T3D Re: digital camera resolution

[michaelk@xxxxxxxxxxxxxxxxxx (Michael Kersenbrock)]

> Michael Kersenbrock wrote:
> 
> > Just a comment that the digital camera article mentions that the
> > cameras *do* have an optical nyquist filter up front.
> 
> I don't know what they mean!!!! There is no need for an optical Nyquist
> filter on a digital or film camera because both the ccd and film are
> integrating detectors!!  
> 
> Maybe nyquist filter = lousy optical quality.  Typically you want your
> optical resolution 3 to 5 times better that the detector resolution so
> that cruddy optics do not degrade the overall image quality but
> conversely you don't purchase needless overkill.

Well, it says:

"Behind the lens and in front of the image sensor, many cameras
employ an optical prefilter -- a piece of quartz that selectively
blurs the image.  This prefilter conceptually serves the same
purpose as a lowpass audio filter.  Because the image sensor contains
fixed spacing between pixels, light wavelengths shorter than twice this
distance can produce aliasing distortion if they strike the sensor 
(Notice the similarity to the Nyquist audio sampling frequency?).  A
similar distortion comes from taking a picture
containing  edge transitions that are too close together for the
sensor to accurately resolve them.  This distortion often manifests
itself as a color fringes around an edge (Figure 3) or as a series
of color rings known as a moire pattern. "

Figure 3 shows a picture of buildings with edges and color fringing
on them.


Mike K.




> 
> --
> John Ohrt,
> Toronto * Ontario * Canada
> 
> 


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