A to Z of Photography: X-Trans Sensor and Xiaoxiao Xu

A to Z of Photography: X-Trans Sensor and Xiaoxiao Xu

With the letter X we move on to looking at Fuji's highly regarded X-Trans sensor which is very different from nearly all the sensors in other digital cameras, before looking at the work of contemporary Chinese photographer Xiaoxiao Xu.

X-Trans Sensor

Camera sensors count photons and the image you see on the screen is simply a representation of those pixel counts. For a black and white JPEG, each pixel contains an 8-bit number that ranges from 0 to 255. If the scene goes from black to white, then the gray value is simply scaled by this number: 0 for black, 255 for white and mid-gray at 128. Other than specialist cameras such as the Leica Monochrom, digital images are color yet there is only one sensor and it can only record one number per pixel. How then does it achieve a full color RGB value? The answer is through the use of a color filter array (CFA) and a little mathematical machination.

Digital sensors are broadly sensitive to visible light, a bit like a black and white film. By placing a filter array over the sensor, each individual pixel records only red, green, or blue light. The process of de-mosaicing separates the samples of red, green, and blue pixels in to individual layers and interpolates (aka estimates) full RGB values at each pixel. Not surprisingly, the algorithms used to de-mosaic range from relatively simple to complex.

The sampling regime used in the CFA is crucial to producing a high quality image and this is commonly the Bayer array, created by Bryce Bayer of Kodak in 1976. It uses a repeating 2x2 block arrangement that is 50% green and 25% red/blue respectively. Daylight perception of the human eye has greater sensitivity to luminance and so green light. Ultimately, a better looking green layer should create a better looking image.

And so to Fuji's X-Trans APS-C sensor. What was unusual about Fuji's first X-series camera, the X100, was it's use of a new sensor and, specifically, a new CFA. Fuji does have their own semiconductor business and has also worked in close partnership with Toshiba, however the world of sensor fabrication is opaque to say the least. Regardless of which exact sensor is used, at some stage fabrication involves the overlay of their own-designed CFA.

Fuji make great fanfare of the unique advantages that the X-Trans CFA can bring. It uses a 6x6 less-regular repeating array, than Bayer's 2x2. The latter's regular arrangement can suffer from interference patterns leading to moire. The solution is typically a low pass filter, however this is at the expense of a loss of effective resolution. The Bayer array also has columns and rows with no blue or red photosites which can lead to false color.

In short, Fuji (and many photographers) believes that, compared to otherwise equivalent sensors, the X-Trans should be sharper and have better color reproduction. Fuji thinks that this makes the APS-C sensor as effective as full frame sensors in competing cameras. This remains to be seen and is largely untested principally because DXOMark's testing suite doesn't support it.

The main drawback of X-Trans sensors (other than their APS-C size) has been the lack of software support for de-mosaicing, with early versions of Lightroom and Bridge particularly poor. There is also a computational overhead which affects both PC and in-camera processing. Even with better raw conversion, algorithms that work well with Bayer array sensors (which is where development is focused) may not work as effectively with X-Trans.

Fuji continues to use X-Trans in it's X-series cameras, excluding the entry level X-A models, however they use a traditional Bayer sensor in their medium format offerings. Fuji remain committed to X-Trans for the time being so it will be interesting to see how the format develops.

Xiaoxiao Xu

Xiaoxiao Xu is an upcoming Chinese photographer who spent the first 15 years of her life in China before emigrating with her family to the Netherlands. Since then she has held a fascination for photographing her homeland in a documentary style, contemplative in nature, as if looking in from the outside.

