[opendtv] Re: Mpels in still cameras

  • From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Sat, 29 Nov 2008 17:51:18 -0500

Cliff Benham wrote:
> But is the digital noise 'random' like the graininess
> of film or is it arranged symmetrically in nice
> convenient little square clumps so it can become
> visible at higher magnifications?
The noise per se is random. Sort of looks like snow in analog TVs. But the 
complete answer to your question is that both effects will occur.
All else equal, a camera with more pixels in the same image area will exhibit 
more noise at high ISO settings than a camera with fewer pixels in that same 
area. So the camera with the lower Mpel spec will be better at taking pictures 
in low light levels than the super high res camera (for a given sensor area).
On the other hand, the camera with the lower Mpel spec will be the one whose 
images, if blown up, will start showing an objectionable pixel structure 
sooner. So basically, you're trading off ultimate sensitivity for higher 
resolution. Which we also had to do with silver halide film.
Also, to be more complete about the diffraction effects with respect to image 
sensor area, here's the deal. For a given field of view, e.g. about 46 degree 
diagonal for a "normal" lens, a camera with a larger sensor area requires a 
longer focal length lens than a camera with a smaller sensor area. Since the f 
stop is that ratio of focal length to iris diameter, a given f stop represents 
a bigger iris diameter in a longer lens than in a shorter one.
So, let's say at f/16, the diffraction effects for a "normal" lens in an small 
sensor camera will be more pronounced than the diffraction effects of a 
"normal" lens in a larger sensor camera. And diffraction is what reduces 
contrast ratio, hence perceived sharpness.
That's why it's not difficult to make affordable HDTV cameras out of digital 
SLRs. They tend to have large sensor areas. If you go to medium format still 
cameras, which are even bigger, lenses start becoming more expensive again, due 
to the sheer size of glass. And no doubt also because there's much smaller 
demand for them out there in consumerland. Too big.
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