Nikon D40 vs D40x ISO newbie question

Discussion in 'Nikon' started by Private, Apr 28, 2007.

  1. Private

    Private Guest

    From KR review of D40x
    http://www.kenrockwell.com/nikon/d40x.htm
    ISO 100 - 3,200. Per the laws of physics, a 10MP sensor has to run at lower
    ISOs for the same performance as a 6MP sensor of the same physical size.
    Therefore the D40x' ISO range starts at ISO 100, not ISO 200 as the D40. The
    "wider range of ISOs" touted as a feature is actually a downgrade: the D40
    didn't need to go slower than ISO 200 for great results, and at ISO 100 the
    D40x is more likely to have more blur or shallower depth of field.
    Please excuse my ignorance, and explain this law of physics.

    "a 10MP sensor has to run at lower ISOs for the same performance as a 6MP
    sensor of the same physical size. "

    and do you agree with KR's statement above?

    TIA
     
    Private, Apr 28, 2007
    #1
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  2. Private

    Guest Guest

    it is the law of ken rockwell spewing nonsense. ignore it.

    here's some credible reviews:
    http://www.dpreview.com/articles/nikond40x/
    http://www.dpreview.com/articles/nikond40/
    http://www.imaging-resource.com/PRODS/D40X/D40XA.HTM
    http://www.imaging-resource.com/PRODS/ND40/ND40A.HTM
    http://www.bythom.com/d40review.htm
    no. he's making up stuff, as usual.

    the 6mp chip in the d40 starts at 200. the 6mp chip in the canon 10d
    started at iso 100. it has nothing to do with the number of pixels.
     
    Guest, Apr 28, 2007
    #2
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  3. There is no actual change in sensitivity in terms of quantum
    efficiency, but smaller pixels collect fewer photons.

    See:
    http://www.clarkvision.com/imagedetail/does.pixel.size.matter

    http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 28, 2007
    #3
  4. Private

    C J Campbell Guest

    Of course ISO 100 will have more blur or shallower depth of field, but
    it has nothing to do with the number of pixels. It is just that
    reducing the ISO requires a slower shutter speed or wider aperture.
    This would be true of any camera.

    Ken is making a few assumptions, one being that a smaller pixel does
    not gather as many photons and therefore it is likely to generate more
    noise. We see that in point and shoots, but it is an open question
    whether the much larger sensors in SLRs have even begun to approach
    that limit. Following Ken's reasoning the Canon 1D Mark II would have
    terrible pictures, which is obviously not the case.

    The ISO 100 is mostly there because Nikon users griped about having
    only ISO 200. Sometimes you want blur or shallow depth of field.
    Personally, I would like to see an ISO 24 setting. It would sure beat
    using neutral density filters to slow down the shutter speed.
     
    C J Campbell, Apr 28, 2007
    #4
  5. Private

    Bengt Cyren Guest

  6. Private

    John Sheehy Guest

    Cameras that start at ISO 200 generally do so because of high quantum
    efficiency and/or small photosites with large microlenses. They collect
    photons quickly enough that the wells fill in the highlight areas before an
    ISO 100 exposure can be obtained.


    If it is quantum efficiency that is the difference, then at any given ISO
    there will be less shot noise with the higher QE. If the difference is
    small photosites with big microlenses, then there is no benefit; just a
    missing low ISO.
    --
     
    John Sheehy, Apr 28, 2007
    #6
  7. Private

    ben brugman Guest

    This law is next to the law that larger cars drive slower than smaller cars.
    (Larger cars have more weight and therefore drive slower).
     
    ben brugman, Apr 28, 2007
    #7
  8. Private

    C J Campbell Guest

    C J Campbell, Apr 28, 2007
    #8
  9. Private

    Private Guest

    Private, Apr 29, 2007
    #9
  10. Private

    Private Guest

    Thanks to all for the responses and links.
     
