[opendtv] Re: Line Pairs/millimeter vs. Price vs. Image Format Area

  • From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 29 May 2007 20:29:31 -0400

Dan Grimes wrote:

I may have miss understood, but what I heard from Bert was
that considering two lenses with the same qualities but
covering a different image area, a larger format lens is cheaper
while Craig said the opposite is true.

Not quite. The point I was trying to make was and is that when covering a larger area, a lens can *afford to be less good*, to achieve the same image quality as that expensive and tiny lens. So, as digital TV cameras go to larger sensors and therefore larger lenses, the lens specs can be reduced, and with that the cost. Especially because they tend to the popular consumer lens sizes when they go up in size.

Mark's lens examples made that clear. Check out how the lp/mm spec was capable of being reduced as the size went up.

While it is true that the MTF of the system needs to be considered
for the overall performance, when considering the lens only, it
is more difficult and expensive to make a lens to cover a larger image
area and still maintain all the positive qualities (max. resolving power,
light transmittance, colorimetry, MTF, etc.)

Well, I'll certainly agree that cost is not linear with lens size. Cost goes down, then up again. There is the sweet spot.

But I don't think that MTF can be dismissed as something else. It's just like audio equipment. If I tell you this amp is good for, say, 100 KHz of bandwidth, you would be wise to ask me, "Is that the 3 dB point?" Because otherwise, the bandwidth may even be 100 KHz sort of, but oh by the way that's 20 dB down at 100 KHz, and even at 20 KHz the output power is 6 dB down (just an example). Which is not good at all. So it matters what the contrast between these lines you are counting is. A larger image area promotes higher contrast between the lines, all else equal.

Bert

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