[opendtv] Re: Line Pairs/millimeter vs. Price vs. Image Format Area
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 31 May 2007 15:03:51 -0400
Craig Birkmaier wrote:
>> Bottom line: increasing the sensor area makes it easier
>> to provide a given number of Mpels. Dispute that, to begin
>> with.
>
> DUH!
>
> Now what has that got to do with HDTV lenses?
Lots. As Mark Schubin replied, as his lens specs showed, as I discussed
in reponses to Dan, the bigger sensor permits use of lower quality
lenses to achieve the same image resolution. Use of bigger sensors also
promotes higher MTF.
For example, I showed where the lens for a standard 18 X 24 mm movie
camera, at merely 70 lp/mm, can provide theoretically as much as 8.5
Mpels of equivalent image resolution. If that lens is used for HDTV, at
merely 2 Mpels, the MTF should be excellent. The contrast between lines,
as projected on the 2 Mpel large sensor, will be very high, compared to
what you can do with a lens that barely meets muster.
If you read the articles you posted, this would have become apparent.
They made it quite clear that the smaller portable HDTV cameras did not
perform as well as the heavy studio stuff. They made it clear that the
small hand-held cameras were not a good replacement for studio cameras,
even if they are cheaper overall. They are cheaper AND LESS GOOD.
The opposite side of that coin is that the lens designed to spread light
over the larger sensor has to be bigger, all else equal.
Here is some high school lens physics to prove this.
Hi/Ho = Si/So. The ratio between object and image sizes (Ho and Hi) =
the ratio between object and image distance (So and Si).
1/Si + 1/So = 1/f. For equal object distance, the lens with a longer
focal length creates its image further away compared with a shorter
focal length lens.
From these two, you can derive Hi/Ho ~ f/So. For a given object
distance, the focal length of the lens is directly proportional to the
image size you want to create. So, bigger image = longer focal length,
and longer image distance.
f-stop = aperture diameter/f. For a given f-stop, a longer focal length
lens requires a bigger aperture. Which means, if you want to put as much
light on the big sensor, compared with the light projected on that
smaller sensor, you need to build a bigger lens.
So that's why the lens cost goes down as sensor size goes up, to a
point, but then you start getting into seriously big glass (and heavy
cameras), and overall cost will go up again.
Bert
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