Optical Design value Analysis - SIGMA 45mm F2.8 DG DN [No.028]

This is a performance analysis and review article of the SIGMA 45mm F2.8 DG DN

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.


The SIGMA 45mm f/2.8 is a small standard single focal length lens exclusively for full-frame mirrorless cameras.

Looking back to the end of 2019, the year this lens was released, Leica, Sigma, and Panasonic formed a partnership and began selling cameras and lenses based on a common mount system.

The name of this organization was called the L-mount Alliance, and I have fond memories of the "much misunderstood buzz" that occurred over the name "L-mount," which is familiar to all old lens fans.

In the midst of all this, Sigma released the new fp camera, which is very compact for a 35mm full-frame camera, and at the same time released a dedicated standard lens, the 45mm F2.8. This lens is the subject of this analysis.

Speaking of SIGMA lenses in the 2010s, the Art series, which started with the 35mm F1.4 in 2012, is representative of this series. The concept of this series was characterized by large aperture, high performance, and massiveness, but this lens is compact in size to match the camera body.

In addition, the lens also features a mechanism that seems to have been designed with old lens fans in mind, such as the revival of the aperture ring, which has often been eliminated in recent years.

However, the focal length of 45mm is a mystery, and the Fno of 2.8 is a little dark for a recent single focal length lens, but what features are hidden?

Private Memoirs

First of all, what does the focal length of 45mm come from? Let's consider a few things.

One of the ways to determine the standard focal length of a photographic system is to combine the diagonal length of the image sensor with the focal length.

If this is the case, the diagonal length of a 35mm full-size sensor is "about 42mm," so it can be seen as matching this length.

As you know, the standard length of a full-size lens is generally considered to be 50mm. There are several theories about this, but the most popular theory is that "Leica, the founder of the 35mm camera, was the first to adopt this standard.

There are some theories as to why Leica chose 50mm as its standard, but unfortunately, there is no accurate record of this.

Incidentally, Leica's standard focal length was 51.6mm, and in the era of rangefinder cameras, all companies used this focal length, so the actual focal length of 50mm lenses is 51.6mm, even today.

In this case, the focal length difference between the SIGMA 45mm lens and the 50mm lens is 6.6mm (approx. 7mm), which is different enough from the 50mm lens in practical use to provide a different angle of view from the 50mm lens.

Another way of looking at it is that there is a conflict between the "50mm group, which originated from Leica" and the "35mm group, which is considered to be close to the field of view of the eye and to produce natural images" in terms of standard lenses.

It may be that it is just the best balance for the first mirrorless product, which allows more leeway on the image sensor side….

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SIGMA 45mm F2.8 DG DN

Document Survey

It is a modern lens and can be instantly found by searching. Patent Publication 2019-211703.

The text suggests two types of focusing configurations: a general configuration with a single focusing group and a floating configuration with multiple focusing groups.

There is no description of floating on the official website of the SIGMA 45 F2.8 product, so we reproduce below the example 1 of the single group focusing configuration as the design value.


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 45 F2.8.

The lens is composed of 8 elements in 7 groups, and 2 aspherical lenses are used. As mentioned in past analysis articles, 50mm lenses generally have the best performance, small size, and efficiency in what is known as a Gaussian-type configuration.

At first glance, this lens appears to be far from Gaussian, but the front and back of the aperture have a concave surface with a tight curve that seems to be a remnant of Gaussian. The five-element lens on the subject side is probably the origin of the Gaussian configuration. It is as if a converter with a negative focal length is attached to the image sensor side.

This design of placing the converter on the image sensor side is unique to mirrorless lenses and can be imagined to be an ingenious design that allows free space on the image sensor side.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

The spherical aberration is oriented in the opposite direction from the full-corrected type, which bulges to the minus side like a typical Gaussian lens. According to the explanation on the official website, the lens "emphasizes bokeh" or "aims for a classical bokeh rendering," so it seems that the aberration is intentionally left in the opposite direction from normal.

I am very much looking forward to verifying this in actual shooting.

Axial chromatic aberration is almost completely suppressed, as is typical of modern lenses.

Image Surface Curvature

Image curvature is ideally corrected up to the middle of the graph, although there is a large fluctuation at the top of the graph at the very periphery of the image.

In the case of this f/2.8 lens, the bokeh amount is slightly insufficient due to the dark f/number, so it is possible that the bokeh is emphasized by sweetening the peripheral areas of the image.


Distortion is unusual, but seems to be slightly threadbare. Normal Gaussian type lenses will be slightly barrel shaped.

Is this also aimed at some kind of visual effect? Or perhaps it is a phenomenon unique to mirrorless lenses, and is influenced by the strong negative lens group on the image sensor side. Or maybe I am just reading too much into it…

Chromatic Aberration

Magnification chromatic aberration is nicely integrated, typical of modern lenses.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

Let's look at it as a lateral aberration.

This is a beautiful graph, although the Fno is darker. If the Fno is originally dark, the aberration in the sagittal direction shown on the right is drastically reduced. Looking at the tangential direction graph on the left, the effect of the two aspherical lenses is sufficient to correct the aberration. The graph is clean around the central axis, so the resolution performance will probably improve further when the lens is stopped down.

Spot Diagram

Spot Scale 0.3 (Standard)

Here are the results of the optical simulation, but let's start with the spot diagram.

The spot seems to be slightly larger because the spherical aberration is intentionally left out, but it may be a ploy.

The idea may be similar to the NIKON NIKKOR 58mm F1.4, which is famous for its 3D high fidelity design in the cases we have analyzed in the past.

Spot Scale 0.1 (Detail)

This is an enlarged spot with a different scale.

There is little sagittal coma flare, and the spot has a somewhat unique shape.

Is this the taste of Sigma's inexpensive lenses for the mirrorless era?


Maximum Aperture F2.8

Finally, let's check the results of the MTF simulation.

Although spherical aberration is intentionally left in the lens, the balance of the lens is excellent, and the lens is sharp from wide-open.

Small Aperture F4.0

The resolution is almost aberration-free, although it is only one stop smaller than F2.8 to F4.0.


We had a hunch that this might be a crunchy lens featuring only a small size for SIGMA's ambitious new camera, but it seems to be a chic lens with spherical aberration seasoned to create a playful gem.

We would like to confirm this by taking actual sample photographs at an early date and will add an explanation in the future.

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SIGMA 45mm F2.8 DG DN

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.