In praise of AA filters

Started 10 months ago | Discussions
Tom Axford
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In praise of AA filters
10 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).

RhysM
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Re: In praise of AA filters
In reply to Tom Axford, 10 months ago

Damn, i guess i'm going to have to stop shooting all those concentric black and white circles i love so much and start shooting real world objects/scenes where the effect of having no AA filter doesn't become apparent and the trade off of extra sharpness is an advantage.

Also did you know that theres a new sub-forum on dpreview called "Photographic Science and Technology" for all these boring, let's hyper analyse our gear, MFT charts, etc... type posts?

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Nigel Wilkins
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Re: In praise of AA filters
In reply to RhysM, 10 months ago

RhysM wrote:

Damn, i guess i'm going to have to stop shooting all those concentric black and white circles i love so much and start shooting real world objects/scenes where the effect of having no AA filter doesn't become apparent and the trade off of extra sharpness is an advantage.

Also did you know that theres a new sub-forum on dpreview called "Photographic Science and Technology" for all these boring, let's hyper analyse our gear, MFT charts, etc... type posts?

It does however, highlight the fact that it's photos of manmade patterns that are most likely to cause problems, since many things are manufactured this way, but these type of regular patterns rarely occur in nature.  Hence, wedding, fashion or architectural photographers might hit problems, but nature & landscape photographers probably won't.

To the OP, do a google image search for moire for examples.

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Tom Axford
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Re: In praise of AA filters
In reply to Nigel Wilkins, 10 months ago

Nigel Wilkins wrote:

It does however, highlight the fact that it's photos of manmade patterns that are most likely to cause problems, since many things are manufactured this way, but these type of regular patterns rarely occur in nature. Hence, wedding, fashion or architectural photographers might hit problems, but nature & landscape photographers probably won't.

Very true.

With regard to examples, there are lots of examples of moire patterns.  What I failed to make clear was that I was looking more for other types of examples as it seems to often be assumed that moire is the only disadvantage of removing the AA filter. Also, as can be seen from the images in my OP, the inclusion of an AA filter only reduces the moire effect, it generally doesn't completely remove it.

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GodSpeaks
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Wonderful
In reply to Tom Axford, 10 months ago

I will make a point to make sure the next 11K pixel camera I buy HAS a good AA filter.

Meanwhile my 36MP Nikon D800E happily does not have an AA filter.

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GarageBoy
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Re: In praise of AA filters
In reply to RhysM, 10 months ago

RhysM wrote:

Damn, i guess i'm going to have to stop shooting all those concentric black and white circles i love so much and start shooting real world objects/scenes where the effect of having no AA filter doesn't become apparent and the trade off of extra sharpness is an advantage.

Also did you know that theres a new sub-forum on dpreview called "Photographic Science and Technology" for all these boring, let's hyper analyse our gear, MFT charts, etc... type posts?

Have fun when you get something like a fine tweed fabric, tie, or mesh in your shot!

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DaveOl
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Re: In praise of AA filters
In reply to GarageBoy, 10 months ago

Just looking at the target image hurts my eyes!  How did you capture that image without a camera with or without an AA?

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StevenMajor
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Re: In praise of AA filters
In reply to Tom Axford, 10 months ago

Thanks for the helpful and informative post....I thought there was software to correct the moire problem?   Yes?

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Tom Axford
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Re: In praise of AA filters
In reply to DaveOl, 10 months ago

DaveOl wrote:

Just looking at the target image hurts my eyes! How did you capture that image without a camera with or without an AA?

The original image was computer generated.

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Tom Axford
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Re: In praise of AA filters
In reply to StevenMajor, 10 months ago

StevenMajor wrote:

Thanks for the helpful and informative post....I thought there was software to correct the moire problem? Yes?

Yes, there is, but it relies on guidance from the user as to where to correct the moire.

It is not possible to automatically totally correct moire in software as the software has no way of knowing for sure whether the moire was generated by the camera or whether it was in the image being captured. For instance, if you used your camera to take a shot of your computer screen showing the images in my OP in this thread, you wouldn't want the software automatically removing the moire that is shown in the images.

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Tony Beach
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Re: In praise of AA filters
In reply to Tom Axford, 10 months ago

Tom Axford wrote:

...assuming that the image is not degraded in any way by the camera lens)

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

As camera sensors reach the point where they are resolving more than any lens you can put on them, the lens has become the AA filter and therefore adding more blur only degrades the image quality.  This is why Sony and Nikon are both getting rid of AA filters on their latest generation of sensors -- they are not fools.

