What compact digicam has the biggest CCD pixels?

Discussion in 'Digital Photography' started by Paul Rubin, Apr 22, 2005.

  1. No. Ilya forgot to include read noise. Read noise is currently
    approximately the same for all good sensors, somewhere in
    the 7-15 electron range. As your signal gets smaller,
    e.g. smaller pixels, or shadow areas, read noise becomes
    a larger fraction of the total noise from the sensor.
    See:
    http://www.clarkvision.com/imagedetail/does.pixel.size.matter

    Note too the diffraction spot diameter: for f/2.8 it is
    3.2 microns for blue (4700 angstroms) light, 4.1 microns for
    red light (6000 angstroms). Pixels smaller than 4 microns
    have images that are diffraction limited (many high megapixel
    P&S have smaller pixels than this)! Go to f/5.6, which is what
    many zoom lenses are, and your at diffraction spot sizes of
    6 to 8 microns. Diffraction and numbers of photons are why
    large pixel size sensors produce better images. Simple physics
    that will not change unless a major, Major, MAJOR, *MAJOR* rewrite
    of fundamental physics happens.

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 23, 2005
    #21
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  2. Paul Rubin

    prep Guest

    Do you have data to back up this claim? If it was as you say, then
    they simply would not work at all. Even the tiniest leakage of loss in
    readout sends the signal to zero. If you wonder about that, try
    calculating 0.999^2000 and see how many 9s you need to get a
    reasonable answer.
    How do CCDs get hot? They have no electronicss on the chip, unlike CMOS
    chips, and rare static during exposure.

    --
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    prep, Apr 23, 2005
    #22
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  3. Paul Rubin

    paul Guest



    I don't know if it's CCD/CMOS dependent but I heard astronomers cool
    their sensors with liquid nitrogen.
     
    paul, Apr 24, 2005
    #23
  4. Paul Rubin

    paul Guest


    Here are some links:
    http://nordicgroup.us/digicam/dslrcriteria/#Minimum 6 megapixel resolution
    -table of DSLR's showing "pixel pitch" of around 8 microns. I don't know
    how pitch is calculated, your calc giveas around 50 microns for these
    cameras.

    http://www.outbackphoto.com/dp_essentials/dp_essentials_01/essay.html
    http://www.outbackphoto.com/dp_essentials/dp_essentials_02/essay.html
    Biased by the author's Foveon connection but otherwise very lucid
    discussion of sensor/pixel sizes. Interestingly though, he talks about
    Foveon downgrading to average out for better SNR at fewer MP like you
    discuss. I'm still not clear this can't be done with a simple
    downsampling in photoshop but I don't know.

    I think sensor size is one part of the formula but there are large pixel
    cameras with bad noise & more pixels really does count for more (within
    limits). A table showing actual signal to noise ratio (SNR) would answer
    this question. I don't know if SNR is as easily graphed as MTF
    (resolution) but still MTF like pixel count is more important than
    noise. Dynamic range is the other big factor which probably is closely
    tied to pixel size. And the quality of the lens is awfully important
    too. That's how MTF is usually used though I suppose the sensor quality
    effects MTF. Unfortunately none of these critical factors are spelled
    out for the smaller MP cameras. I agree it'd be nice to have a super
    quality compact 1 to 3MP but I guess the reality is the smaller cameras
    are just not taken seriously in terms of quality control so have poorer
    lenses, more noise, etc. and the pixel size is targeted as small as
    possible for the day's technology. If it has bigger pixels at the low
    end, it's because they can't afford to make better smaller pixels.
     
    paul, Apr 24, 2005
    #24
  5. They do, because they want to take very long exposures. CCDs have
    leakage currents that *double* for every 8 degree C rise in temperature.
    If your exposure time is 1/60 second, the leakage in that time is small
    compared to the signal you're trying to measure, so it can be ignored.
    For somewhat longer exposures (a few seconds), you can take a dark
    exposure and subtract it to eliminate most of the effect of leakage.

    But astronomers take exposures that are tens of minutes or hours. At
    room temperature, CCD leakage would saturate the CCD wells and you'd get
    no image at all. So they cool the CCD as cool as practical to reduce
    leakage to almost nothing.

    Even some astronomical cameras used by amateurs use Peltier devices to
    cool the CCD well below room temperature.

    But none of this means that a CCD *heats up* during exposure - it just
    remains at whatever temperature it is. As "prep" wrote, the CCD isn't
    even clocked during exposure, it just has static voltages on it.

    Dave
     
    Dave Martindale, Apr 24, 2005
    #25
  6. Paul Rubin

    ASAAR Guest

    I didn't look at the link, and didn't examine any formula, but
    simple "mind math" showed that if the former was a linear
    measurement and the latter was area, they'd be very close, and this
    was proved by a calculator. 4 * 4 * pi == 50.265. Of course this
    may just be a coincidence.
     
