Techniques, Reviews and Commentary
Is removing the aliasing filter really such a smart idea?
11/06/13For those not versed in the intricacies of digital sensors I will try to illuminate upon the reasons for the filter, it's supposed deficits and widely accepted impact it has on image quality, followed by a little reality and an actual photographic example.
Virtually all sensors use a matrix of red green and blue pixels and as the word matrix implies they are laid out in a grid pattern. The structure is completely different to film where the light sensitive elements, (silver halides) are randomly deposited upon the films surface.
The normal method of arranging the pixels is known as a "Bayer Pattern Pixel Matrix" its served us well offering efficiency and relative ease of image data conversion into the final photographic image. Some manufacturers have attempted to use different RGB layouts and designs, primarily Foveon (Sigma Corp) and Fuji. These alternative designs have proven successful in use, but regardless remain just minor players in the sensor world as they bring other complexities to the table, like designing raw converters that can actually work with them and much higher unit cost.
The Bayer layout has it's issues though. When you shoot subjects that contain repeating patterns the patterns often mis-align with the sensors pixel spacings and create moire patterns, it's easier to see than explain so have a look at the attached picture. The Bayer layout can also cause related issues with colour accuracy, but as pixel counts have increased this later issue has become less of a problem.
Moire patterns most frequently show up on architectural subjects and clothing fabrics, they are however rare with natural objects unless they have significant repeating patterns, which in practice means mainly macro shots of cacti etc.
AA filters (anti aliasing) filters reduce or eliminate the issues by acting as a slight soft focus filter directly in front of the cameras sensor. In essence the filter subtly blurs the resulting image, which effectively means it lowers the actual resolution a little bit. The losses are typically recovered by re-sharpening the image after it has been initially processed.
Unfortunately the sharpening processes are not a free lunch often creating problems of their own, sometimes referred to as artifacts. Most typically we see halos around details giving images that "digital look " but there are others problems as well. In some cases the sharpening if overdone can actually obliterate very fine detail and even make image noise appear far worse by aliasing the noise patterns themselves.
AA filters are also often responsible for poor wide angle lens resolution in the image corners. Its simple really, the filter sits in front of the sensor but there is a small air gap between it and the sensor. Light rays traveling to the edge of the sensor from the rear element of most wide angle lenses do so at a much sharper angle than those at the centre. As a result the light rays on the edges of the frame are more softened by the filter than the ones "on centre" simply because they are passing through a thicker layer of glass and a larger air gap. This issue also exacerbates chromatic aberration and is probably why some lenses that exhibited no CA (Chromatic Aberration) on film cameras can be utterly awful CA monsters on certain digital cameras.
For the above reasons the depth of the air gap and thickness of the filter are quite critical, but are always at best are a compromise because what might be ideal for a tele lens could be complete overkill for a wide angle one.
Like many aspects of digital imaging the reality of the AA filters role is not quite as straightforward as is often believed by those on web forums advocating "lets dump the AA filter".
The final clarity of an image is the result of many factors, any of which could have a far more significant effect on the final detail in the image than the AA filter does by itself. The most obvious issue is lens performance, in fact if you shoot with a soft lens it is highly unlikely you would ever need an AA filter.
Full frame image displaying colour morie and aliasing
Close up of the problem, yes its really crook. NEX 5N 18-55mm standard zoom
But there is more, much more.
Poor focus will lower resolution enormously and in my experience a very significant proportion of regular images are not critically focused. The differences between sharp and really sharp focus is incredibly fine.....much finer than most auto focus systems are capable of.
Small scale Camera movement will subtly blur the image reducing potential aliasing.
JPEG compression can seriously mess with resolution post capture, blurring out fine details that would otherwise show aliasing.
Noise, all noise, degrades potential resolution and thus reduces aliasing.
All noise reduction methods sap the life out of fine details and mask detail aliasing significantly.
