This is a performance analysis and review article of the PENTAX -D FA *85mm F1.4.
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.
Overview
"Among PENTAX lenses, there are the" "Star" "series, which is an ultra-high-performance flagship product, and the" "Limited" "series, which balances size and performance with the pleasure of ownership."
This article's PENTAX D - * FA 85 mm F1.4 is one of the * series. It is a large diameter medium telephoto lens with durability, dust and drip-proof without compromising the performance.
In the PENTAX lens series, the 85 mm focal length lens, which has been missing for a long time, was introduced in 2020 with the appropriate expression "long lasting".
Before this HD PENTAX 85 mm F1.4 went on sale, various rumors were spread on rumor boards and summary sites, such as that the development had been delayed or that it was going to be OEM (development / production by an outside manufacturer), and I was excited.
As a result, as summarized in this article, there is a patent application filed by PENTAX, so it is assumed that a lens made of pure PENTAX was released.
First, let's look back at the history of the 85 mm F1.4 in PENTAX.
First of all, PENTAX lens used to be called Takmar in old days, and it seems that F1.8 was the maximum aperture of telephoto lens in that period.
In the Takmar era Based on the information about the number of telephoto lenses, I can imagine that it is a "large Gaussian type".
And time has passed. In 1984, in the era of the PENTAX A series, the 85 mm F1.4 specification lens will be introduced for the first time as a star lens.
The following is the release date and the number of units of the 85 mm F1.4 specification lens released after that.
First gen: SMC PENTAX A *85mmF1.4 (1984) 6 /7
2nd gen: SMC PENTAX FA *85mmF1.4 (1992) 7/8
Third gen: HD PENTAX D FA *85mmF1.4 (2020) 10/12
groups/elements
All of the 85 mm F1.4 lenses are sold as high-performance series *(star).
In this article, we analyze the third generation lens that was released 28 years after the second generation lens was released.
Private Memoirs
About 15 years after the previous (second generation) SMC PENTAX FA * 85 mm F1.4 was sold out, the PENTAX 85 mm was finally renewed in 2020.
Why was the 85 mm F1.4, which is regarded as a standard item by each company, left out for a long time? I don't know the internal circumstances, but I feel PENTAX's determination when it is re-released as a set with the "Optical Finder Declaration" which PENTAX suddenly swore in 2020.
In the past, we analyzed the SMC PENTAX FA 85mmF1.4, the second generation 85mmF1.4.
SMC PENTAX FA 85mmF1.4
By reducing the size and weight of the focusing lens group, the second generation 85 mm lens realizes a truly stylish and smart optical system. This product is like the foundation of modern large-aperture single-focus lenses.
Let's analyze how PENTAX came up with the answers from the 28th year, whether it was because the walls of the 2nd generation were thick or because of the weight of *.
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Document Survey
The analysis of the predecessor, the second generation 85 mm, was conducted as an event to commemorate the release of the third generation.
In Japan, it takes about a year and a half to two years for a patent application to be published.
If a patent is filed immediately before a product is released, the longest time that a patent is published is one year after the product is released.
Of course, there are cases where patents are not applied for for some reasons, and there are also manufacturers who hardly apply in the first place.
PENTAX is basically a manufacturer that applies for patents at the time of product release, so I was going to wait patiently for a while.
However, the day came much earlier than expected.
The third generation 85 mm went on sale on June 26, 2020, and its patent, JP-A 2020-154060, was published on September 24 of the same year.
I would like to express my gratitude to PENTAX and the Patent Office for their timely response that is not too early nor too late.
Assuming that Example 3 similar to the cross-sectional view is commercialized from this patent document, the design data will be reproduced below.
Notes!
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 HD PENTAX D FA 85 mm F1.4.
It seems to have 12 lenses in 10 groups, one aspheric lens on the side closest to the image pickup device, and two special low dispersion glasses (ED).
Based on the configuration of the second generation 85 mm optical system, which focuses on the rear group of the image pickup device side, which is slightly distant, we can see that we have increased the number of lenses and improved performance.
What is quite shocking is that the first surface of the first lens, which is the side closest to the subject (left side), is concave.
IN OTHER WORDS, WHEN YOU LOOK AT THE LENS FROM THE FRONT, THE LENS SURFACE IS CONCAVE.
It doesn't mean that there is no concave lens in front.
There are many mirrorless lenses these days, but it's quite rare for the lens for single-lens reflex camera with mirror.
For example, the current Nikon F-mount lens is said to have a concave shape on the front side. (There is a Z.)
I thought it was rare, but there was a description of it in the patent document.
[Claim 5]
Cited from JP-A 2020 / 154060
The first lens unit is a negative lens, the concave surface of which is directed toward the object side located closest to the object side.