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Tom Axford
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Re: In praise of AA filters
In reply to Tony Beach, 10 months ago

Tony Beach wrote:

Tom Axford wrote:

...assuming that the image is not degraded in any way by the camera lens)

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

As camera sensors reach the point where they are resolving more than any lens you can put on them, the lens has become the AA filter and therefore adding more blur only degrades the image quality. This is why Sony and Nikon are both getting rid of AA filters on their latest generation of sensors -- they are not fools.

Yes, indeed.

I don't know for sure, but I suspect many cheap compact cameras use a lens of relatively poor quality to act as an AA filter. In fact, if the sensor is small enough, then even if the lens is perfect, diffraction blurring will act as an AA filter.

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Tony Beach
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Re: In praise of AA filters
In reply to Tom Axford, 10 months ago

Tom Axford wrote:

In fact, if the sensor is small enough, then even if the lens is perfect, diffraction blurring will act as an AA filter.

By about f/8 diffraction blurs enough to avoid moire on the D800E.

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crames
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Re: In praise of AA filters
In reply to Tom Axford, 10 months ago

Tom Axford wrote:

...

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.

...

It would be more realistic to include the effect of fill factor, which in modern aa-less cameras approaches 100%. Instead your simulation approaches 0% fill factor (point sampling). You can use the Pixelate/Mosaic filter in Photoshop to add pixels together before resizing, in order to simulate fill factor.

AA-filterless with 100% fill factor looks more like this:

Without AA, 100% fill factor, shown 200%

As you can see, fill factor does a lot of the work of an AA filter. Not nearly as bad as your point-sampling example.

Cliff

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unknown member
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No need as I shoot with Sigma cameras :)
In reply to Tom Axford, 10 months ago

No need as I shoot with Sigma cameras

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unknown member
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Re: Wonderful
In reply to GodSpeaks, 10 months ago

GodSpeaks wrote:

I will make a point to make sure the next 11K pixel camera I buy HAS a good AA filter.

Meanwhile my 36MP Nikon D800E happily does not have an AA filter.

Neither do my Sigmas.

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Lee Jay
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AA filters are necessary even for Sigma cameras
In reply to Tom Axford, 10 months ago

Offering me a choice of with and without AA filter would always result in me choosing the camera with the AA filter.
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rovingtim
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Alias artifacts don't only occur in man-made objects
In reply to Tom Axford, 10 months ago

Once you take out the AA, the software of the camera necessarily much finish off partial signals. If it gets it 'right', it is rarely noticed. However, when it gets is wrong, we used to call that an artifact. Regardless, it is software invented 'detail'.

As someone point out to me, ever taken a shot of a stubbled man with a no AA camera and seen the occasional massive beard hair? Ever noticed how a camera with no AA will happily capture sharp detail of brickwork and then suddenly it go blank like someone blurred it in photoshop? Then a little further on, more sharp detail. No gentle transition.

Ever notice that some leaves on the trees look 'crunchy'? Ever notice that the colour shifts in busy areas of a landscape? For example, a clump of trees looks to be in a faintly purple shadow where none exists.

Ever notice that some 'photographers' are constantly striving to improve their ability to overlook the increasing alias artifacts of modern cameras?

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ryansholl
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Good point
In reply to Tom Axford, 10 months ago

My photos of concentric circles from far away are going to be terrible!  I can't believe I preordered the a7r!

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unknown member
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Re: Alias artifacts don't only occur in man-made objects
In reply to rovingtim, 10 months ago

rovingtim wrote:

Once you take out the AA, the software of the camera necessarily much finish off partial signals. If it gets it 'right', it is rarely noticed. However, when it gets is wrong, we used to call that an artifact. Regardless, it is software invented 'detail'.

As someone point out to me, ever taken a shot of a stubbled man with a no AA camera and seen the occasional massive beard hair? Ever noticed how a camera with no AA will happily capture sharp detail of brickwork and then suddenly it go blank like someone blurred it in photoshop? Then a little further on, more sharp detail. No gentle transition.

Ever notice that some leaves on the trees look 'crunchy'? Ever notice that the colour shifts in busy areas of a landscape? For example, a clump of trees looks to be in a faintly purple shadow where none exists.

Ever notice that some 'photographers' are constantly striving to improve their ability to overlook the increasing alias artifacts of modern cameras?

That doesn't apply to Sigma cameras with their Foveon sensors.

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