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Posted by Dave Martindale on 11/06/07 21:46
"David McCall" <mccallmail@verizon.net> writes:
>Often people confuse this measurment of resolution with
>the number of pixels in rows and columns of the picture.
>They are related, but not the same.
>Lines of resolution take into account not only the pixel resolution,
>but also the optics of the camera. A single line consist of a
>black line as well as the white space between the black lines.
>A simplistic way of looking at it would be to say that 800 lines
>of resolution would require 1600 columns of pixels.
>One pixel for the black line and one for white space.
>Unfortunately it isn't that cut and dried. But that should get you started.
Unfortunately, you're confusing things further.
When still photographers and optical designers talk about "lines", they
generally do mean "line pairs", which is one black and one white line.
These are the same size as "cycles", one cycle of a sine wave, which is
used in MTF measurements, so measuring resolution in line pairs or
cycles gives close to the same value.
In the video world, "lines" counts black and white lines separately.
So you need to multiply by two when converting line pairs to video
lines. And, in theory at least, resolving 800 TV lines takes at least 800
pixels, not 1600 pixels. (In practice, with real equipment, using
decent anti-aliasing filters, the number is closer to three pixels per
line pair or per two TV lines).
But wait, there's more complexity. TV resolution is generally quoted in
terms of "lines per picture height", even when it is measuring
horizontal resolution. If you want to know the total number of lines
*per picture width* that can be resolved, you need to multiply by the
aspect ratio. For example, a SDTV camera with "500 line" resolution can
actually resolve 500*4/3 = 667 tv lines, or 333 line pairs, across the
width of the screen.
another Dave
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