SIGMA

Optical Design value Analysis - SIGMA 85mm F1.4 DG HSM Art [No.009]

This is a performance analysis and review article of the Sigma 85 F1.4 DG HSM.

We analyze the lens performance through simulation by inferring the design values of the optical system from patent information and actual photographic examples.

Professional lens designer Hitoshi Takayama carefully unravels design information such as optical path diagrams and aberration characteristics, which are rarely seen by the general public, and explains them in depth and gently.

Please enjoy the special information that can only be read on this blog.

Overview

Sigma's Art lens series is a flagship model that combines a high-grade appearance with metal parts and high resolution performance.

The 85mm F1.4 DG HSM Art lens introduced in this section is a large-aperture medium telephoto lens that boasts extremely high resolution performance.

The 85mm F1.4 DG HSM was released in 2016, well after the start of the Art series, so there is no blurring with the concept of the Art series, and it has a massive texture and high resolution performance that seems to embody the true goal of purely pursuing performance. It is a lens.

The focal length of 85mm is naturally considered a portrait lens, but the fact that the manufacturer's official website describes itself as "the ultimate portrait lens" shows the strong pride it takes in its performance.

Private Memoirs

Since I only take portraits of my grandchildren, I have few photos to publish, and I am wondering what to do with the sample pages…

Continuing from the article on the SIGMA 35mm F1.4 Art, we will analyze the Art series single focal length lenses.

This will be part of our effort to create a standard (benchmark) for modern optical design values.

The reason is that Sigma's Art single focal length lenses are designed with a very straightforward concept of "performance-oriented, size-neutral" and are therefore easy to use as benchmark standards.

There is a slight problem here. I have been looking at the design values of the Art series for a while now, and my senses have gone completely haywire with the performance of the Art series, which I think may be too high.

One example of this is that when I view photos taken with Art lenses at "equal magnification" on my monitor, I feel a "good sensation" from the high resolution.

I am well aware that there is little point in viewing a photo at magnification with a recent high-resolution camera, but all I can do is look at the high resolution of the photo.

I have always believed that resolution is not the only attraction of a photographic lens, but to become "pleasantly surprised by the resolution of a lens" is a very serious disease…

Document Survey

Now, when I checked the patent literature, I easily found patents that I thought would be relevant since it is a modern product. Assuming that Special Publication 2018-5099 Example 4 is close to the product in appearance from the atmosphere of the cross-sectional drawing, and assuming that it is a design value, let's reproduce the design data 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 figure shows the optical path diagram of the SIGMA Art 85 F1.4.

The final lens, which is closest to the image sensor, has an aspherical surface to correct spherical aberration and curvature of the image plane at the same time, and two special low-dispersion materials to correct chromatic aberration well.

It looks horribly like the optical system of an old semiconductor lithography system…

The lens group on the subject side also has a double-Gaussian shape, and the lens group behind the aperture on the image sensor side also has a double-Gaussian shape.

The two sets of front and rear lenses are double-Gaussian, which means double double-Gaussian or double-Gaussian? Quad Gaussian? Should we call it?

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

The spherical aberration is so small that there is no point in explaining it, and the d-line (yellow) is an abbreviated straight line characteristic diagram.

I also feel that there is a slightly larger axial chromatic aberration in the f-line (light blue), but this may be a problem with my reproduction data.

The example data in the optical system patent does not disclose the name of the material, so I estimated it from the information on refractive index and dispersion.

If custom-made materials are even used, it becomes difficult to completely reproduce the design values.

When data reproducibility becomes an issue, chromatic aberration is the most likely deviation.

Image Surface Curvature

If the image curvature is this small, the level of aberration is almost completely suppressed.

Distortion

Distortion is almost zero because of the low distortion specification for the focal length.

Chromatic Aberration

The magnification chromatic aberration is skillfully balanced so that each color is all averaged into a small group.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

Halos in the sagittal direction are extremely small considering the Fno1.4 specifications. In the tangential direction, there is a slight halo/coma feeling at the intermediate 6mm image height, but it seems to be corrected at a level sufficient for practical use.

Spot Diagram

Spot Scale 0.3 (Standard)

The spot shape is almost a dot from the center to about 12mm in the middle of the image height, and there is a slight V-shaped effect due to sagittal coma flare when the image height exceeds 18mm on the periphery.

Spot Scale 0.1 (Detail)

MTF

Maximum Aperture F1.4

The MTF is already high enough characteristic throughout the entire screen at fno 1.4 maximum aperture.

As expected, there is a little image curvature around the 21mm image height, but it is an area where there is no problem for most photographic use.

Small Aperture F4.0

If you stop down to fno4.0, the performance will feel aberration-free.

Conclusion

Although it is quite heavy, this is an ultra-high-performance lens that does not make you feel its disadvantages. When you look at the actual images, you will be impressed by the high resolution that is pleasant from the maximum aperture of Fno.

Many people place great importance on the bokeh effect in portrait lenses, but the way a high-resolution subject appears to float in the shallow depth of field characteristic of large-aperture lenses may be called a new spectacular sight.

As expected from the official website, the performance is described as "ultimate," and for this blog, which enjoys aberrations, it is somewhat difficult to write about it because it is so good.

Sample Picture

If you are looking for analysis information on other lenses, please refer to the table of contents page here.

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