What is a Low-Pass Filter?

A low-pass filter, also known as anti-aliasing or “blur” filter, was designed by camera manufacturers to eliminate the problem of moiré by blurring what actually reaches the sensor. While extreme details are lost in the process, the problem of moiré is completely resolved. Since most cameras are designed to be used for day-to-day photography, where moiré pattern is very common, most cameras on the market today use a low-pass / anti-aliasing filter. While this surely benefits most photographers out there, it is a big blow on landscape photographers that never see moiré and yet end up with blurred details. Because of this problem, some companies on the market started specializing in removing the low-pass / anti-aliasing filter from modern DSLR cameras, specifically targeting landscape photographers. Most digital medium-format and some high-end cameras do not have a low-pass filter, because they want to deliver the best performance from their sensors. While those cameras are affected by moiré, manufacturers leave it up to the photographer to decide on how to avoid it or deal with it in post-processing. Below you will find two examples of low-pass filters used on typical Nikon DSLRs and on the Nikon D800E.

A typical low-pass filter contains of 3 or more different layers, as shown on the top illustration below:

Nikon D800 vs D800E Low-Pass Filter

As light rays reach the first “horizontal low-pass filter”, they get split in two, horizontally. Next, they go through an infrared absorption filter (illustrated in green color). After that, the light rays go through the “second vertical low-pass filter”, which further splits the light rays vertically. This light ray conversion process essentially causes blurring of the details.

With the Nikon D800E DLSR model, Nikon took a different approach. The full low-pass filter cannot be completely removed, because it would cause the focal plane to move; plus, the camera still needs to be able to reflect infrared light rays. Instead of making a single filter with one layer, Nikon decided to still use three layers, but with two layers canceling each other out. As light rays get split into two with a vertical low-pass filter, then through the IR absorption filter, those same light rays get converged back when passing through a reversed vertical low-pass filter. Hence, instead of getting blurred details as in the first illustration, we get the full resolution.

I am not sure if the above method is the best way to deal with the issue, but I suspect that Nikon decided to take this route for cost reasons. It would probably be more expensive to produce a single IR absorption filter layer coated on both sides, than continue to use the same layers, but in a different configuration.

Here is a sharpness comparison between the Nikon D800 and D800E (Image courtesy of Nikon):
Nikon D800 vs D800E Sharpness


Avatar of Nasim Mansurov About Nasim Mansurov

is a professional photographer based out of Denver, Colorado. He is the author and founder of Photography Life, along with a number of other online resources. Read more about Nasim here.

Comments

  1. 1
    ) EricD

    ‘Anti-aliasing filter’ on Wikipedia is interesting, too.
    http://en.wikipedia.org/wiki/Anti-aliasing_filter

    The birefringent crystal material mentioned there is Lithium Niobate – it expands and contracts in electric fields, which may be how the self-cleaning function works (Surface Acoustic Waves – also used in focussing !). Conversely, it also generates voltages from vibration or temperature changes, which might attract dust – so it probably has an outer conductive coating, too, to prevent this.

    Amazing how much can be done in one component.

  2. So is the answer to increase the sharpness in a D800, is it?

    My camera is set to the D800 default, around mark 3. I have been told a lot of my shots look soft so should I up the sharpening to 8 or 9 for landscapes.

    Would be grateful for your comments.

    Great website, keep up the good work.

    Mike

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