Raw conversion software can have an enormous influence on aliasing, those that are better at extracting fine detail typically exacerbate alaising
So here is the thing, removing filter will not in all honesty gain you anything unless all of the above have been dealt with and WELL controlled. Should you actually take the time to sort all the above issues then you may well find yourself being very grateful for that AA filter.
You don't believe me?
Well let me tell you about how I shoot with my current general purpose tool, the Sony Nex 5n.
Currently I use a NEX for the great majority of my non-professional work and occasionally for available light pro stuff, I love it and have shot around 6000 frames in the last 3 months with the micro beast. The 5n does have an AA filter, but it is known to be a fairly weak one.
So has the filter limited the resolution I am getting?
Not in the least!
Have I had issues with aliasing as a result of the weak filter?
I see aliasing often and find myself constantly having to deal with it, removing the filter entirely would just make my editing life miserable.
Here is why.
I shoot a lot of architecture and also subjects that are likely to create aliasing issues, but more importantly, remember that list above, I have basically removed every single one of those items as a factor in limiting resolution.
I shoot with sharp legacy lenses at optimal apertures, the focus is as dialed as I can possibly get it using 14.8x live view, I use a tripod whenever needed, electronic first curtain shutter is used to eliminate vibration. Going further I don't shoot jpegs, rarely shoot above 400ISO, use raw software that extracts all the detail in the file with all noise reduction turned off and beyond that I often shoot using sensor balanced capture.
Ultimately for me the only possible source of resolution loss left is that AA filter and it frankly is not a factor, quite the opposite. With all issues dealt with the weakness of the 5n AA filter is a constant problem. How much of problem? I would calculate that of the 9000 or so files shot at least 400-500 show some aliasing issues unless I compromise something along the workflow to negate it....which usually means I do it in the raw processing.
The inconvenient truth is that in many cases I cannot fully control the aliasing, that dear reader is a universal truth about aliasing, if you have it, getting rid of it can be really really hard, and trust me I know all the methods out there to do so. Basically aliasing has to be dealt with at the sensor, doing anything else is shutting the gate after the horse has bolted.
The camera makers know this and have always known it, hence the fact they virtually all had AA filters, till recently. Frankly I think the only real reason for makers offering cameras without AA filters is marketing, they have worked out the a camera "sans AA filter"offers a pathway to a few extra sales with almost no extra manufacturing costs involved.
The NEX 5n is not by any mean the only camera I have used or tested that exhibited aliasing when all the detail/resolution masking issues are removed. So here is my take on the AA filter.
Generally I feel AA filters should be maintained and as much as possible optimized for the camera in question. Photographers who want or need more resolved detail should examine and modify their technique or tools before jumping to the conclusion that the AA filter is the cause of their problems. Lets be clear here, if you for example use the Sony Kit lens at focal lengths between 35-55mm I doubt you could ever see alaising, even without a filter, the lens simply does not resolve well enough, and this is the case with a lot of similar lenses. Removing the filter will not make a poor lens resolve any better.....period!
If your technique is slack and you only shoot jpegs but use really good lenses then perhaps the lack of an AA filter might be a bonus as it will remove one of the minor potential limits.
Should you think, it won't worry me as I only shot landscapes, well yes perhaps your right, but most folk shoot a wide variety of subjects, and you'd be surprised at how many of them have aliasing inducing patterns present. I have seen aliasing on corrugated iron, brickwork, mesh, fences, windows, cars, and hundreds of other items, trust me in a resolution optimized system aliasing is nowhere near as rare as most armchair webtoghraphers would have you believe!
As resolution of sensors climb it is likely the AA filter will not need to be as strong, but I doubt it can be fully dispensed with. Higher resolutions will enable larger print sizes aliasing will still eventually show up although it's effect will occur on ever finer repeating patterns than it does now.
I like to think of the AA filter as analogous to the safety features on your car, like airbags, traction control etc. You don't need them all the time or often, but when you do they will save your bacon. And it's much much harder to sort out a bingled car than it is to have avoided the accident and injury in the first place.