    Private, Apr 29, 2007
    #10
  11. Yes, they have reached that limit. See Figure 6 at:
    http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary

    For all tested digital cameras, and by inference, similar performance
    of other digital cameras, the signal-to-noise ratio digital cameras
    achieve is photon noise limited for signals greater than a few tens
    of photons. That means the noise is given by the square root of
    the number of photons converted to electrons by each pixel.
    Just like a bigger bucket collects more rain drops in a rain storm,
    larger pixels collect more photons. Because digital cameras
    have reached these fundamental limits, it lead to a host of
    predictable properties, ranging from high ISO performance to
    depth of field limitations.

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 29, 2007
    #11
  12. Private

    C J Campbell Guest

    The trouble with your charts (and this has always been the trouble with
    your charts) is that they do not seem to have anything to do with
    actual results.

    Theory is great, so long as it can predict results. According to your
    charts the Canon 1D Mark III should produce worse pictures than the EOS
    350D or even the Nikon D70. This is demonstrably untrue.

    I cannot tell you what is wrong with your charts. But it is obvious
    that there is something seriously wrong with them.
     
    C J Campbell, Apr 29, 2007
    #12
  13. Private

    C J Campbell Guest

    Actually, there is something seriously wrong with Roger's analysis. It
    simply does not accurately predict results. According to his theory,
    the Canon 1D Mark III will produce worse pictures than a Nikon D100.
    This is demonstrably untrue.

    I have no problem with scientific analysis. I just expect that it
    should have reproducible results.
     
    C J Campbell, Apr 29, 2007
    #13
  14. Actually they do. Some examples, Noise levels are just as predicted:

    http://www.clarkvision.com/imagedetail/does.pixel.size.matter2

    http://www.clarkvision.com/photoinfo/dof_myth
    This is not theory; it is measured results with models that
    are pretty standard in the electronics industry. Further,
    the data I present is a compilation of my own analyses,
    those of others and sensor manufacturers from their data
    sheets. They are all consistent and all doing quite well
    in predicting results. In astronomy the measured parameters
    and models predict nicely what one gets in difficult conditions
    in astrophotography. Then, DSLRs using these models are doing
    photometry, and quite well.
    Where do you get that idea? The 1D Mark III is not on the charts.
    Canon states the photo sites collect the same number of photons as
    the 1D Mark II (closer pixel pitch but better micro lenses), so
    the 1D Mark III should plot at 7.4 microns on a line horizontal
    to the 1D Mark II. That puts it still performing better
    than a 350D or a D70.
    Perhaps you are mis-interpreting something?
    If you can be more specific, perhaps I can clarify it, or perhaps
    I will see an error (again noting some of the data on the
    charts are from sensor manufacturers data sheets).

    rOGER
     
    Roger N. Clark (change username to rnclark), Apr 29, 2007
    #14
  15. No, you are incorrect. See my other post.
    The 1D Mark III is not on the charts.
    Canon states the photo sites collect the same number of photons as
    the 1D Mark II (closer pixel pitch but better micro lenses), so
    the 1D Mark III should plot at 7.4 microns on a line horizontal
    to the 1D Mark II. That puts it still performing better
    than a 350D or a D70.

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 29, 2007
    #15
  16. Private

    C J Campbell Guest

    It appears that your charts are an argument that more photons reduces
    picture quality and that there is nothing you can do about it. Yet here
    you claim that better micro lenses can correct the problem.

    So, either the limit on sensor size has been reached as you said, or it
    can be increased further with better micro lenses or other engineering
    solutions, which you also said. It seems to me to be a contradiction,
    so please understand if a simple tax accountant seems confused.
     
    C J Campbell, Apr 30, 2007
    #16
  17. Private

    C J Campbell Guest

    Argh. I meant more pixels, not more photons. I really hate typing as a
    means of communication.
     