    ASAAR, Apr 24, 2005
    #26
  7. Pixel pitch is the linear distance from one pixel to the next.
    For example, on the Canon 1D Mark II, the sensor size is
    28.7 x 19.1 microns, which have 3504 x 2336 pixels.
    Pixels spacing (pitch) = 28.7*1000/3504 = 8.2 microns x
    19.1*1000/2336 = 8.2 microns. Assuming a square area,
    that would be 8.2x8.2 = 67.4 square microns. Pixels
    do not necessarily have to be round. The question is,
    what is the fill factor (the percentage of the area that
    is sensitive to light)? For smaller sensors, it has to be
    less. Probably at least a micron or so is needed between the
    sensitive areas to keep the charge from leaking across
    pixels. If someone has info on fill factor, please post it.

    This site http://www.fast-vision.com/cameras/camerasensor.HTM
    claims 75% for their CMOS sensors. If similar to the
    1D Mark II, the the Mark II would have about 50 square microns
    of sensitivity.

    Now a small sensor still needs that wall between pixels, and if CMOS
    room for electronics, so efficiency of the active area must drop.

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 24, 2005
    #27
  8. [A complimentary Cc of this posting was sent to

    ???

    perl -wle "print 0.999**2000"
    0.135199925397499

    Comparing the camera of 2000 and cameras of today are comparing days
    and night. The sensors of today are within an order of magnitude of
    theoretical limits. If you take large sensors (those used in dSLRs)
    then they are within 7x or 8x of theoretical limits.

    Cameras of 2000 were a pure junk comparing to what is available today
    (at least compacts; I did not have my hands on a dSLR then).

    Hope this helps,
    Ilya
     
    Ilya Zakharevich, Apr 25, 2005
    #28
  9. [A complimentary Cc of this posting was sent to
    Roger N. Clark (change username to rnclark)
    You mean "smaller pixels with the same exposition"; but this is not
    necessarily how cameras are used. If you increase exposition to
    compensate for smaller pixels, you get the same contribution of read
    noise. Example:

    a) 8 microns square pixel; you expose it as ISO1600; you get (e.g.)
    2500 electrons for white; noise of white is 50 electons; read noise
    is (e.g.) 12 electrons; total noise is 51 electron;

    b) 2 microns square pixel; you expose it as ISO100; you get the same
    2500 electrons for white; noise of white is 50 electons; read noise
    is (e.g.) 12 electrons; total noise is the same 51 electron.

    Of course, cheaper cameras come with lousier sensors (even when the
    performance is rescaled for the size); probably, the read noise is
    worse too. E.g., with the same throughput QE a 2/3'' sensor should
    perform as 8 times less sensitive than half-frame sensor; however, in
    my measurements, 50ISO with 2/3'' sensors is only marginally better
    than 800ISO with haolf-frame (with the same QE it should behave as
    400ISO).
    What planet are you on? Today's 2/3'' can produce "throughput" 75%
    MTF at 150 lp/mm (with Adobe demosaicer and sharpening mild enough to
    not introduce visual artefacts with line art images [no artefacts at
    least when viewed on CRT; I did not check yet on LCD]). See the
    thread on "MTF of monitors" for an example.

    Hope this helps,
    Ilya
     
    Ilya Zakharevich, Apr 25, 2005
    #29
  10. True enough. But the difference is that ISO 1600 is near the top end of
    the useful ISO range for the DSLR. You can choose to expose at ISO 800,
    400, 200, and perhaps 100. Each halving of ISO doubles the number of
    electrons per pixel, improving the signal to noise by sqrt(2), and
    giving more dynamic range to boot.

    On the other hand, ISO 100 is at or near the *bottom* of the ISO range
    available in a small-sensor camera. You can't reduce it much without
    saturating the sensor, since you're already close to the full-well
    capacity of the tiny sensor elements. You can select higher ISOs, with
    even noisier results.

    So your example shows that the read noise of a DSLR at just about its
    worst ISO (in terms of image noise) is equal to a small-sensor P&S
    camera operating at or near its best.

    Dave
     
    Dave Martindale, Apr 25, 2005
    #30
  11. Dave Martindale wrote:
    []
    This is part of how I see the P&S versus DSLR trade-off. Bigger, more
    inconvenient and heavy package but with the advantages of higher
    sensitivity, lower noise and interchangeable lenses. Decide what you want
    to do, pay your money and take your choice.

