This is a performance analysis and review article of the FujiFilm Fujinon XF 56mm F1.2
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.
FujiFilm's X Series interchangeable lens digital cameras began with the FujiFilm X-Pro1 released in 2012.
The most notable feature of this camera system is the film simulation.
This is a scary camera system that is gradually expanding its power even in the stagnant camera market with a special function that grabs the hearts of camera owners including me.
By the way, the XF56 1.2 that I am introducing this time was released in 2014, so it is a large aperture middle telephoto lens that has existed since the early stage of the series.
The image sensor size of the Fuji X Series is the APS-C size, which is one size smaller than the full size, so the focal length must be "converted".
When the focal length of XF56mm is converted to the full size, it is equivalent to about 85 mm, which is the angle of view of the so-called portrait lens.
This lens is available in two models, "Mujirushi" and "APO". APO is a model with a built-in apodization filter, which makes blurred images smoother.
This article is the first analysis of the APS format lens in this blog, so this time I will explain the relationship between the difference in sensor size and aberration.
First, as a basic characteristic of the optical system (lens), there is a proportional relationship in which the aberration is reduced (reduced) when the entire lens is reduced.
The diagonal length of the full-size image sensor is about 21 mm, and the diagonal length of the APS-C size image sensor is about 14 mm. Therefore, a simple calculation shows that if the full-size lens is reduced to about 66%, it can be used as a lens for the APS-C sensor.
Even if you say 66%, I do not feel that it is a big difference, but if the length and width are reduced by 66%, the weight will be reduced to less than 1/3, so there is a huge difference. (It is just a simple calculation.)
The figure below shows a comparison between the typical APS-C size and the full-size image sensor size. (Figures vary slightly from company to company.)
Now, this is the main point.
There is a question of "How can I compare the performance of the APS lens with the full-size lens?" However, in this blog, we decided to make it possible to compare the full-size analysis results and graphs side by side by "making the scale about 66%" of the aberration diagram, etc.
Intuitively, I'll compare the lenses as if they were the same viewing size (print size) for both the full size and the APS-C.
However, there is a point to be noted. Although this comparison method is appropriate as a comparison of aberrations, for example, "when A3 printing is performed from the full size" and "when A3 printing is performed from the APS", since the stretching magnification is different, the small APS of the image sensor is more disadvantageous than the amount of aberration due to the degradation caused by stretching.
In addition, please understand that true comparison is difficult because the smaller the size of the image sensor is, the larger the noise becomes.
Since the XF lens is a modern lens, if you look for a patent, it will come out for a while. Assuming that Example 3 of Japanese Patent Application Laid-Open No. 2015 / 141384, which has been found, is commercialized, the design data is reproduced below.
In addition, I discovered several XF lenses at the same time as this lens, so I would like to analyze them one by one.
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 Fuji XF 56 mm F1.2.
It is composed of 11 lenses in 8 groups. The 2nd and 3rd lenses use ED lenses (special low-dispersion material) to correct chromatic aberration well, and one aspherical lens is introduced.
Three lenses are attached to the image sensor side of the stop. The configuration of attaching three lenses is rarely seen today. With the development of coating technology, it is not necessary to attach three lenses as a measure against the decrease in the amount of light due to reflection. Since the number of lenses in the configuration has increased compared to the past, it is not necessary to attach three lenses forcibly.
Even so, the fact that 3 sheets are pasted is probably an extremely effective structure for correcting aberrations. With 3 sheets, the lenses are arranged in the order of convex and concave, but the front and back convex lenses seem to use the latest ultra-high refractive index material with a refractive index of over 2.0. The key to this product is this 3 sheets.
Since the Fno is as large as 1.2, the entire lens has a larger diameter than the image pickup device. If this lens is used in the full size, it will be considerably heavy, but it fits in a cute size due to the advantage of the APS.
