Anti-aliasing filter in 1DX

[mrmarks]

Will the 1DX have a strong anti-aliasing filter (optical low-pass filter) as in the 5D3? Is there any indication of this?

[neuroanatomist]

No way to know for sure, but Canon did tout reduced moiré for the 1D X, just like the 5DIII, which suggests a strong AA filter.

[JR]

Yikes!  That's no good!  I find the AA filter of the mkiii stronger then the mkii to the point it can impact IQ.  Am I imagine things or seeing things that are not there?

I wish there was a way to remove some of the impact of a AA filter in post processing?

[PeterJ]

Hope this isn't taking things too OT, I'm familiar with anti-aliasing in general and the use of low-pass filters when it comes to sampling traditional analog sources (such as audio) but I'm having trouble wrapping my head around how it works in optical systems.

You have different wavelengths of light, which presumably aren't low-pass filtered because you'd lose colour information, so the only other thing I can think of is that you'd be filtering is more like a 2D domain to average out the effect of sharp edges etc, but I thought that would be fairly trivial to do in the digital domain.

Anyway I'm sure there's a good reason, I just need someone to enlighten me .

[JR]

Anyway I'm sure there's a good reason, I just need someone to enlighten me .

Makes 2 of us!

 

[daveswan]

The OLPF is designed to blur the incoming image so as to remove high spacial frequencies which woulc cause interference with the fixed Bayer patern of the sensor

[neuroanatomist]

In an optical system, the AA filter basically blurs the image slightly by passing the light through two layers of a birefringent material (lithium niobate).  A birefringent material separates incoming light into two rays, essentially polarizing it.  The two layers are oriented orthogonally to one another.  There's a 1/4-wave plate in between the two layers to circularly polarize the light after it passes through the first one (else it would be blocked by the 90° polarizers).  The thickness of the layers determines the degree of separation of the light rays, and that thickness is varied according to the pixel density of the sensor, to match the Nyquist frequency of that sensor.

[JR]

Thanks for the explanation Neuro.

[Z]

I wish there was a way to remove some of the impact of a AA filter in post processing?

This is exactly what Canon claims is done by the latest revision to DPP, 'Digital Lens Optimizer'.

The EOS 5D Mark III comes with Canon Digital Photo Professional (DPP) v3.11 software to enable high-speed, high quality processing of RAW images. DPP v3.11 includes a new Digital Lens Optimizer (DLO) tool that is designed to improve image resolution. DLO imitates lens performance, with a series of mathematical functions replicating each stage of the journey of light through the optical path. Using this information DLO can correct a range of typical optical aberrations and loss of resolution caused by a camera’s low pass filter, by applying an inverse function to each shot to take the image nearer to how the scene appears to the naked eye. This creates detailed, high-quality images with manageable file sizes, providing photographers with maximum image quality and greater flexibility.

source: http://cpn.canon-europe.com/it/content/education/tipsandtricks/3204.do

In reality one must remember the adage 'garbage in, garbage out'

[JR]

Well thanks Z as i did not realize the dpp had this.  I thought they meant something to do with fixing lens curvature.  Since i use LR4 i had not tired dpp.  I will try it out to see if it make a difference!

Thanks.

[Stephen Melvin]

A well-tuned low pass filter does not really lose any detail, whereas a camera without a low pass filter will generate false detail. A filterless camera is destructive to detail.

[jrista]

I had thought that Canon had put on a thinner, not thicker, AA (low-pass) filter in the 5D III. I remember it being explicitly called out in this blog:

http://blog.jeffascough.com/photographers/2012/03/canon-eos-5d-mark-iii-review.html

As I understand it, Jeff Ascough is pretty tight with Canon, and would likely have fairly in-depth knowledge about the 5D III, so I'm not sure his claim about a thinner AA filter can simply be written off as hearsay or some such.

[ereka]

A well-tuned low pass filter does not really lose any detail, whereas a camera without a low pass filter will generate false detail. A filterless camera is destructive to detail.

Now I'm confused

What would be the advantage of having a different version of the D800 without a low pass/AA filter then? I somehow gained the impression that it was for e.g. landscape photographers who want more detail in their images. If absence of a filter generates false detail/is destructive to detail, what would be the advantage of leaving the filter off?

[7enderbender]

Thanks for all the background info provided here. A few more questions:

Is this problem less pronounced in CCD sensors? As far as I know the Leica M9 doesn't have such a filter and I still think that it's one of the cameras that delivers the best results of them all (I don't have one and there may be other factors at play including the lenses).

Also, wouldn't it be possible to leave out the filter and deal with any moire effects in post? Or would it be possible to arrange the photosites more randomly? I'd like to see something that was more along the lines of film grain anyway...

[jrista]

A well-tuned low pass filter does not really lose any detail, whereas a camera without a low pass filter will generate false detail. A filterless camera is destructive to detail.

Now I'm confused

What would be the advantage of having a different version of the D800 without a low pass/AA filter then? I somehow gained the impression that it was for e.g. landscape photographers who want more detail in their images. If absence of a filter generates false detail/is destructive to detail, what would be the advantage of leaving the filter off?

Low-pass filtration is a bit of a controversial topic. It boils down to spatial frequencies (wavelets in two dimensions), and how they are recorded. When you have spatial frequencies smaller than the nyquist frequency of the sensor (that really just means three rows of pixels are necessary to resolve one "cycle" of spatial frequencies, a ratio of 1.5 pixel rows to line pairs), the effect those frequencies have on image detail is largely indeterministic. The general result is that at very fine levels of detail, noise and aliasing increases. The intent of adding a low-pass filter is to blur spatial frequencies that are smaller than the size of a pixel, while simultaneously leaving spatial frequencies as large or larger than a pixel unaffected. Designing a perfect low-pass filter can be difficult. You don't want to design it too strong such that it blurs useful frequencies, however you don't want to design it too weak such that it doesn't blur enough non-useful frequencies that you still get noise and aliasing.

When it comes to landscape photography, the random nature of detail mitigates the impact these "useless" spatial frequencies impose when they are recorded by the sensor, which is why its not really a big deal to eliminate the low-pass filter for such photography. However when you have fine detail in repeated patterns...such as with fabric, the impact those "useless" spatial frequencies impose tends to result in semi-random coloration...color moire and concentric color banding or color sparkle. That is as much a consequence of using bayer-type sensors as it is due to the removal of the low-pass filter, though. With a Foveon-type or monochrome sensor, the effects of not having a low-pass filter are FAR less. The original Foveon sensors did not actually use a low-pass filter, if memory serves, and it was only in the more recent generations were low-pass filters added.