    C J Campbell, Apr 30, 2007
    #17
  18. The number of collected photons is a fundamental limit to picture
    quality. Dynamic range can be no greater than the number of collected
    photons, and noise is set by Poisson statistics. That is independent
    of the efficiency of collection. You can never do better than
    Poisson statistics. Thus, in any light collection system, the system's
    dynamic range and noise is set by the number of photons you actually collect.
    Other noise sources in the electronics only add to the noise, so noise
    due to the random arrival times of photons is the best one can do.
    This is a fundamental physics limit, and in general is called photon noise,
    or photon noise limited.

    Let me try and explain and please ask further questions if what I say
    is not clear to you (or anyone else reading).

    So we've established photon noise is a fundamental limit. The noise in
    a light signal is the square root of the number of photons collected.
    So if you collect 90,000 photons, the noise is 300, thus the
    signal-to-noise ratio = 90,000/300 = 300. The dynamic range is
    90,000. Let's say we have no other noise sources in our sensor or
    electronics, so we say the system is photon noise limited and
    has reached a fundamental limit. A fundamental limit is not
    an absolute best that can be done limit.

    But that does not mean it can't be improved. Let's say the sensor
    converts 1/3 of the photons that are incident on the pixel, and
    let's say the dead space on the sensor is about 20% (the lines
    between pixels and support electronics on the pixel that are
    not sensitive to light). We say the Quantum Efficiency (QE)
    is 33% and the active area, or fill factor, of each pixel is
    80%.

    So we can do a couple of things: we can improve the QE or
    the fill factor. Micro lenses collect light from a larger
    area and focus it down to smaller spot. Manufacturing 8 to
    10 million tiny lenses is not easy and manufacturing processes
    to do that have been improved over the years. That is what
    Canon announced with the 1D mark III: they shrunk the pixel size
    and improved the micro lenses to keep the total light collected
    the same.

    An analogy is collecting rain drops in buckets in your yard
    during a rain storm. Lets say you place 100 buckets on a 10x10
    grid spaced every foot. The buckets are 0.5 foot in diameter.
    Let's say the rain fall is constant all day. Let's also say
    the rain drops arrive at random intervals and are modeled
    by Poisson statistics (probably a good assumption in reality).
    Say we collect for one hour.
    The amount of rain in each bucket is not the same. The noise
    we get in measuring the amount of water in each bucket is
    the square root of the number of rain drops in the bucket.
    Our data regarding measuring the amount of water is
    "rain drop noise limited."

    Now let's replace the 0.5 foot diameter buckets with buckets
    measuring 0.75 foot in diameter but still spaced every foot.
    That increases our collection efficiency from 20% to 44%.
    We collect more water in the same interval, and our measured
    result will be a little more precise because we collected
    more rain drops. We can maximize out efficiency by designing
    square buckets that are 1-foot in outside dimension increasing
    our collection efficiency to almost 100% (the width of the buckets
    limits us a little).

    Another way we could improve efficiency with our 0.5-foot diameter
    buckets is to put funnels over each bucket. Say the funnel was
    1-foot square at the top. We've increased efficiency to near 100% without
    changing the bucket.

    If we want to collect even more photons, we must use larger buckets
    (or buckets + funnels) spaced at larger intervals. For example we
    could use square buckets 2-feet on a side and double the area our
    buckets cover. We would collect 4-times the number of rain drops
    as our 1-foot square buckets. All the system have been Poisson
    noise limited.

    Our buckets have been 100% efficient (assuming nice metal or plastic
    buckets). Lets say we make the buckets out of some absorbing
    material, so we lose some water. The water lost to absorption
    is not counted when we measure the depth of water in the
    bucket. The noise we measure from bucket to bucket is the
    square root of the number of drops making up the water in the
    bucket. We are still Poisson statistics limited, and we still
    say we are "rain drop noise limited."

    So we can have photon detectors that are not perfect and which could
    be improved, but the noise we see from the system is photon noise limited
    and improving other electronics will not improve the noise we see
    in our images. We say the system is photon noise limited.

    Dos this help?

