[opendtv] Re: Line Pairs/millimeter vs. Price vs. Image Format Area
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 29 May 2007 14:17:39 -0400
Dan Grimes wrote:
> 1) Very few professional SLR still digital cameras have
> 24mm x 36mm imagers. Most are around 16.7mm x 25.1mm
> (which is the APS-C format) or smaller. Based on the
> trend in still photography cameras, the standard size
> for professional SLR imagers will be in that area.
This does not change the situation appreciably. In fact, 18mm X 24mm is
the standard for 35mm movie frames, and Mark Schubin did indicate that
one camera had a sensor the size of 65mm movie film, which is similar to
"medium format" still cameras. Quoting:
"The NAC and Dalsa cameras use imagers slightly larger than a 35-mm
movie frame. The Vision Research Phantom 65 has an imager the size of a
65-mm movie-film frame. The Photron Ultima APX uses an imager slightly
smaller than a 35-mm movie frame. The Weinberger Cine SpeedCam and the
Weisscam HS-1 use imagers significantly smaller than 35-mm but
significantly larger than 2/3-inch video."
In any event, a lens capable of 70 lp/mm with a sensor 18 X 24 mm should
be able to resolve 2520 X 3360, or 8.5 Mpels, which is still more than
adequate for 1080p (2 Mpels).
> 2) Very few still lenses that cover 24mm x 36mm are capable
> of 100 LP/mm. Good ones are 70 LP/mm. Most are in the 50s.
> And that is for fixed focal lengths. Zooms are far worse and
> very inconsistent over the zoom range.
Being an amateur photographer, I have tracked this sort of performance
figure for a very long time. I realize that not all 35mm camera lenses
are that good, but the ones that are are also not all that expensive. A
few years ago, I bought a Zeiss 50/1.4 for $160 street price, and a
Zeiss 85/1.4 for a few hundred more, about $500. Peanuts for
professionals. Both are better than 90 lp/mm at image center. (Pop Photo
used to publish those figures, but now they went to a "subjective
quality factor" rating which homogenizes all the hard numbers.)
> 3) There is trade off between image size, price and resolving
> power. When you go to a medium format (larger) image area
> (say 6 cm x 7 cm), the image quality increases because of the
> film grain reduction, but the LP/mm for the lenses are typically
> less and only the same if you are willing to spend more money
> for the lens on the order of 10X the cost.
True, but when the image area goes up so much, the lp/mm can afford to
be less. I know that medium format lenses don't measure very high
compared with 35mm lenses. So assume 40 lp/mm in a 45 X 60mm frame (the
smallest format of "medium format" cameras, and similar to 65mm movie
film). That's still 17.3 Mpels equivalent in the 45 X 60 mm frame. And
again, these are priced for amateurs, so they should be no big obstacle
to professional film makers. (Medium format lenses go for multiple
hundreds of $, in the "normal" ranges.)
I agree with your point about film grain being less of an issue with the
medium format frame, but that same point applies also to the lens
quality. More glass might mean more cost, but less resolution demanded
from that glass lowers the cost at the same time. The sweet spot used to
be in the 35mm SLR lenses. Maybe now it went to APX. Either way, it's a
good fit for digital TV cameras if they use this basic frame size.
> 4) In the digital still camera world, because of the reduced
> imager sizes, they can make the image area smaller and thus
> manufacture lenses cheaper for the same resolution. If you
> want to compare apples to apples, compare Canon lenses (of
> the same generation) that cover APS-C imagers with lenses that
> cover the full 35mm still image area.
I don't consider APS to be hugely different from 35mm, so I don't think
the lens price/performance will be all that different. Before APS came
to be, it seemed pretty obvious that 35mm lenses were the best
performing at the lowest cost. That may have changed slightly now in
favor of APS, I don't know.
> 6) A lens designed for a larger film area will look very poor
> on a smaller imager. It proves the difference in LP/mm trade
> off when going to a larger image size. I've done it many times
> (for various reasons) and rarely has the image quality been
> acceptable with regards to resolution.
Again, that will depend on how big the difference. Digital SLRs tend to
have the same lens mounts as their 35mm film cousins. You can use 35mm
Nikon SLR lenses on digital Nikon SLRs, for example, even though the
digital Nikon has an APS-size sensor. And that works quite well,
according to the reports I've read.
So as long as TV cameras use APX or similar sized sensors, in favor of
the tiny sensors of the old days, lens costs should not be an issue for
1080/60p cameras, I don't think. We can quibble about 24 X 36 mm vs 18 X
24 mm, but lenses for these formats are affordable and easily capable of
handling 2 Mpels, I do believe.
Bert
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