    Cheers,
    David
     
    David J Taylor, Apr 25, 2005
    #31
  12. Yes, if you want to compare iso 1600 images to iso 100 images,
    which is hardly a fair comparison. Compare the same ISO and
    the smaller pixels then read noise dominates more and more as the
    pixel size drops. And the effect is worse in the darker
    portions of the image.
    1) Sensor size is irrelevant. It is pixel size that matters.
    2) I've shown that modern P&S cameras, like the S60 are photon
    noise limited. It is a fundamental property of scaling
    pixel size that no matter what the camera throughput and
    quantum efficiency of the sensor, larger pixels will
    produce higher signal-to-noise images.
    3) Your 8 to 10x numbers are based on flawed calculations.
    Improvements can come by 2 methods: improving the
    transmission of the optics (lens, blur filter, color filters
    over the pixels), and sensor quantum efficiency.
    Both of these are unlikely to change unless a radical new
    detector comes out, and then it is likely only a factor
    of 2 better for small as well as large sensors. Transmission
    is already high and is unlikely to change. Fill factors
    are already effectively 100% from what I'm reading.
    Here is a page listing quantum efficiency:
    http://www.britastro.org/vss/ccdtable.html
    (see spectral response rows). Here is a page
    showing fill factor:
    http://www.fillfactory.com/htm/technology/htm/high_fill.htm
    I'm in the real world with real physics. You are in the
    world of fake math. Look up the diffraction spot size of
    a lens.
    Diffraction spot diameter = 2.44 * w * f_ratio,
    where w = wavelength, and f_ratio is the f/ratio
    of the optical system.

    Here is a table from:
    http://clarkvision.com/imagedetail/does.pixel.size.matter

    ================================================
    red= Green= Blue=
    0.6 0.53 0.47
    micron micron micron
    ================================================
    f/ratio diffraction spot diameter in microns
    ================================================
    2 2.9 2.6 2.3
    2.8 4.1 3.6 3.2
    4 5.9 5.2 4.6
    5.6 8.2 7.2 6.4
    8 11.7 10.3 9.2
    11 16.1 14.2 12.6
    16 23.4 20.7 18.3
    19 27.8 24.6 21.8
    22 32.2 28.5 25.2
    32 46.8 41.4 36.7
    45 65.9 58.2 51.6
    64 93.7 82.8 73.4
    ================================================

    For decent photography where you want some control over depth of
    field, you want to get to f/8 (and even that is not much depth
    of field in many situations), diffraction strongly influences
    image quality, and it should be obvious that diffraction is
    less of a problem with a larger pixel.

    The diffraction limit for an f/2.8 lens at 5000 angstrom (green)
    wavelength is: 585 lines/mm = Rayleigh Resolution Criterion,
    which is ~9% MTF. That requires pixel spacing if 585*2/1000
    = 1.2 microns. But that is only 9% MTF! Yes, you could
    see lines on a test target, but that does not make for a
    clean sharp image. The 0% MTF is the Dawes limit, which
    for a diffraction limited f/2.8 lens is only 550 lines
    per mm for 6500 angstrom light (red). So pixels spacing
    less than this will show now detail at all. Yet you say
    on your web page that the optimum pixel size is less than
    a micron.

    Do you know of ANY diffraction limited f/2.8 lens on a P&S
    camera (diffraction limited over the whole field of view)?
    I do not.
    No it doesn't. You are providing more confusion.
    Here is the problem. You cite a number that you pull from
    somewhere without consideration of other factors. For example
    deriving optimum pixel size that derives quantum efficiencies
    10 times less than manufacturer's specifications, and ignore
    diffraction. I have honestly tried to help you pointing
    out some of these things. You reject them all and then
    declare your erroneous result again. This is becoming
    no different than the film versus digital religious wars.

    Theory is fine when it includes all components in the real world.
    Then there is the real world.....

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 25, 2005
    #32
  13. Paul Rubin

    paul Guest


    Ah! I doubt all pixels are square & they need some space between. I
    wonder how much that efficiency of packing them tighter has changed. The
    older cameras have twice the pixel pitch of new ones & were pretty
    crappy about noise & such.

    I also doubt one could easily find the actual pixel size of consumer
    digicams for a comparison.


    Interesting. They do confirm that CMOS is packed less tightly than CCD.
    And that there are quite a few factors involved.
     
    paul, Apr 25, 2005
    #33
  14. Most still camera sensors have a pixel grid that's square or very close
    to it. If not, you simply have separate horizontal and vertical pitch
    numbers.

    Also, some CCDs use lenslet arrays in front of the sensor. These
    collect virtually all incoming light and focus it onto the
    light-sensitve portion of each pixel, giving an effective fill factor of
    nearly 100% even though the silicon itself (without the lenses) has a
    much lower fill factor.
    My Canon cameras put the pixel pitch values in the EXIF data of every
    image they shoot. Even without that, it's not hard to get an estimate:

    The size of sensors is usually expressed as one over some number in
    inches, for example 1/1.8". This is an odd notation, but what it does
    tell you is that the actual image capture area diagonal is about
    16/1.8 = 8.9 mm. From that and the sensor aspect ratio (usually 1.33,
    but sometimes 1.5) you can calculate the horizontal and vertical size.
    In our example, it's about 7.1 x 5.3 mm.

