in Canon rumors there is talk of a 50mp camera with a low pass filter. what is a low pass filter?

[Skirball · Accepted solution]

Technically, the news is that there will be one without the low-pass (aka anti-aliasing, aka AA) filter.  Most cameras, and all of Canon's dSLRs do have them.  In fact most all have something of one, but certain cameras like Nikon's high megapixel 800E had a very light one.

Low-pass filters 'catch' the high frequencies, the detail in a picture, and blur it slightly.  Although it reduces the sharpness slightly, it also reduces moire and aliasing issues, so the tradeoff is considered worth it.  A slight reduction in sharpness for a big reduction in moire and aliasing.

But there are many ways to reduce moire, really any kind of blurring will do it - such as a cheap lens.  The theory here is that as the megapixels get higher, the size of the pixels gets smaller, and diffraction increased - which acts as a kind of blurring effect. So high megapixel (small pixel) cameras are naturally more resistant to moire.  Since AA filters reduce sharpness slightly, it would make sense to remove it, if this proved sufficient..

The problem is, that as lenses get sharper the more you'll see things like moire.  And most people that own high end high megapixel cameras are going to be slapping sharp glass on there.  And there's certain to be an uproar if Canon released a new high end camera that had unusable moire with certain lenses.

Another great example of the fact that everything in photography is a tradeoff...  increase sharpness, increase chances of moire and aliasing artifacts.  Take your pick.

[TCampbell · Accepted solution]

The rumor speaks of Canon making a version with and another without the low pass filter.

The low pass filter is directly in front of the sensor and it's job is to very fractionally diffuse the light to deal with the "moiré" issue.  It's sometimes called an anti-aliasing filter.

It technically creates a very tiny amount of blur.

As a result, a camera that doesn't have a low pass filter can possibly produce a fractionally sharper issue (assuming a very high resolution sensor, becuase at lower resolutions you probably wouldn't notice it.)  However... such a camera would be more likely to have the issue with "moiré" patterns showing up in your images.  Only specific patterns can generate moiré.   In landscape photography it is unlikely you'd run into this issue.  In fashion or architecture photography I should imagine you'd run into this issue more often.

Edit:  Looks like Skirball was posting at the same time.  Sorry for the redundant post.

[Skirball · Accepted solution]

Technically, the news is that there will be one without the low-pass (aka anti-aliasing, aka AA) filter.  Most cameras, and all of Canon's dSLRs do have them.  In fact most all have something of one, but certain cameras like Nikon's high megapixel 800E had a very light one.

Low-pass filters 'catch' the high frequencies, the detail in a picture, and blur it slightly.  Although it reduces the sharpness slightly, it also reduces moire and aliasing issues, so the tradeoff is considered worth it.  A slight reduction in sharpness for a big reduction in moire and aliasing.

But there are many ways to reduce moire, really any kind of blurring will do it - such as a cheap lens.  The theory here is that as the megapixels get higher, the size of the pixels gets smaller, and diffraction increased - which acts as a kind of blurring effect. So high megapixel (small pixel) cameras are naturally more resistant to moire.  Since AA filters reduce sharpness slightly, it would make sense to remove it, if this proved sufficient..

The problem is, that as lenses get sharper the more you'll see things like moire.  And most people that own high end high megapixel cameras are going to be slapping sharp glass on there.  And there's certain to be an uproar if Canon released a new high end camera that had unusable moire with certain lenses.

Another great example of the fact that everything in photography is a tradeoff...  increase sharpness, increase chances of moire and aliasing artifacts.  Take your pick.

[TCampbell · Accepted solution]

The rumor speaks of Canon making a version with and another without the low pass filter.

The low pass filter is directly in front of the sensor and it's job is to very fractionally diffuse the light to deal with the "moiré" issue.  It's sometimes called an anti-aliasing filter.

It technically creates a very tiny amount of blur.

As a result, a camera that doesn't have a low pass filter can possibly produce a fractionally sharper issue (assuming a very high resolution sensor, becuase at lower resolutions you probably wouldn't notice it.)  However... such a camera would be more likely to have the issue with "moiré" patterns showing up in your images.  Only specific patterns can generate moiré.   In landscape photography it is unlikely you'd run into this issue.  In fashion or architecture photography I should imagine you'd run into this issue more often.

Edit:  Looks like Skirball was posting at the same time.  Sorry for the redundant post.

[wln · Accepted solution]

thanks to both of you that replied. Both answers helped me to understand . 

Thanks,

WLN

[TCampbell]

If you go to images.google.com and do a search for terms like "examples of moire" you'll see lots of examples that show what the low pass filter is trying to eliminate.

[Skirball]

There's actually a really good one in the thread on CR that is discussing the 50 mp camera; I assume that's the thread the OP is referring to.

Edit - This one: