Cleaning a DSLR

"Rather than simply omitting the anti-aliasing filter when building a digital camera, manufacturers have to use a filter that doesn't have the normal anti-aliasing properties, but still retains the ability to block infrared light, for example. So a new filter design is required."

So there still is a filter !

Aliasing is a general term in Digital Sampling Theory (it applies to digital sampling of any function) and refers to 'Aliases', or false components in the reconstructed function as a result of the sampling process. A description of the aliasing effect can be found Here

Aliasing occurs whenever the sampling rate (either time or spatial sampling) is insufficiently high to cope with the level of detail in whatever is being sampled. The mathematics are a little complicated, but if you want details then Googling Nyquist and Shannon will tell you more.

The digital camera is just one example where an anti-aliasing filter is used. In digital audio, the anti-aliasing filter is an electrical low-pass filter with a cut-off frequency at around half the sampling frequency. For example, audio CDs use a sample rate of 44.1 kHz, and the anti-aliasing filter used in the recording process blocks audio frequencies above approximately 20 kHz (the upper limit of human hearing). In a digital camera, where the sampling is in the spatial domain, the strength of the anti-aliasing filter depends on the the resolution of the sensor. Aliasing occurs if the detail in the image presented to the sensor is greater than half the resolution of the sensor.  

The anti-aliasing filter in a DSLR can therefore be regarded as a 'blurring' filter, but its purpose is to prevent false detail appearing in the captured image.

Some DSLRs are made without anti-aliasing filters for those people who want higher apparent sharpness, but the perception of sharpness in the resulting image does not represent reality, since the sharp details are mostly false (aliases).

There is only one camera I know of where the anti-aliasing is done without a physical filter in front of the sensor. This is the Pentax K-3 where the sensor is vibrated minutely to blur the image. Not only can the anti-aliasing be switched off completely if required, but the degree of anti-aliasing is controllable. 

I realise I have rambled off the topic of  'Cleaning a DSLR', but the anti-aliasing filter should be regarded as an essential part of the camera, and is not merely an 'effects' filter.

Hi Ruth,

We'd appreciate if you didnt keep implying that the forum threads problem is the result of a hack or virus, as we have already told you it isn't. It is a technical issue (in house) that we are already working on. If you are still not convinced, that is your prerogative, but we'd appreciate it if you didn't bring it up again in the forum as it will do no good for the site and really is unnecessary.

Regarding any members photos/comments that have disappeared, this would only be as a direct result of either them or us removing them, and nothing else. There is a privacy issue there so we won't be more specific on that, and we'd appreciate it not being discussed on the PoE forum out of respect for any members involved. Thank you for your support in this matter,

 Chris & Sarah

A rather ambiguous reply, but now I understand why, everything falls into place. I've managed to sort the problem out myself. 

Having spent a few hours searching around the Internet and making a few phone calls I've managed to locate and get in contact with John. I now know exactly what's happened, why his photos have gone, and indeed his account. I now realise why there are holes in some of the forums. Anyway it's a long story.

Anyone who wants to PM me, I have John's email address, I know some of you want to keep in touch, he is a good friend. I also know where he is posting his pictures now and he's quite happy to share the details with his friends.