameras, and was highly evaluated in the market.
Sonnar is something like a brand name and not necessarily "this shape".
In modern terms, for example, SIGMA's Art line and NIKON's S series.
This time, I will introduce the most famous composition in Sonnar.
The above figure shows a 50 mm f / 1.5 optical system with a standard angle of view, which is the most famous of all Sonner-type optical systems.
What is characteristic and easy to understand is that the 2nd, 3rd, and 4th lenses are attached, and the 5th, 6th, and 7th lenses are also attached.
Especially the 2nd and 3rd lenses are the same convex lens, but they are stuck together. It has a unique look that is rarely seen in modern times.
Why does he look like this? In the first place, there is no point in attaching convex lenses together in terms of imaging performance.
Why did we bother to stick them together? The reason is to improve the transmittance.
First of all, the lenses are colorless and transparent, and they allow light to pass through, but at the same time they reflect a little.
When the transmittance is low, the energy of the light that reaches the image sensor decreases no matter how bright the Fno is, and as a result, the exposure time must be lengthened.
In addition, the reflected light is irregularly reflected inside the lens barrel, resulting in harmful light called ghost or flare, which reduces the contrast of the photo.
This reflection of light occurs at the "interface where there is a difference in refractive index." Air has a refractive index of about 1.0, and a typical lens (glass) has a refractive index of 1.5 to 2.0. In other words, a large reflection occurs at the interface where the air and the lens come into contact.
On the other hand, if you put lenses together, the reflection will be reduced and the transmittance will be improved.
In the days when coating technology was still in its infancy, lenses were bonded together as much as possible to prevent a drop in transmittance, and image quality was also improved.
Note that there is an approximate 4% light loss at the air-lens interface.
If the optical system is composed of five sheets with no coating and no bonding surface, how much transmittance reduction will occur?
Since there are five single lenses, there are 10 interfaces between the lens and air. If there is a 4% loss on each surface, the optical system as a whole will have a light loss of 0.96 times 10 times (10 surfaces), and the light reaching the imaging surface will be reduced to about 60% of the input energy.
It can be said that nearly 40% of the energy acts as harmful light.
On the other hand, since only the refractive index difference of the glass is reflected at the bonding surface of the lenses, the loss due to reflection is reduced to less than half (depending on the combination).
In the modern era, multilayer coating technology has become common, and even an optical system consisting of about 20 lenses does not cause a noticeable decrease in transmittance.
Before the development of coatings, the Sonner type was highly evaluated in the market for its high transmittance and high-contrast imaging performance.
By the way, even if you say Sonnar, there are various improvement types. Please note that this configuration diagram is not the only one that is called Zona.
Well, since it's a recent product, the patent is easy to understand. It's Patent Publication No. US9491348B2.
By the way, if you look at the patent applicant, it seems that the Zeiss lens has been applied for by Sony. I see. It's the same method as the Planar FE 50 mm F1.4. It's like "training in Germany". Let's refrain from expressing it any more. Now, assuming that Example 1 has been commercialized, the design data will be reproduced as follows.
The following design values have been selected and reproduced from the appropriate patent literature and do not correspond to the actual product. Naturally, the data is not guaranteed, and I am not responsible for any accidents or damages that may occur by using this data.
Analysis of Design Values
Optical Path Diagram
The above diagram shows the optical path of the Sony FE 55 mm F1.8 za.
It is composed of seven lenses in five groups, and three aspherical lenses are used to improve the imaging performance.
I can only think of the NIKON NIKKOR Z 58 mm F0.95, which has three aspherical lenses for a focal length of around 50 mm.
Before being impacted by an aspherical lens In the first place, if you look at the cross-section.
WHAT WAS Sonnar LIKE IN THE PREVIOUS CHAPTER?
I WONDER IF I REALLY NEEDED TO EXPLAIN IT SO MUCH.
It looks totally different.
It is completely different from the double gauss type which is said to be the origin of modern lenses.
However, if you look closely, it looks like a symmetrical optical system in which negative lenses are placed before and after the reverse Tesser, and it seems to have a reasonable configuration as an optical system for mirrorless that enables short-back.
In addition, it can be said that it is the origin of the new configuration often seen in mirrorless optical systems in which the first surface is a concave surface, which is not rare recently.
Another feature is the use of internal focusing, in which only one fifth lens is moved during focusing, to achieve quiet focusing.
The general double gauss brings out the whole, so the noise and vibration may be a big concern, so it is a great progress.
Graphs of spherical aberration, image surface curvature, and distortion
Now let's check the aberration.
Spherical aberration is horribly linear.
I think they aimed at thorough correction by putting in 3 aspherical lenses.
In the same way, the field curvature has a very restrained characteristic.
A typical double Gaussian optical system has a distortion of less than 2%, but the distortion is much smaller.
Lateral Chromatic Aberration (Magnification Chromatic Aberration)
I wouldn't say that the lateral chromatic aberrations is extremely small, but it is moderately restrained, straight, and has no peculiar shape. The phenomenon that the color bleeding differs depending on the image height position of the screen does not occur.
(Left)Tangential direction, (Right)Sagittal direction
Let's look at it as lateral aberration.
It seems that the coma aberration in the tangential direction has been thoroughly suppressed. The Fno is a moderate 1.8, so the sagittal coma flare is also at the minimum level.
Spot Scale 0.3 (Standard)
The results of the optical simulation will be shown from here, but let's look at the spot diagram first.
The aberration is thoroughly suppressed, and the Fno is modest at 1.8, so the spot display of the standard scale is too small and too good to see.
Spot Scale 0.1 (Detail)
Because the sagittal coma flare is suppressed, the V-shaped feel around it is also thin, and it seems to be a naturally arranged description.
Maximum Aperture F 1.8
Finally, let's look at the results of the MTF simulation.
As shown in the aberration diagram, the performance is sufficiently high.
It is slightly inferior to the NIKON Z 50 mm F1.8, which has been found to have the highest performance among the lenses analyzed in the past, and it can be said that it has the same level of performance in practical use.
Small Aperture F4.0
To be honest, I thought it would be a standard performance because it was released relatively early as a lens for mirrorless, but I was surprised that it was an original optical type and very high-performance.
As expected, the name of the Zeiss lens was not Date.
As the analysis of the 50 mm F1.8 lens has progressed in recent years, I would like to produce a comparative analysis article in the future.
Example photos are in preparation.
If you are looking for analysis information on other lenses, please refer to the table of contents page here.