Episode 6: The SIGMA 45mm F2.8 DG DN | Contemporary

In July 2019, SIGMA launched the 45mm F2.8 DG DN | Contemporary, a compact full-frame prime lens with a new concept. “45mm F2.8” might seem like unspectacular at first glance, but in truth this lens has a wealth of stories. In this column, I want to touch upon the engineering background, the features and the performance of this unique lens.

An F2.8 aperture

When the 45mm F2.8 DG DN | Contemporary became available, its high-grade metal housing and its compact size drew much attention. At the same time, there were quite a few people who felt uncertain about the relatively dark F-stop of 2.8. After all, an aperture opening of F1.4 had become the quasi-standard during the single-lens reflex age, and even cheap lenses usually offered F-stops of 1.8 to 2. An F-stop of only 2.8 is mostly reserved for high-resolution macro lenses and perhaps standard zoom lenses. In order to find an F-stop of 2.8 on a standard prime lens, you would almost have to go all the way back to cheap rangefinder lenses of the 1950s.
And yet we have decided that F2.8 was the best choice in order to create a daily-use lens for full-frame mirrorless cameras. We have, above all, prioritized four goals: small size, high optical performance, a short minimum focus distance, and exceptional build quality. In order to achieve all four of these goals, an F-stop of 2.8 proved necessary.
The only reason why standard lenses had to have high apertures in the first place during the single-lens reflex age was to provide a bright image with shallow field of depth in the viewfinder. Since we switched to LCD/EVF finders, the image in the viewfinder has become much brighter than it had ever been on single-lens reflex cameras, and considering the capability of mirrorless digital cameras to reproduce depth of field accurately, we judged that a compact standard lens with F2.8 would be perfectly sufficient. In a wider sense, you could say that our concept resembles Sony’s Sonnar T* FE 35mm F2.8 ZA.

A sturdy metal barrel

The first thing about the lens that catches the eye is its high build quality as well as the metal lens barrel and its exceptional operability. Instead of high-productivity aluminum cast parts, we machined the lens barrel, aperture ring, front ring, lens hood and other outer parts using an aluminum alloy that is also used for the exterior of our cinema lens series. As a result, we were able to combine exceptional rigidity and durability with a beautiful metal appearance. Thanks to high-precision machines and years of mechanical design experience, we were able to accomplish aperture rings and focusing rings that provide great accuracy and feel great to use.
We also paid utmost attention to the lens hood and its attach-and-release function. Normally, the hood can be made of a single plastic part, but this lens has a luxurious and precise structure that fastens a bayonet part to the aluminum hood body with four screws. As a result, we succeeded in achieving both a beautiful metal appearance and a smooth and crisp attachment / detachment feeling.
In short, it looks great, feels great, and works great.

The most important feature: optical performance

The first difficult problem we ran into during the development of the lens was regarding its optical performance. To be frank, with a total length of 46mm to work with, the number of lens elements we could use was limited, and it seemed difficult to achieve superior MTF performance throughout the entire image as good as that of our Art series lenses. Furthermore, in order to shorten the minimum focusing distance with an internal focusing design, the movement of the focusing lens element limited the available space, and adding more lens elements was not possible. No matter what we tried, we were only able to slightly outperform – or only same – our competitors’ lenses. Furthermore, if all attention is spent on high optical performance, it tends to end up with an unpleasing bokeh. Given the small bokeh permitted by a 45mm F2.8 lens, the bokeh needs to look beautiful, otherwise the images will have a rough atmosphere.
There we were, unable to raise the MTF performance and unsatisfied with the bokeh, when we received a surprising proposal from our optical engineers in charge: to develop the lens with the highest priority given to its expression of bokeh.

Beautiful bokeh and image rendering

The key to beautiful, smooth bokeh are the spherical aberrations of a lens. The plan that our optical engineers proposed was not to correct the spherical aberrations but to use and control them. That way, the close to medium range, where bokeh is most striking, the spherical aberrations would generate flare, and cause the image to blur. When I first heard of this plan, I must admit I was bewildered. “This is just like a classic lens!” I wondered.
However, as I studied their optical design more closely, I understood the decisive differences to the way classic lenses render their images:

1 — The overall MTF performance is lower due to the flares caused by spherical aberrations, but all other aberrations – especially color aberrations – are wonderfully suppressed, resulting in unexpectedly great MTF performance in high frequencies. In other words, the image is covered by a very thin veil, but its optical resolution is extremely high.

2— The remaining aberrations are spherical in nature, meaning that an aperture stopped down two stops already results in an MTF profile to rival our Art series lenses.

3— With our optical design, the spherical aberrations are low towards the infinity focus and increase towards the minimum focusing distance, so we achieved sharpening the distant view area with few bokeh and making soft and beautiful bokeh at the distant view area where many bokeh occurs.
Once I understood the design of the lens I actually began to feel very excited. This lens and its unique design, clearly distinguished from classic lenses yet making use of aberrations – what kind of images will it deliver?

Photos: Yasuhiro Ohsone

True beauty reveals itself in photos

Finally, in February 2019, I was able to test a prototype of this lens. I was able to really experience the performance which was already exciting just looking at the aberration chart. When I saw the first images, my hopeful expectations turned into confidence. They were beautiful, truly beautiful. As predicted, the optical performance in the close-up range was not that high, and we also could not rule out the possibility of being called a poor lens. But upon seeing the soft, rich flare created by spherical aberrations, and the seamless, creamy bokeh they cause, I almost gasped. Once more I was reminded that MTF measurement charts alone do not tell the whole story of a lens.
Furthermore, SIGMA’s Art series thankfully already offer prime lenses that prioritize optical performance and high apertures.
The 45mm F2.8 DG DN | Contemporary wanders a different path to its Art series counterpart, and I have come to realize how valuable this is

A new standard lens

The SIGMA 45mm F2.8 DG DN | Contemporary was created to fulfill two roles – to reduce the size of full-frame interchangeable lenses, and to serve as a standard lens for the upcoming SIGMA fp, the world’s smallest full-frame camera. With its exceptional build quality, the stunningly beautiful images it renders, and a minimum focusing distance of only 24cm, the 45mm F2.8 DG DN | Contemporary is a truly rare and unique lens. It is true that we needed to overcome many problems and make difficult decisions in order to achieve. When our customers hold the lens in their own hands, I would be delighted if they felt that our decisions lead to the one perfect answer.

Yasuhiro Ohsone

Head of product planning at SIGMA

Entered SIGMA in 1987. Has worked in the development of optics and mechanisms as well as collaborating with many companies. Became head of product planning in 2013.

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