I am reminded of Sting's track "Englishmen in New York" where he sings "I'm an alien, I'm a legal alien." Xu's photos show a sense of the familiar, yet still wanting to identify with, and understand, what the camera shows. For her graduation work at the Photo Academy in Amsterdam she traveled back to her home town of Wenzhou. She says of this

I think that is when I began to tell stories, in this case, telling my own story, a story about homesickness, melancholy, and about how a rapidly changing city differs from the one in my memory

Fifteen is a difficult age to be torn from a home of familiarity and friends, to fine oneself in an alien land. Xu says she felt "locked up" and it was during this period that she found photography. Perhaps most emotively of all

photography allows me to crawl out of my shell and gave me a voice to express my feelings

For Xu, the curriculum at the Photo Academy enabled her to confront her memories, motives, and desires and how these impacted upon her artistic interests and "aesthetic preferences." However it was the continual cycle of assignments and presentations that forced her to go out in to the world, returning to present her understanding of it — "to reveal my own vision of life and my environment." One photographer who impacted upon her during this period was Claude Cahun, the inveterate surrealist self-portraitist who was particularly active in the 1920s and 30s.

Xu started shooting with an entry level Canon DSLR, but during her degree switched to analog, initially a Hasselblad, to which she has added a Mamiya 645, Mamiya 6x6, Holga, Yashica, and Fuji Mini Instax. Her favorite lens is a 50mm standard. Whilst many might associate documentary photography with black and white, not withstanding work such as Egglestone's, she purposefully shoots in color because of its "vividness and playfulness" and it being "closer to reality."

What is interesting about Xu's work is that she predominantly shoots in China, the land of her birth. Whilst many immigrants will photograph the country they settle in, possibly to help them understand their new home, Xu's images demonstrate a sense of fascination — a desire, a longing, to better know and understand her homeland. She says

I am alienated from the country but at the same time deeply attached to it

Perhaps this sense of separation is the result of the age at which she moved and so the inability to grow to adulthood, and integrate, with her native peers. This alienation may well be magnified with China. She says: "The madness and absurdity of China strengthened my fascination even more. The most impossible scenes happen… and yet the people live in harmony with it."

"Aeronautics in the Backyard" was her breakout work which garnered considerable international interest. For this project she visited eight farmer-aeronauts, living with them, interviewing them, and photographing them. In their spare time they have designed and built their own aircraft, some less successfully than others, bearing the visible scars. She believes the success of the project was "because the subject is in its purest form about imagination and the pursuit of your dreams." These are humble people and it touches upon their inner most desires. Whilst it may seem as if many of her images are posed, this is not something Xu sets out to do. She observes and if a "pose" naturally presents itself she might ask them to hold it.

Her projects have taken the form of a road trip, such as "Chinese Wall" where she traveled 25,000 km in three journeys. The logistics are far more complex in these scenarios and so, prior to each trip, she plans out the entire route. The schedule was tight, given the distance and desire to visit as many villages as possible. She was usually on the road from 7am to 7pm every day, stopping frequently and talk to locals. This is now forming the basis for her next photobook. And after that? "I want to start a project about India."

Other Xs

Other X's that didn't make the cut this week include xerography and XnView.

A to Z Catchup

Alvarez-Bravo and Aperture

Bronica and Burtynsky

Central Park and Lewis Carroll

Daguerrotype and Frederick Douglass

Exposure and Harold Edgerton


Family of Man

Nan Goldin and the Golden Triangle

Hyper-lapse and Horst P. Horst

Image Stabilization and Into the Jaws of Death

JPEG and William Jackson

Lenna and Leica

Inge Morath and Minolta

Noise and Helmut Newton

Paul Outerbridge and the Orton Effect

Panorama and Pillars of Creation

Wayne Quilliam and the Queen

Reflex Camera and Tony Ray-Jones

Shooting Sex and Strip Photography

Tilt-Shift and Train Wreck at Montparnasse

Ultimate Confrontation and Umbo

Von Wong and Vivitar

Weegee and Wet Plate Collodion

Lead image a composite courtesy of Skitterphoto and brenkee via Pixabay used under Creative Commons and Wikipedia, in the public domain. Body images courtesy of Xiaoxiao Xu and via Wikipedia in the Public Domain.

Mike Smith's picture

Mike Smith is a professional wedding and portrait photographer and writer based in London, UK.

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I love the range of topics in all these posts! Can't wait for the last one, although I hope another series like this continues on with the education!

Thanks for the kind comment! Much appreciated as it's been quite a journey to get to X. I've certainly learnt a lot along the way