> > So I saw a news item the other day that mentioned > a tie Bush was wearing > had such a tiny repeating pattern that the cameras > got confused and there was > a lot of unwanted play of light. > > I had earlier been told not to wear a blouse with > tiny checks or pinstripes > when I went to appear on a kind of local antiques > road show because of the > moire effect. > > Is this something that has always been a problem > with video technology or > is it (a) just since the digital revolution > and (b) only something to worry about in MOVING > images?
No, and no. It's not quite the same sort of thing as a Moire effect, although both fall under the general umbrella of what is referred to as 'aliasing'. The Moire effect is caused by interference between two spatial frequencies - it's the spatial analogue of the temporal wagon-wheel reversing effect in movies. Moire fringes show up in still images - no motion required. The problem with fine detail (pinstripe shirts, etc.) is caused by the fact that the bandwidth of the signal needed to exactly reproduce that image exceeds the bandwidth allocated (in the case of TV broadcasts). This usually shows up as a chroma crawl artifact, as some of the received signal gets misinterpreted. There are also some Moire effects (as you will see sometimes if you take a digital still photograph), but the bandwidth limitations of TV broadcasts will have orders of magnitude more effect on the result. > Are some of the effects we are seeing in images > here that displease > or perplex due to this kind of thing? I doubt it. Perhaps if you take a lot of photographs of fine meshes (or resolution charts :-) you might see artifacts on one or two of the shots, if you happened to be exactly the wrong distance away from the subject. Maybe the wedding photographers might have the occasional problem with a bridal veil. > How does a tight closeup of a starling's wing, for > instance, get affected? Far less than a wider shot with the starling occupying a smaller portion of the frame. You get problems when the size of a detail element is comparable to the size of the "pixel" (for still images) or bandwidth limit. Moving in for a tight closeup means the detail elements cover a larger area of the sensor, and so are easier to depict.