    Roger
     
    Roger N. Clark (change username to rnclark), Apr 30, 2007
    #18
  19. Private

    John Sheehy Guest

    *IF* having more pixels means having smaller photosites, then those
    photosites will have more shot noise, due to collecting less photons each
    (all other things remaining equal). The role of the microlenses is to
    "funnel" photons into the photosites which would normally fall outside of
    them, and turn into heat or reflect or diffuse off of the non-sensitive
    areas of the sensor. They make the sensor perform the same way with less
    light, and give a higher native ISO (if a microlens doubles the photon
    capture, then the native ISO is double what it would be without them, but
    the DR and noise performance of both will be the same at the
    lowest/native ISO).

    Canon claims that the 1DmkIII photosites capture as many photons as the
    mkII. That can be confirmed as soon as there are convenient RAW samples
    to measure from.

    If it is true, then the mkIII will have the same shot noise at the pixel
    level as the mkII. This means the same image shot noise (as opposed to
    pixel-level shot noise) if you crop into it for 8MP, and slightly less
    image shot noise using the entire image. From RAW samples I've seen, the
    read noise seems to be the same at ISO 100 in both cameras, so the same
    follows there for read noise and total noise. There are claims that the
    higher ISOs have less read noise. That can be tested when RAW dark
    frames are available for the mkIII. The high-ISO samples I've seen so
    far are all JPEGs, and seem to me to be possible with more post-capture
    noise reduction in the JPEG engine(s), so I will not be persuaded until I
    see the RAWs.

    In any event, even if everything remains the same, RAW-noise-wise, at the
    pixel level, more pixels will mean less total image noise, if you use the
    full frames, and display at the same size.





    --
     
    John Sheehy, Apr 30, 2007
    #19
  20. Private

    C J Campbell Guest

    Some. However, I did not make the leap where the noise we get in
    measuring the amount of water in each bucket is the square root of the
    number of rain drops in the bucket. I did not see the justification for
    that conclusion.

    As I understand it, noise in digital photography is of two types: fixed
    pattern and random. Fixed pattern noise is caused by imperfections in
    the sensor. All cameras exhibit some fixed pattern noise and it differs
    from camera to camera as well as from model to model. It is easily
    screened out by the camera software. The random noise is caused by
    several things, including heat buildup, electron transfer between
    sensors, electromagnetic interference, etc. The less light there is and
    the longer the exposure time the more you are likely to be recording
    something other than light. This does not seem to me to be dependent on
    sensor size. Making the pixels larger seems to also make the sensor
    more likely to pick up stray non-light signals as well as light. The
    signal to noise ratio would remain constant, would it not? Placing the
    pixels closer together would make them more likely to interfere with
    one another, but I don't see how larger pixels helps that -- in fact,
    it might aggravate the problem.

    Anything that would make a pixel more likely to catch photons would
    make it more likely to catch other particles as well. This seems to me
    to be fundamentally a filtering problem rather than a pixel size
    problem. I can see how micro-lenses boost the signal to noise ratio;
    but this is a filtering problem.

    For a given size of sensor, however, more pixels should catch more
    light, not less. Consider placing stacks of coins in a chest. You can
    put a lot more coins in the chest if they are smaller because the space
    between the stacks is less.

    On a sensor, the performance of individual pixels might be less with
    smaller and more closely spaced pixels, but the overall performance of
    the sensor should remain the same or increase, should it not? You lose
    fewer photons to the spaces between the photo-sites, so overall your
    image quality and signal to noise ratio for the entire sensor should be
    improved.

    Taking your rain bucket example, if I fill up an area of 40x30 feet
    with ten gallon buckets as closely packed as I can put them, I will
    catch less rain than if I fill up the same area with quart buckets as
    closely packed as I can put them. Each quart bucket will catch less
    rain than a ten gallon bucket, but the total amount of water collected
    by all the buckets will increase. The only limit that I can see would
    be if each bucket was smaller than a raindrop and therefore could not
    collect any rain at all. However, photons are very tiny and we are not
    anywhere near making a photo-site that is so small that a photon cannot
    enter it.
     
    C J Campbell, Apr 30, 2007
    #20
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