SIGMA

Optical Design value Analysis - SIGMA 20mm F1.4 DG HSM Art [No.014]

2022-04-28

This is a performance analysis and review article on the Sigma Art 20 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 check magazines and the Internet, you probably only find 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 that are not generally visible, and explain the taste and descriptive performance of lenses in a deep and gentle manner.

Please enjoy the special information that you can read only on this blog in the world.

Overview

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

The 20mm F1.4 DG HSM Art lens introduced in this section is a large-aperture wide-angle lens that boasts extremely high resolution performance.

It was released relatively early in 2015 as a single focal length lens in the Art series, and at that time, it was the first lens in the world with a focal length of 20mm and Fno1.4 specifications. From SIGMA HP

Currently, only the 14mm wide-angle and 135mm telephoto focal lengths in the Art series are Fno1.8, while the other focal lengths remain consistent at Fno1.4.

The weight is said to be an abbreviated 1Kg (950g), which is like a legendary weapon for those who choose to carry it, but the price is rather reasonable considering the specifications.

Generally speaking, the weight of an optical system is proportional to the image quality, so the performance will surely not disappoint you.

Private Memoirs

This is the end of the SIGMA Art lens analysis series, which has been going on for quite a while now. At the time of writing, the 14mm and 40mm lenses are already on the market, but since they have special specifications, we do not think it necessary to analyze them as benchmarks. We will analyze them at another time.

Now, we will continue our analysis of the Art series single focal lengths from the SIGMA 35mm F1.4 Art article.

This is part of our effort to establish a benchmark for modern optical design values, since 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 a benchmark standard.

I will conclude here, but thinking back, the analysis of the Art series was really a painful process… It was hard because it took a lot of time and effort to create reproducible data because of the large number of lenses, but there was nothing to comment on because of their good performance.

Amazon Product Link
Click here to check prices

SIGMA 20mm 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. Assuming that Example 1 of Patent Publication 2016-117419, which is visually similar to the product shape, is the 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

Above is the optical path diagram of the SIGMA Art 20 F1.4.

It has 15 elements in 11 groups, 2 aspherical lenses, and 7 elements of anomalous dispersion material for good correction of chromatic aberration.

This is a development of a typical wide-angle lens arrangement, such as a Gaussian-type lens system on the image sensor side and a wide-angle converter on the subject side. The aspherical lens of the second lens is aspherical on both sides, but has an aperture close to Φ60mm. Aspherical lenses of this size are not easy to own.

Compared to the 24mm focal length lens we analyzed in the previous issue, it is about 1.5 times larger in size. The front element is protruding in a so-called "protruding" shape, which prevents the attachment of ordinary screw-in filters. However, since the hood is integrated into the lens, the risk of accidentally touching the front lens element can be avoided to a certain extent.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

Spherical aberration and axial chromatic aberration may be easier to correct at wider angles, but they seem to be adequately corrected.

Image Surface Curvature

Conversely, the wider the angle of view, the more severe is image curvature. The wider the angle of view, the tighter the angle of light incident on the lens, and the more difficult it is to correct. This can be seen from the optical path diagram. There seems to be a slight variation from the area near the center of the image, but the absolute value is within a small range.

Distortion

Distortion is a barrel distortion with some remaining correction, but as expected with a focal length of 20mm, it is barely suppressed to about 3%. Many high-power zoom lenses have distortion of up to 5%, so this lens should at least be better than zoom lenses.

Chromatic Aberration

The magnification chromatic aberration seems to be adequately corrected, but I feel that it is difficult to correct because of the wide angle of view. Although some image curvature is present, it is not at a level that would be considered bad in practical use.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

The sagittal flare is as large as that of the 28mm f/1.4 analyzed previously. Sagittal flare is an aberration that improves when the lens is stopped down, so I personally do not mind this level of flare, but it will be a concern for those who shoot stars and other objects.

Spot Diagram

Spot Scale 0.3 (Standard)

As you could see in the lateral aberration, the spot is extended laterally due to the sagittal flare of the high image height. It would be better to stop down the aperture by two stops for star photographers.

Spot Scale 0.1 (Detail)

MTF

Maximum Aperture F1.4

The image curvature is as per the image curvature variation, and it seems that the image curvature is also affected while the MTF drops a bit towards the periphery of the image.

Small Aperture F4.0

This is the MTF stopped down to Fno4.0. Image curvature is not improved, but the resolution performance of the photo itself improves to the ideal value level because the height of the peaks is increased.

Conclusion

Examples will be provided after production.

Amazon Product Link
Click here to check prices

SIGMA 20mm F1.4 DG HSM Art

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.

-SIGMA
-