After all, this first lens is the liver of this optical system. The explanation that it contributes to the good correction of the aberration is also described in the text.
Let me explain a little why I am so surprised by the concavity.
When I saw this shock concavity, a "legendary master" descended in my mind. (Note: "Legend" is a story in my brain, and it's just an ordinary person.)
Several decades ago, when I was just starting out, my master in optical design taught me the following. The figure below is his memory chart.
"Boy, don't make the first lens concave, or you'll be excommunicated."
(This story is so old that it has been heavily dramatized, including the illustrations.)
In response to this statement, I, a young man, said, "I don't have any other suitable job, and it's not good for me to be excommunicated." and swore that I would stick to this teaching without considering any reason.
By the way, Master, you look very familiar. I don't think it was floating back then, but.
Now, returning to the main point of the subject, I will consider the reason of "The concave is the excommunication" again today.
First of all, if the first side of the lens is convex, it looks great and you feel like your soul is being sucked in, and you will say, "I think I can keep looking." (Note: this effect varies greatly from person to person.)
On the other hand, if the first surface is concave, the effect of sucking in the soul is not only weak, but also the effect of making the inside of the lens look smaller by illusion by looking through the concave lens becomes stronger.
In the old days, most of the lenses were small, partly because they had manual focus. Therefore, in order to create a high-class look, there was a desire to make them look bigger and better. Therefore, the concave surface on the first side was probably prohibited.
However, as time passes and only super-large lenses are allowed in this modern age, they may have broken the boring custom by saying, "The performance will improve and I want to make it look even a little smaller with the concave effect."
Since I feel that the reason for "A concave face is an excommunication" is wrong, I may be criticized by my master.
Longitudinal Aberration
Graphs of spherical aberration, image surface curvature, and distortion
Spherical Aberration , Axial Chromatic Aberration
Spherical aberration is corrected very well. * I feel the spirit of the lens.
Axial chromatic aberration can be said to be almost ideal.
Field Curvature
The field curvature has also been reduced to a minimum.
Distortion
The distortion is "0".
I'll write it again, "0."
The official website also says 0 distortion, and that's right.
In general, manufacturers avoid expressions like 0 or infinity.
This is because we are afraid of being called an exaggerated advertisement if there is a "case where it is not 0" due to variations in manufacturing and measurement conditions.
However, as far as this data is concerned, it is small enough to make you think, "I have no choice but to say 0," so they probably think there is no problem.
It is assumed that the designer was under intense pressure to say, "This third generation lens also needs to be reduced to 0!" because the second generation 85 mm lens had distortion aberration of almost 0.
Lateral Chromatic Aberration (Magnification Chromatic Aberration)
Lateral chromatic aberrations is small enough in absolute terms.
I feel that I want to make the g line (blue) a little more positive, but I want to judge accurately by spot diagram, etc., even if it is so small.
Transverse Aberrations
(Left)Tangential direction, (Right)Sagittal direction
Let's look at it as transverse aberration.
As a whole, there is little coma aberration and it is well organized.
Because of the large aperture, the sagittal image remains at the corner of the 21 mm image height, but this is not noticeable unless the camera is used for special purposes such as shooting stars.
Spot Diagram
Spot Scale 0.3 (Standard)
Now we are going to look at the optical simulation results, but at the spot diagram first.
The standard scale is too small to tell.
Spot Scale 0.1 (Detail)
I can finally understand on the scale of enlargement.
The spot diagram slightly leaves the c line (red) from the axis, but it is at a level where you can finally see it on an enlarged scale, so it is not at a level where you are concerned about the actual picture.
Even lateral chromatic aberrations, who was a bit worried, does not have a noticeable atmosphere in spot diagram.
MTF
Maximum Aperture F1.4
Finally, let's review the results of the MTF simulation.
The MTF of the open Fno is high enough to be suitable for a new generation star lens. At the corner of the image height of 21 mm, there is a displacement (defocusing) of the mountain, but the MTF value remains at the central mountain position, so sufficient resolution can be obtained.
Small Aperture F4.0
If you narrow down to F4, you don't need to comment anymore.
Conclusion
As the third generation of the 85 mm * lens, the new 85 mm F1.4 lens was born with the burden of realizing high performance. Naturally, it was an ultra-high-performance large-aperture medium telephoto lens.
This product is sure to be called "a rare product" in the future because it is equipped with standard additional functions such as dust and drip-proof in addition to this high performance.
As an individual, I was shocked by the unexpected lens shape and the memory of the legendary master.
I would like to continue to do a comparative analysis with the second generation.
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Sample Picture
Example photos are in preparation.
If you are looking for analysis information on other lenses, please refer to the table of contents page here.