SIGMA

Optical Design value Analysis - SIGMA 105mm F1.4 DG HSM Art [No.010]

2022-03-27

This is a performance analysis and review article of the Sigma Art 105 F1.4.

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 Jin 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 105mm F1.4 DG HSM Art lens introduced in this section is a large-aperture medium telephoto lens that boasts extremely high resolution performance.

Furthermore, it is a lens that Sigma itself calls the "BOKEH-MASTER" among the Art large-aperture single focal length series that Sigma is proud of.

I have heard that the culture of calling the blurring of an image caused by a large-aperture lens "bokeh" originated in Japan, and that it is called "bokeh" in the whole world.

I do not have any foreign acquaintances, so I do not know if this is true or not…

Private Memoirs

I believe that "100mm focal length = f/2.8 macro" has been the conventional wisdom for the past 20 years or so.

Since every photographer must own at least one macro lens, it is hard to find a reason to purchase a 100mm lens in addition to a macro lens, isn't it?

It was under these circumstances that the 105mm F1.4 was introduced, and my first reaction when I saw it for the first time was "What? I remember thinking, "There is a tripod mount….

A tripod stand is a kind of attachment to mount a lens on a tripod.

Generally, when you use a tripod, you use the camera's tripod screw holes to fix the lens, but for heavy lenses such as telephoto lenses, a part with screw holes for fixing a tripod is attached to the lens side.

If the lens is heavier than the camera, the lens side must be secured to the tripod to be stable, and the weight of the lens will damage the camera.

This may not be familiar to those who do not use telephoto lenses.

This lens weighs about 1.6Kg, so a tripod mount is certainly necessary.

Also, the filter size is said to be Φ105, which is in line with the focal length? Is this the size normally distributed in the first place? It is too large a size to have heard of.

I checked and it is sold, but it seems to cost around 8,000 yen even with a protect filter…you could buy one decent used lens.

Is it for professional use? Is it for academic or experimental use? It is an amazing price.

Now, continuing from the article on the SIGMA 35mm F1.4 Art, we will analyze the Art series single focal lengths.

This is part of our effort to establish a benchmark for modern optical design values, because SIGMA's Art single focal length lenses are designed with a very straightforward concept of "performance-oriented, size-neutral" and are easy to use as a benchmark standard.

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SIGMA 105mm F1.4 DG HSM Art

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. From the atmosphere of the cross-sectional drawing, we assume that the design values are similar to the design values of the product, JP2019-144477 Example 1 is visually similar to the product, and try to reproduce the design data as follows.

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 figure below shows the optical path of the SIGMA Art 105 F1.4.

The lens has 17 elements in 12 groups, with an aspheric lens placed on the last element closest to the image sensor to correct spherical aberration and curvature of the image plane at the same time, and five elements made of special low-dispersion material for good correction of chromatic aberration.

The basic medium telephoto layout is doubled, or rather, a small medium telephoto within a large medium telephoto… Staring at this lens, you may feel as if you are looking at an optical illusion.

It is naturally difficult to design a small number of images, but with today's faster computers, it is easier to understand the end result.

On the other hand, designing a lens with such a large number of lenses can quickly become uncontrollable unless you are a skilled designer.

Although the computer assists in the design of lenses, the designer must have faith in his or her own design to arrive at an appropriate answer.

The number of lenses increases the degree of freedom of design and allows for advanced aberration correction, but the number of manufacturing error factors also increases, which multiplies the difficulty until the lens is launched.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

SIGMA has an 85mm F1.4 lens in its Art series, and I thought it would be the best performer… but, somewhat unbelievably, this 105mm lens has less spherical aberration…

Image Surface Curvature

Image curvature is at a level where aberrations are almost completely suppressed.

Distortion

Distortion is threadbare, which remains slightly on the positive side because of the telephoto range, but it is not at a level that affects the photograph in absolute terms.

Chromatic Aberration

The magnification chromatic aberration is also well corrected at a level that is hard to believe for 105mm F1.4 specifications.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

In terms of lateral aberration, I have the impression that it is smaller and more corrected than the 85mm f/1.4.

Spot Diagram

Spot Scale 0.3 (Standard)

Slight sagittal flare is a concern, but is adequately corrected.

Spot Scale 0.1 (Detail)

Here is a graph to further expand the scale and see the details.

MTF

Maximum Aperture F1.4

The characteristics are high enough throughout the entire screen at f/1.4 maximum aperture. As expected, there is some image curvature around the 21mm image height at the very periphery of the image, but this is only at the corners, and overall high resolution can be expected.

Small Aperture F4.0

When stopped down to fno4.0, the performance is extremely high, which can be perceived as aberration-free.

Assuming the current image sensors for cameras, there may be no way to physically produce images with higher resolution than this.

However, if we consider compositing multiple images, it is another matter…

As noted at the beginning of this article, this lens has another name, BOKEH-MASTER, but from the sharpness of the aberrations, it might as well be KIRE-MASTER? I thought…

Conclusion

This lens was featured as a typical example of modern optical design, and the aberrations corrected more than I had imagined were breathtaking.

However, the introduction of a large number of aspherical lenses runs the risk of increasing the blur of what is known as "onion blur," so I assume that the lens was designed using as many spherical lenses as possible.

The Art 105mm F1.4 will be difficult to use if you are not confident in your physical strength, but once you use it, you will not be able to stay away from its high performance.

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SIGMA 105mm F1.4 DG HSM Art

Sample Picture

SIGMA Art 105 1.4 examples 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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