The FujiFilm X series is a mirrorless system, but we analyzed the SONY 85 mm F1.4 GM as a mirrorless mid-telephoto in the analysis data in the past.
The Fno is different between the 1.4 and 1.2, but it happens to be an example of a full-size lens with the same number of lenses, so please take a look.
Graphs of spherical aberration, image surface curvature, and distortion
Spherical Aberration , Axial Chromatic Aberration
The scale of the aberration diagram is set to approximately 66% of the full-size analysis so that it can be compared with the full-size lens.
Looking at the spherical aberration, it seems that it is a large aperture lens of the F1.2, but it has a fairly amazing correction with the characteristics of a nearly linear level.
Although the time and focal length are different, if you look at the analysis result of the Zuiko 50 mm F1.2 as the lens of the F1.2 analyzed in the past, I think you can see how amazing the correction is done.
The axial chromatic aberration g-line (blue) is slightly larger than the SIGMA Art series, which is easy to understand as a modern design example, but this is a large aperture of F1.2, so it can be said that it is doing well.
Field curvature has also been corrected to match the beauty of spherical aberrations.
The maximum value of the vertical scale in the field curvature and distortion graphs is 14.1 mm, which was calculated and determined from the vertical and horizontal size (23.5 x 15.6 mm) of the image pickup device listed in the FujiFilm X-pro3 specifications.
A lens with a focal length in the mid-telephoto range has a focal length that makes it easy to reduce distortion, but it seems to be finished to a level that can be said to be almost zero.
Lateral Chromatic Aberration (Magnification Chromatic Aberration)
As expected of lateral chromatic aberrations Although the g-line (blue) is slightly larger in the 14 mm corner of the screen, it has been sufficiently corrected up to the 11 mm periphery.
(Left)Tangential direction, (Right)Sagittal direction
Let's look at it as lateral aberration.
There is coma aberration in the tangential direction at the intermediate image height, but it is sufficiently corrected considering the large aperture of the F1.2.
I can feel the pain of axial chromatic aberration a little.
Spot Scale 0.2 (Standard)
The results of the optical simulation will be shown from here, but let's look at the spot diagram first.
The sagittal coma flare is also well corrected. It is hard to believe that it is a large aperture of F1.2.
Spot Scale 0.07 (Detail)
This is a scaled up view of the spot diagram.
Maximum Aperture F 1.2
Finally, let's look at the results of the MTF simulation.
The MTF of the open Fno1.2 is extremely high, which is hardly comparable to that of the F1.2, and the degree of coincidence of the vertices is also excellent. This indicates that the resolution performance is extremely high throughout the entire screen.
Since the scale of the evaluation frequency is reciprocal, the more the number of prints, the finer (more severe) it is. Therefore, the ratio of the full size and APS is on the larger side. Therefore, if the normal index of 20 prints / mm is multiplied by 1.53 and the result is evaluated more severely as 30 prints / mm (rounding off), it becomes the condition of "viewed with the same printing magnification".
By setting it to 30 / mm, it can be evaluated side by side with a full-size lens (20 / mm).
Since it is a large aperture lens, I also confirmed the F2.0, which is in the middle. At this point, the characteristics have already been improved to a level close to the ideal value.
Small Aperture F4.0
Check the MTF with a small aperture of Fno down to F4. In general, this reduces aberrations and improves resolution.
The focal length is equivalent to 85 mm on a large aperture F / 1.2, so before analyzing it, I thought it was a "slightly tasteful lens," but I was surprised at the extremely high performance.
In addition, it is not strange that the weight reaches 1 kg with this specification of a full-size lens, but this product is surprisingly 405g in weight and φ 62 in filter size.
Although it is a benefit of the APS size, this exquisite product size and weight, and the balance and high level of perfection as a tool born from it, is a part of the secret of the FujiFilm camera system loved by professionals.
Example photos are in preparation.
If you are looking for analysis information on other lenses, please refer to the table of contents page here.