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by Bart Arondson

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It seems as though it is becoming more common for new digital cameras to come without low-pass filters. I learned quite a bit from reading the excellent answer here: What is the difference between the newly launched Nikon D800 and D800E? I took away from that this key point:

With the sensor resolution now sitting at over 36MP, though, there are a lot fewer instances where the detail you are trying to record cannot be resolved and reconstructed accurately -- especially if you are working in a studio

The D800 obviously has massive amounts of resolution, but other cameras such as the Nikon Coolpix A with 16MP or the Pentax K-5 IIs with the same 16MP sensor - are not really in the same 30MP range. So why now are digital cameras switching to remove this filter?

This might be a duplicate of Why is a physical anti-aliasing filter still needed on modern DSLRs? but I guess I am asking the opposite.

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Technically, the D800e does have AA filters. But they are oriented at 180 degrees to each other and cancel each other out. The D800 has the normal arrangement where the second polarizes light at 90 degrees to the first. –  Michael Clark Mar 8 '13 at 2:39
    
shutterbug.com/content/… –  Michael Clark Mar 8 '13 at 8:47

3 Answers 3

up vote 3 down vote accepted

It is because pixel density compared to lens resolving power has increased to the point where aliasing occurs less frequently and users are becoming increasingly demanding when it comes to resolution.

AA filters are there to prevent aliasing. Aliasing is worse when the signal frequency is very close to the sampling frequency. If you increase sampling frequency (by using sensors with higher pixel counts) for the same signal frequency (by keeping the same lenses) then aliasing is less of a problem. Effectively lens aberrations are doing the job of the AA filter.

Even if you use newer and better lenses, ultimately there will be a point where the AA filter is redundant as lens resolution is ultimately limited by diffraction.

There is also a non-technical side to the recent trend. In the beginning with very low pixels counts of 3 and 6 megapixels, aliasing would have been a huge problem with certain subjects (e.g. man made fibres). The problem would get gradually better as pixel counts increase, however manufacturers would be wary of being the first to remove the filter for fear of any negative press about image artifacts. Once satisfied that consumers accepted the filterless cameras more manufacturers followed suit.

This is all in consumer land, of course. Digital medium format bodies have been doing without AA filters for a long time.

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When talking about aliasing, pixel pitch is much more important than overall resolution. For the most part, the newer cameras without AA filters have pixels that are approaching a size smaller than details the lenses used on them have the ability to resolve. Pixel size for the 36MP D800 is 4.84µm. The smaller sensor of the 16MP Pentax K-5IIs and the Nikon Coolpix A has pixels 4.75µm wide. The newly announced 24MP D7100, which will not have an AA filter, will have a pixel pitch around 4µm.

When, in the future, sensors are capable of recording finer resolution than the lens can resolve, the lens itself will become, in effect, the AA filter.

As the processing power available in-camera increases, software solutions can also deal with moire much better than in the past. The demosaicing algorithms used to produce images with resolution equal to the total number of RGB pixels on a sensor are already blurring things to a degree, which also helps deal with moire.

Another thing I believe is influencing the trend is the way most images are viewed now. Most images are re-sized before being presented for viewing through digital delivery methods. What we often see in images that we assume was caused by aliasing at the time the image was captured are actually scaling errors introduced when the image was re-sized. Just as software can deal with the moire introduced by scaling errors, it can deal with moire introduced by the data collected on the sensor.

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By that argument, no compact camera should have an anti-alias filter, yet they almost all do. I think the resolution matters most in this cause because that represents how a scene is sampled, no the pixel-pitch. –  Itai Mar 8 '13 at 2:43
    
The last part of your answer is a good addition. Some moire-free images end up with moire once scaled down to the desired size. I've seen this many times. Only difference is that the scaling software has the higher-resolution data to work out where moire should not be. –  Itai Mar 8 '13 at 2:45
    
The software used to re-scale the image does have the original data to work with. The software used to display the image does not. I'm thinking in terms of the software a GA (video card) uses to render an image. –  Michael Clark Mar 8 '13 at 2:52
    
When compact cameras first came out, the resolution was less than 1MP and the pixel sizes for 1/2.3" sensors were in the 6µm range. Getting sensitivities high enough to allow usable shutter speeds without generating so much noise as to make the images unusable was the early emphasis, along with the MP race. Neither processing power, especially in-camera, nor the software to utilize it were anywhere near the level they are today. Once they started putting optical low-pass AA filters in the cameras... –  Michael Clark Mar 8 '13 at 8:58

There are much smaller cameras without anti-alias filters. The Olympus PEN E-PL5 uses a 4/3 sensor and the Fuji X20 uses a 12 megapixels 2/3" sensor which is only slightly larger than sensors of compact cameras and even lower resolution.

What is seems like to me - and I am speculating - is that most digital cameras had anti-alias filters because everyone else was doing it. Then, when Nikon used it to give an edge to the D800E over the D800, other manufacturers followed suit in order to gain a comparable edge as well.

The truth is that an anti-alias filter was never absolutely needed. It compromises on image quality, trading fine details for a lower risk of moire. Yet, even cameras with anti-alias filter can still show moire because the filters may not be strong enough to avoid moire completely. You can see this in my comparison of the Pentax K-5 IIs and K-5 II. Play with the aperture controls in the demo towards the bottom of the page and you will see how both cameras exhibit moire.

While it would be possible to create an anti-alias filter to avoid moire completely, it would result in unacceptable softness, particularly when comparing with cameras having weaker anti-alias filters.

The next step in evolution is that some cameras will have built-in processing to reduce moire and that will encourage more cameras without anti-alias filters.

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The smaller sensor of the Olympus PEN E-PL5 has a pixel pitch of 3.73µm, compared to the larger 4.84µm pixels of the D800. Most higher resolution compact point-and-shoots fall in the 2-3µm range. In contrast the 16MP Nikon D4 has 7.21µm pixels and the 18MP Canon 1D X has 6.92µm wide pixels. –  Michael Clark Mar 8 '13 at 2:45
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Of course the AA filter was never absolutely needed, you could always get an image without it, but I'm pretty sure the early 3MP and 6MP DSLRs would have suffered from extreme colour moire problems without AA filters! –  Matt Grum Mar 8 '13 at 10:26
    
I particularly like the conclusion that in the end it doesnt matter, as you cant see the difference. That's what I thought when I could only see the difference when pixel peeping. Only rare cases where you need to zoom by cropping to 100% or print large will it be detected, and only if you have a comparison. –  Michael Nielsen Oct 21 '13 at 14:14

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