Low- Pass Filter Removal (Part 2)

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7/7/2012 11:33:40 AM

It is also recommended that photographers alter their shooting style when using a camera such as the D800E. This may be by employing tripod mounting, altering technique, lens choice and aperture. Also, bear In mind that moiré and false colour are virtually impossible to spot on a camera's LCD screen and will need to be viewed at 100% magnification on a computer screen to see the artefacts in the image. Also, JPEG and TIFF shooters should avoid a D800E as files are processed in-camera and are virtually impossible to correct later on. The best way to improve an image that shows moiré or false colour Is to shoot RAW and fix it during post-production, when the image file is processed.

Description:  Nikon D800 vs D800E Low-Pass Filter

 Nikon D800 vs D800E Low-Pass Filter

Fujifilm, however, has approached the issue differently with its X-Pro 1. Rather than simply removing anti-aliasing properties (and, more importantly, leaving the probability of moiré and false colour in the images), the company has designed a completely new sensor that takes its inspiration from Fujifilm's long-established practice and experience with traditional photographic film. Fujifilm stresses that the new colour filter array in its sensor completely removes the need for an OLPF.

“Moiré is tackled at its root cause by the innovative colour filter array of the new sensor," the company explains. “By enhancing the aperiodicity (randomness) in the array arrangement, the colour filter minimises the generation of moiré and false colours, eliminating the necessity for an optical low-pass filter in the lens and enabling the sensor to capture the full 'unfiltered' resolution and descriptive quality of the lens.”

The result, according to Fujifilm, is a compact system camera that outperforms the sensors found in many full-size DSLRs. So why does moiré occur in photos taken with a digital camera, when it never appears in photos taken with a silver halide film camera that does not include optical low-pass filters? Fujifilm stress this “is because silver halide particles that make up photographic film form a random pattern. Silver halide particles are an organic compound and naturally have an irregular configuration. Reproducing and achieving low periodicity, seen in silver halide particles on photographic film, with a digital sensor, can prevent moiré without having to use optical low-pass filters. This results in the delivery of the highest-resolution images."

Description: Fujifilm X-Trans CMOS sensor vs. conventional CMOS

Fujifilm X-Trans CMOS sensor vs. conventional CMOS

At the risk of getting too technical, it's important to understand how moiré occurs inside the camera and how Fujifilm has eradicated it from its new sensor for the X-Pro 1. Colour filters generally used in modern camera sensors use something called the Bayer array'. It is a highly periodic array of 2x2 (4-pixel) grids consisting of one R (red), two G (green) and one B (blue) pixels, arranged In a regular pattern.

A new 6x6 grid was developed for the X-Pro 1 to break the cyclic nature of the 2x2 array. It has 36 pixels (6x6), creating a highly complex combination compared to Bayer's four patterns to dramatically reduce the regularity of the regular-repeating pattern. It has been found that the complexity of this array is sufficient to fundamentally prevent the development of moiré in most cases.

When you take a photograph, a sensor receives light and uses the three RGB filters for colour separation. It also performs interpolation (resizing) based on the correlation of adjacent pixel information to create a colour image.

Description: FUJIFILM have developed a new CMOS sensor called the X-Trans

FUJIFILM have developed a new CMOS sensor called the X-Trans

In the Bayer array, each row or column features only two of the three R.G or B filter pixels. In developing the new colour filter array, Fujifilm ensured that all rows and columns are laid out with R, G or B filter pixels resulting in more realistic and accurate colour representation and removal of 'false colour’.

Incidentally, G (green) pixels are largest in number in this new colour filter array. Fujifilm focused on G pixels because green has the highest sensitivity in the visible spectrum (i.e. the most easily recognised by human eyes). The new colour filter array also has a balanced mixture of R (red) and B (blue) to enable natural colour reproduction, close to what the human eye can see.

In real-world terms, this can even result in images in the colour spectrum that other cameras cannot even produce. Tatsuya Tanaka is an internationally published nature and astronomical photographer who was stunned at the results of this new sensor. "The red stars of the Rosette Nebula, shaped like a rose, are difficult for cameras with low-pass filters to capture because the red wavelength gets cut off. I was excited about how those stars would appear with the X-Pro 1, a camera without low-pass filters installed. It reproduced them well. It easily captured objects that the human eye could not see in the short exposure time of three minutes. It reproduces a world that could not be made possible with compact digital cameras. Tones that could be reproduced with films were difficult to do digitally, but the X-Pro 1 could reproduce tones in a wide dynamic range from bright to dark areas."

Description: Fujifilm X-Pro 1 Mirrorless Camera

 Fujifilm X-Pro 1 Mirrorless Camera

So. where is this all leading? These cameras may not be for absolutely everyone, but there is a distinct market of both professionals and serious amateurs who will welcome any increase in image quality. It’s a bizarre phenomenon that, despite constant increases in megapixel counts and leaps forward in imaging technology, digital cameras are still susceptible to two serious issues that were never and issue for film - moiré and false colour. It seems even more odd that, until now, the only way to correct this was by inserting optical low- pass filters into the camera that subsequently softened the image. However, with both Nikon and Fujifilm actively providing alternatives to the standard configuration, all that could be about to change.

Both of these cameras, while very exciting, have their own distinct issues.The D800E eliminates anti-aliasing entirely, delivering optimum image sharpness but exacerbating the problems solved by the filter. The X-Pro 1, on the other hand, wipes out the moiré and false colour issues while delivering sharp images - but not everyone will want to invest in a brand-new system or, more importantly, a compact one. Maybe the solution is some kind of hybrid, where Fujifilm licence its technology to be employed in other types of cameras such as DSLRs. This, however, reduces Fujifilm's competitive advantage and it may not be a technology the company is willing to give up-just yet.

Description: Faneuill Hall area in Boston with Fuji X-Pro 1 Fujinon 35mm f/1.4

Faneuill Hall area in Boston with Fuji X-Pro 1 Fujinon 35mm f/1.4

Whatever happens, it is highly unlikely that either company (or their competitors, for that matter) are going to rest on their laurels. The bar has been set for both image sharpness and artefact reduction. The question is, who will raise it next?

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