Artefacts (or interference patterns?) when 'capturing' still VHS on a digital camera

For reasons too complex to go into at the moment I have been using a

Sony DSC F505V digicam to capture stills from some CCTV footage. In

order to reduce banding interference and glare I have set the shutter

speed to 1/15 sec and worked in low light conditions. The images

produced on the memory stick are reasonable. I intend to print these

and also email them to others (it's part of an insurance claim not

dodgy goings on in the workplace...).

Unfortunately the JPEGs have arcs of interference patterns

(artefacts ?) which appear when the JPEG is viewed from a zoomed out

position but disappear in close-up views. These are like those which

appear on some footage of footage (so to speak).

Q - Has anyone else experienced such a problem and is there a

solution. I have similar problems with all of the available

resolutions I have tried (to max res of 3.3Mpixel camera)

Ian, is my understanding correct that you are photographing a monitor? If so, the monitor has a certain "scan" frequency; I would guess that catching the scan mid-screen is causing what you called interference patterns. If so, using an even longer shutter speed should minimize it more.

On the other hand, you may already realize this, since you indicated that you used 1/15 second to "reduce banding interference" and your problem is a different one. If so, you might be talking about "aliasing", where the the pixel dots on the monitor and the pixel sensors on the CCD have got such a relationship between them that something akin to a "beat frequency" called a moire pattern occurs. If so, the ideal solution would be a camera with a better (optical) anti-aliasing filter inside. To do with your current camera, I would suggest try this; use your camera at the highest resolution setting, but try putting the lens at various slightly off focus positions. Hopefully you will find a position that stops the aliasing (IF that is the problem) yet does not hurt image detail too badly. Good luck!

PS, or possibly you are talking about a problem that only exists displayed on a monitor? If so, it's likely the same basic problem (aliasing); I can't suggest any definite cures here.

Thanks for the prompt reply Bill,

Yes I have noticed that the effect diminishes and returns as I move the camera towards the video monitor screen and away again (sorry that it was not clear in my question that I am photographing a CCTV image displayed on a video monitor). I have tried to reduce this effect by finding the anti-nodes (or whatever they may be called in aliasing) and this helps with the initial image. I have reduced the shutter speed to reduce the bands resulting from the CRT scan - as seen on TV pictures of monitors and TV screens and that works ok.

The odd thing is that when I capture the image it is fine on the LCD screen and fine when I display it in high detail (close-up or zoomed in) but the alaising reappears when I zoom out to look at the image as a whole. I have used the camera at the maximum resolution (about 2200 by 1450) but this has little or no effect. I shall look into anti-aliasing filtration for this camera - the camera belongs to a work colleague and I needed the graps fast for a barrister !