SIGMA

Optical Design value Analysis - SIGMA 70mm F2.8 DG Macro Art [No.007]

2022-03-09

This is a performance analysis and review article of the Sigma 70mm f/2.8 Macro.

The design values of the optical system are inferred from patent information and actual shooting examples, and the lens performance is analyzed by simulation.

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

At the time of writing (2020), this is the only macro lens in the Art series.

In recent years, macro lenses have been defined as having a focal length of about 100mm and a maximum magnification of 1:1, and I am wondering what the purpose of the 70mm lens is.

There is a predecessor lens with the same specifications, but it seems to be this one.

The cross-sectional shape of the lens is different from that of the predecessor (EX series), so the optical system seems to have been newly designed.

As a matter of fact, I have not been interested in macro lenses personally, because I can get by with just one.

Therefore, I am not familiar with macro lenses and did not even know that there was a 70mm macro lens until it was reissued in the Art series in recent years.

Moreover, I had no idea that it was nicknamed "razor" by users and feared by many.

I imagine that the 70mm focal length is convenient for both full-size and APS cameras, since it provides a full-size equivalent angle of view of about 100mm when mounted on an APS sensor camera.

I assume that the aim is to be able to use it on an APS camera with the usual 100mm macro feel, and on a full-size camera with the feel of a slightly longer standard lens.

The size of the product is also exquisite, with a short focal length of 70mm, making it a very compact product, and its large aperture makes it stand out for its small size among the many large lenses in the Art series.

Now, let us consider when you would choose this lens.

For example, when you have decided to use only a single focal length lens for wide-angle photography, you can choose the 70mm Macro as another standard to medium telephoto lens to cover both standard and macro photography, which is a convenient and compact combination.

Private Memoirs

Continuing from the previous article "SIGMA 35mm F1.4 Art", we will analyze the Art series single focal lengths.

The reason for this 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 a benchmark standard.

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Sigma 70mm F2.8 DG Macro 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 Special Publication 2019-144441 Example 1 is close in appearance to the product based on 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

Above is the optical path diagram of the SIGMA Art 70 F2.8 Macro.

The lens has 13 elements in 10 groups, with aspherical surfaces on the ninth lens element and the final element closest to the image sensor, as well as four elements made of special low-dispersion material. As far as I remember, there has never been a macro lens with two aspherical elements.

Looking at the way the lens moves when focusing, it looks like the old macro type where the lens on the subject side pops out boldly, but the patent explains that it is a floating type where the lens group on the subject side above the aperture and the three lenses on the image sensor side above the aperture move out in separate movements. The fact that multiple groups can move separately means that there is more freedom in design, and therefore, improved performance can be expected.

This lens has a shorter focal length and the working distance from the front lens element to the subject is shorter because the front lens element moves out. Because of the short working distance, this lens may not be suitable for insect photography, but it is easy to use for still-life subjects, so there may be different tastes among photographers.

The flat plate glass in front of the sensor indicates a low-pass filter or UV cutoff. Some companies list them, while others do not.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

Spherical aberration is a characteristic diagram that is so abbreviated and linear that there is no point in explaining it.

If you look at the spherical aberration diagram a little more closely, you can see that the f-line (light blue) aberration and the d-line (orange) aberration overlap at the upper end of the spherical aberration diagram, so the resolving power is expected to be extremely high. The same is true of the The goal of correcting axial chromatic aberration is to overlap the f and d lines while bringing the other colors together.

Even though the g line is slightly off, it is a small amount in absolute value, so the resolving power must be extremely high, which is why it is called a razor.

Image Surface Curvature

Beyond the middle portion, there seems to be a little image curvature and non-point septation.

Distortion

Distortion is corrected adequately because it is relatively easy to correct distortion when the focal length is longer than 50 mm.

Chromatic Aberration

Although the absolute value of the magnification chromatic aberration is small, the g-line (blue) and C-line (red) seem to have a characteristic of a net increase toward the outside of the image.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

The focal length is short at 70mm and fno2.8, which is dark for a single focal length, so the sagittal is nicely corrected. At an image height of 12mm, coma aberration appears in the tangential direction and halos remain at higher image heights, but the coma aberration can be removed by stopping down a little.

Spot Diagram

Spot Scale 0.3 (Standard)

Spot Scale 0.1 (Detail)

Even when observed on a magnified scale, the area near the center of the image is well organized. From an image height of 18mm, the halo effect seems to increase the spread of the image.

MTF

Maximum Aperture F2.8

The central area is unquestionably high, and there seems to be little image plane variation. When the image height exceeds 12mm, the apex seems to shift due to image curvature and halo.

Small Aperture F4.0

The MTF characteristics at small apertures show an increase in peak height by the periphery and an improvement in the image curvature component. The image will feel almost aberration-free up to 18mm of image height at the periphery.

Conclusion

Since when the word "razor-sharp" has been used to describe the resolution of a lens, and the expression "razor-sharp" was derived from the two meanings of the extremely thin focus plane and the sharpness of the lens.

Recently, the common sense has been imprinted on us that macro = focal length of 90-100mm, but the focal length of this lens, 70mm, is also exquisite. With a 100mm focal length, you feel as if you are a short distance away from the subject, but with a 70mm focal length, you feel as if you are "right there" and can zoom in on the subject, making it easy to use.

And since it feels comfortable when used as a standard lens, you can probably get by with just this one lens on many days. Moreover, it is a very small one for an Art lens.

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Sigma 70mm F2.8 DG Macro Art

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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