MINOLTA

Optical Design value Analysis - MINOLTA AF 85mm F1.4 [No.042]

This is a performance analysis and review article of the MINOLTA AF 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

As you know, MINOLTA developed the world's first full-scale auto-focus SLR camera. It is a camera series named α (Alpha). In other words, MINOLTA's lens is the world's first full-scale auto-focus lens.

(OF COURSE.)

The AF85mm F / 1.4, which is discussed in this section, is the world's first large-aperture, medium-telephoto lens with auto-focus capability for the α series.

(Again, of course.)

First of all, if we trace the development of MINOLTA's 85 mm f / 1.4 specification lens, the ROKKOR lens in the manual focus era did not have a large aperture lens of f / 1.4. It seems that f / 1.7 was the largest aperture.

In 1987, the large aperture lens of F1.4 was produced by the sudden auto focus correspondence, and the history after that is as follows.

  • AF 85mm F1.4 (1987)6/7This article
  • AF 85mm F1.4G( 1993)6/7This article
  • AF 85mm F1.4G(D) (2000)6/7This article
  • AF 85mm F1.4G(D)Limited (2001)6/7
  • SONY Planar T* 85mm F1.4 ZA(2006)6/8

groups/elements

The first AF85mm F1.4 (unmarked) will be released in 1987, two years after the release of MINOLTA's auto-focus camera system A (α) mount, which started in 1985.

The second generation lens in 1993 had the same optical system as the first generation, but the structure around the AF was improved and it was promoted to "G grade".

The third generation G (D) in 2000, equipped with a distance encoder, also adopted the same optical system as the first generation.

And it seems that it was produced until the time when the camera business was transferred from MINOLTA to search SONY, so it is presumed that it was a long-life lens that had been produced and sold for 20 years.

In addition, the 4th Limited in 2001 is a limited edition lens that produced only 700 pieces, but I will talk about this next time.

First, let's analyze the world's first auto-focus 85 mm f / 1.4 lens.

Private Memoirs

Let me explain a little about the focusing mechanism of the lens.

In the manual focus era, the "all group feed method" was the mainstream method of focusing. To put it simply, the entire lens is fed to the subject side.

Note: With the exception of some lenses

However, with the advent of the auto-focus era, a new challenge arose. It was an optical system in which the focus lens group was too heavy to be moved by a motor or the like in the all-group delivery system. This created a new challenge of reducing the weight of the optical system.

In any case, the weight of a medium-telephoto lens with a large aperture is usually about 400 grams. Since it is necessary to move this heavy lens instantaneously and with ultra-high accuracy, it has become necessary to make it as light as possible.

As explained at the beginning, the MINOLTA 85 mm F / 1.4 is the world's first auto-focus lens, but it is also presumed to be the optical system of the heaviest focusing lens group.

Because telephoto lenses are too heavy, a method called "partial focus", in which only a portion of the lens is moved, was proposed early on. However, in the case of a focal length shorter than the middle telephoto, the weight was not a problem if manual focus was used.

Let's analyze the lenses to find out how we overcame this weight problem.

Document Survey

A search of patents prior to the launch of the MINOLTA 85 mm F1.4 reveals two publications.

In my heart, I thought it was an urban legend, but I guess one of these two cases will lead to the Limited lens that only produced 700.

From the product release date and performance, Japanese Patent Laid-Open No. 244010 / 1987 is presumed to be AF85 mm F1.4 (unmarked) in this section. When the reproduced data is made, it is found that Example 1 is an error, printing (scanning) failure, or my copying error. Since the performance is a little strange, Example 2 is reproduced as design data.

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 MINOLTA AF 85 mm F / 1.4.

It is composed of 7 lenses in 6 groups, and does not use special lenses or aspherical lenses.

It seems to be a configuration in which one sheet is added to the image pickup element side of the orthodox symmetrical Gauss type.

The NIKON Ai 85 mm f / 1.4S with manual focus that I introduced before has a lens on the object side (front), but if you add a lens on the object side, the diameter is large and the weight increases.

Therefore, it seems that they are trying to improve the performance while controlling the weight increase by adding a lens to the side of the image pickup device where the aperture can be small.

In addition, in the patent document, it is described that the first lens to the sixth lens are moved at the time of focusing, and it seems that one lens closest to the image pickup device is fixed at the time of focusing.

It was probably a measure to reduce the weight of the focus lens. I can see a tearful effort.

By the way, the PENTAX FA * 85 mm F1.4 was invented in 1992 by a legendary designer as a new focus type middle telephoto that fundamentally overcomes this weight problem.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

The spherical aberration is in proportion to the small number of constituting sheets, and it is a full collection type that greatly expands to the minus side, and the axial chromatic aberration is also slightly large.

Field Curvature

The field curvature has a smaller absolute value than expected, and there is less difference between the tangential and sagittal directions (), giving a good impression of high flatness.

Distortion

The distortion is slightly left on the minus side (barrel shape), but it does not seem to be a level that I worry about when I take a picture.

Lateral Chromatic Aberration (Magnification Chromatic Aberration)

The lateral chromatic aberrations is a little big considering the number of sheets and the trend of the times. I think it doesn't bother you as long as you use it open, but if you use a small aperture, it will stand out a little.

THOSE WHO USE THIS LENS WO N'T USE IT WITH A SMALL APERTURE.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

Let's look at it as lateral aberration.

From the correction of the spherical aberration, I thought that the sagittal coma flare was enormous, but it seems to be restrained moderately so as to balance with the tangential direction.

Coma aberration (asymmetry) in the tangential direction is also considerably suppressed, and it is expected that natural blurring will occur in both the forward and backward directions of focus. It can also be seen that the flatness of the focal plane is high because the halo (tilt) is uniform.

To be honest, until I looked at this lateral aberration diagram, I had thought that it was "a design that was forced into a small metal frame to somehow make it auto-focus," but I was forced to change my mind.

I caught a glimpse of the reason why there are many people who have this lens certified as Meitama.

Spot Diagram

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 spot is a little big, but the d line (yellow) doesn't look bad.

Spot Scale 0.1 (Detail)

If you look at the spot in the out-of-focus area, you can see that the spot in the out-of-focus area is considerably large, and the blur is large and rich, although the aberration may be slightly large from the reference state (focal point position).

MTF

Maximum Aperture F 1.4

Finally, let's look at the results of the MTF simulation.

Looking at the MTF at the open Fno, it can be seen that although the height of the peak is not very high, the degree of coincidence of the peaks is good and the flatness of the focal plane is high.

Small Aperture F4.0

When you narrow it down, the height of the mountain rises enough, but the surrounding area seems to shift to the minus side.

I don't think "people who enjoy chic" like using this lens will be narrowed down this much.

Conclusion

It turned out that the MINOLTA AF 85 mm F / 1.4 lens was not only the world's first auto-focus lens, but also an innovative lens designed to find balance point in both "flatness of focus throughout the entire screen" and "blurriness."

There was a good reason for the lens to be loved in the market for a long time.

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.

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