[opendtv] Line Pairs/millimeter vs. Price vs. Image Format Area
- From: dan.grimes@xxxxxxxx
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 29 May 2007 10:08:40 -0700
Bert writes: "I think Mark Schubin indicated that new cameras are
available now with sensors the size of 35mm movie film frames (18mm X
24mm). That should take a lot of cost out of the lens, and should make
those cameras good candidates for 1080p acquisition.
Craig Birkmaier Responds: No, it increases the cost of the lens.
Bert Argues in Response: "Wrong, Craig. You can find any number of
consumer-grade 35mm lenses good for as much as 100 lines/mm at the image
center, and maybe half that much at the edges. The ones I'm referring to
are for 35mm still cameras, so they are for 24mm X 36mm film frames. They
would provide very good performance in the smaller movie frame. I assume
35mm movie cameras can benefit from the same lens technology. (Nikon, Carl
Zeiss, Schneider are some good brand names.)
Dan Grimes Responds: Bert, I'm afraid Craig is right regarding the
increase in the cost of a lens when the image area increases. I've been
involved with imaging most of my life and have worked with film (still and
motion picture) lenses for over a two decades.
A few facts in the still and film lens world:
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.
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.
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. When the image area
is increased, the design and manufacturing tolerance are much more
critical, and thus much more expensive, to get the same LP/mm. However,
this isn't a hard and fast rule. I have a 58mm lens that cost $100 (used)
with only four elements that covers 6cm x 7cm and can get 100 LP/mm with
excellent contrast. The key was the design and manufacturing technique.
But that is very rare indeed.
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. Also, there is a huge
difference in lens pricing between the consumer level lenses and the
professional level lenses. Only the professional lenses can get the
resolution necessary to make use of 10 to 12 MP camera.
5) Lens quality and cost have drastically reduced because of newer
materials and manufacturing techniques that allow aspherical lenses.
Aspherical lenses need fewer corrective lenses. But it doesn't cancel the
physics of the design for image area and the trade offs.
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.
Dan Grimes
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