In praise of AA filters

Started 11 months ago | Discussions thread
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Tom Axford
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In praise of AA filters
11 months ago

There have been several interesting threads recently discussing AA (anti-aliasing) filters, Moire patterns and similar effects. My views on the desirability of AA filters have gradually changed. At one time I may have been tempted by cameras without an AA filter on the sensor, but I now regard the AA filter as essential.

However, it is quite hard to find really convincing images to show the advantages of using an AA filter, not least because I do not own two cameras that are identical except one has an AA filter and the other doesn't (and I suspect very few photographers do).

Instead, I have attempted to simulate the effect of removing the AA filter on an image. I have taken a pretty extreme example in that the image consists of black lines and text on a white background (i.e. 100% contrast) and have simulated the effect of capturing this image with a sensor consisting of a square array of identical pixels (i.e. monochrome recording).

The image to be captured is shown below. I assume that this image is formed on a small section of the sensor (only 95 pixels wide).

The image to be captured by a 95x112 pixel section of the sensor

If there is no AA filter in front of the sensor (and also assuming that the image is not degraded in any way by the camera lens), then the captured image is something like this:

Captured image without AA filter, shown at 200% actual size

If there is an AA filter present, then the captured image will be more like this:

Captured image with AA filter, shown at 200% actual size

Neither image looks much like the original because the original contains far more detail than it is possible to show in a 95x112pixel image. However, the first looks much sharper and deceives us into thinking it contains quite a lot of detail. Most of the detail is false and quite misleading. The second image appears more obviously blurry and soft, so we are not deceived into thinking that it contains a lot of useful detail.

It is more surprising if you look at the two lines of text below the circular disc. In the first image it is effectively impossible to guess what these are. In the second image, despite it being more blurry, it is now possible to make a reasonable guess at some of that text!

I think this example illustrates very strongly the case for using an AA filter.

This example is not produced by a real camera, it is purely a software simulation, and I would not claim any great accuracy, but I think it does illustrate the principle of what happens with an AA filter.

I'd be very interested in comments from others about whether they agree that it is a reasonable (if rather extreme) example.

What I did to simulate the effect:

The first 'captured' image was produced by resizing the original image to 1/8 size with no interpolation.

The second was produced by blurring the original with a focus blur of radius 8 pixels, then resizing to 1/8 size with no interpolation.

If a real camera was used to produce similar images, the difference would be much less because the camera lens would inevitably degrade the image considerably before it reached the sensor (bearing in mind that we are looking at only 95x112 pixels, so the actual image would be very small).

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