    Then divide those numbers by the image size in pixels to get the pixel
    pitch. If it's a 4 megapixel camera with 2272 x 1704 images, then the
    pixel pitch is 7.1e-3 / 2272 = 3.1e-6, or 3.1 micron.

    The example numbers above are for a Canon G2. The actual sensor
    resolution from the EXIF data is 8114.29 pixels/inch, which is 319.46
    pixels/mm or a pixel pitch of 3.1303 microns.

    Dave
     
    Dave Martindale, Apr 25, 2005
    #34
  15. Paul Rubin

    Paul Rubin Guest

    There have been millions of relatively inexpensive compact full frame
    35mm point-and-shoot cameras made with high quality lenses, sometimes
    even zooms. I don't see why compact digicams can't be made with large
    sensors. It must just be a cost thing.
     
    Paul Rubin, Apr 25, 2005
    #35
  16. Paul Rubin

    Big Bill Guest

    It would seem to me that the lenses used on compact, small-CCD cameras
    are made specifically to fit eh CCD size used. Many are about 1/3"
    diagonal. The lenses are smaller, because they only need to cover that
    small CCD.
    Putting a full 35mm size CCD in the same camera body will require a
    much larger lens (and all incidental hardware). This *will* cost more.
    There's no way around it.
    Plus, the compact cameras aren't empty; there's 'stuff' already in the
    nooks and crannies that exist. To make room fro the larger CCD and
    lens, the 'stuff' would need to go somewhere; if miniaturization is
    used to reduce the size of the 'stuff', that also increases cost. If
    not, it increases size.
    At least that's how I see it.
     
    Big Bill, Apr 26, 2005
    #36
  17. Paul Rubin

    Big Bill Guest

    Almost.

    From:
    http://www.dpreview.com/learn/?/Glossary/Camera_System/Sensor_Sizes_01.htm

    "Sensors are often referred to with a "type" designation using
    imperial fractions such as 1/1.8" or 2/3" which are larger than the
    actual sensor diameters. The type designation harks back to a set of
    standard sizes given to TV camera tubes in the 50's. These sizes were
    typically 1/2", 2/3" etc. The size designation does not define the
    diagonal of the sensor area but rather the outer diameter of the long
    glass envelope of the tube. Engineers soon discovered that for various
    reasons the usable area of this imaging plane was approximately two
    thirds of the designated size. This designation has clearly stuck
    (although it should have been thrown out long ago). There appears to
    be no specific mathematical relationship between the diameter of the
    imaging circle and the sensor size, although it is always roughly two
    thirds."
     
    Big Bill, Apr 26, 2005
    #37
  18. Paul Rubin

    Paul Rubin Guest

    Well, larger, I don't know about "much larger". The lenses in small
    35mm P/S cameras are not that large.
    Take a small P/S digicam like a Canon SD200. That clearly contains
    all the stuff in those nooks and crannies. Now take a small 35mm P/S
    like an Olympus Stylus. That clearly contains a full frame "sensor"
    (film frame) plus a lens capable of covering the sensor. (It also
    contains space for the film cartridge and takeup spool). Now imagine
    a digicam the size of the SD200 and Olympus Stylus put together. The
    result is still a pretty small camera, maybe the size of a Canon A70.
    But it clearly has space for a full frame sensor and the associated
    electronics. Yes it would cost a lot, but I'm not even sure that's
    prohibitive. High end 35mm P/S cameras like the Contax T cost more
    tha low end DSLR's of today, so people are willing to pay that much
    for compact cameras.
     
    Paul Rubin, Apr 26, 2005
    #38
  19. Paul Rubin

    Big Bill Guest

    I'm not so sure that you can compare the innards of a film camera with
    that of a digital camera.
    The film camera needs the cannister/takeup reels, but the digital
    needs the extra electronics to process the image,and of course, a card
    to hold the image files (with some pretty amazing things done to the
    smaller cards, so that hey are easier to lose! :) ).
    Well, OK, maybe the same volume.
    I still wonder why, if it's so possible for the same price, it's not
    being done. I think the above has somewhat to do with it.
     
    Big Bill, Apr 26, 2005
    #39
  20. Paul Rubin

    Paul Rubin Guest

    As explained, a camera the size of a small 35mm AND a small digital
    PUT TOGETHER, should be able to hold everything that's in both of them,
    and still be a fairly small camera.
    It couldn't be done at the same price as today's compact digicams.
    But it could probably be done at the price of today's entry level
    DSLR's. It's an open question whether anyone would pay that much for
    a compact digicam with a large sensor. People still seem to want
    gimmick features like extreme pixel counts.
     
    Paul Rubin, Apr 26, 2005
    #40
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