Light fall off on dSLRs - an experiment

Discussion in 'Digital SLR' started by Kennedy McEwen, Mar 16, 2006.

  1. Kennedy McEwen

    Paul Furman Guest

    here's another:
     
    Paul Furman, Mar 17, 2006
    #61
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  2. Kennedy McEwen

    Kyle Jones Guest

    But by removing the photons that would be arriving from many diffeent
    angles might he also have removed some side-effect of photons
    interacting that would change the result? I'm not saying, I'm asking.
    I know I don't really understand quantum mechanics, but what I've been
    able to understand tells me that deconstructing a particle system in an
    experiment like this _might_ produce a counterintuitive result.
     
    Kyle Jones, Mar 17, 2006
    #62
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  3. Kennedy McEwen

    Stacey Guest


    Why not just shoot the same scene using a wide angle lens on both a digital
    and a film camera body, scan the film and compare them like that? If they
    both end up with the same fall off, you've answered the question.

    This other test sounds like a BS way of "proving" this, especially given you
    wanted a specific outcome to your test before you did it.
     
    Stacey, Mar 17, 2006
    #63
  4. Kennedy McEwen

    bjw Guest

    When the sensor is at an angle to the source, foreshortening
    means fewer photons are incident on the sensor per area.
    If the surface is matte, the reflected photons are redistributed
    over all angles, with an intensity given by the cosine law, which
    is a statement about Lambertianness (Lambertiality?) But
    this can't change the fact that there are fewer photons incident;
    the incident intensity depends on the angle of illumination,
    although the fraction reflected may not.

    <http://en.wikipedia.org/wiki/Lambert's_cosine_law>
    "This means that although the radiance of the surface
    depends on the angle from the normal to the illuminating
    source, it will not depend on the angle from the normal
    to the observer." (in the section on Lambertian reflectors)
    Independent of Lambertianness, think of how it behaves before
    getting all the way to the extreme angle. Imagine that the angle
    of incidence is 70 degrees off normal - there are cos(70) = 0.34
    times as many photons incident per area. This is hard to see
    by eye, but it's real. It's also why incident light meters use those
    little white domes rather than a flat white disc.

    It's like the higher latitudes in winter. The sun's rays are
    incident on the earth at a greater angle, so there's less
    energy/area, so it gets colder.
    All I'm questioning is the mapping between the pixel numbers
    in the jpeg and the number of photons detected per pixel, which
    is what we really want to know. One could take an underexposed
    noisy picture, multiply all the pixel values by 100, and it would still
    be an underexposed noisy picture.

    The other possibility is that due to fluctuations in whatever
    (shutter speed? LED output?) the test didn't detect the ~15%
    difference that should be there due to a statistical fluke.
    I kind of doubt this. But if it were true, it would mean you'd
    have to take a number of exposures to figure out what the
    noise in each measurement was. (Maybe you did this and
    didn't say.)
     
    bjw, Mar 17, 2006
    #64
  5. No, I am not. There don't seem to be many sensor designers who do say
    that there should be light fall-off due to microlens arrays - only those
    in the 4/3 consortium, and none of the articles saying that come from
    the sensor manufacturers, only the camera manufacturers. Also, as
    already mentioned, the problem might even be more of an issue with their
    smaller pixels than it is with larger, less noisy pixels.
    I would be very surprised if some sort of similar test wasn't actually
    performed at least on a batch basis by the sensor manufacturer. I know
    that we do something similar to this with our sensor production, which
    is what prompted the idea in the first place. Then again, our sensors
    routinely operate with f/1 lenses and faster, so extreme angular
    response is important.
     
    Kennedy McEwen, Mar 17, 2006
    #65
  6. Except that a sensor element with a microlens is a bit more complex than
    a little square hole.

    Does the anti-alias filter have any effects that depends on the angle of
    light?
     
    Philip Homburg, Mar 17, 2006
    #66
  7. No - the light from the lens is incoherent so constructive and
    destructive interactions cancel each other out. If the lens was
    photographing a laser illuminated scene then perhaps there could be some
    issues, but that is rather unusual and people are claiming this effect
    is significant in normal light.
     
    Kennedy McEwen, Mar 17, 2006
    #67
  8. But if his test is repeatable, he does have point. How can a DSLR show
    additional light fall-off compared to film when the sensor itself show
    not exhibit that property?
     
    Philip Homburg, Mar 17, 2006
    #68
  9. Not sure I understand what you are describing here, but I measured about
    half the loss you are estimating, so it might not be very far from an
    emipirical explanation.
     
    Kennedy McEwen, Mar 17, 2006
    #69
  10. Why not just shoot the same scene using a wide angle lens on both a digital
    and a film camera body, scan the film and compare them like that? If they
    both end up with the same fall off, you've answered the question.
    [/QUOTE]
    Because, as others have pointed out, that is much more complicated to
    get quantitative results than your nave question suggests.
    Actually, I expected to find about half a stop difference - I was
    extremely surprised by an entirely negligible difference.
     
    Kennedy McEwen, Mar 17, 2006
    #70
  11. (sorry - long post) (O:
    To do that I would need to know what angle you attained - you haven't
    said yet..?
    And then I would have to examine a typical wide angle lens for that
    camera to determine where the *opposite* outer edge of the rear element
    would be - when it was focused at whatever distance puts the rear
    element closest to the sensor (infinity I presume).. Not owning a 5d
    and suitable wide angle, I can't do that.

    And to show I'm being fair, I would also need to know how much light
    from various areas of the lens will actually *hit* the sensor from
    those angles - does the opposite edge actually contribute much? I
    suspect not, but without knowing the makeup of the light spread across
    the sensor, it remains an unknown variable that would, or at least
    could, affect the result..

    And no matter how you look at it - there is a bit of a problem with
    your results - (compounded by the fact that you didn't actually state
    *what* incident angle you achieved)... That problem is that all of the
    actual, published technical data that I can find (which isn't much
    admittedly) on ccd and cmos sensors, indicates that they suffer
    noticable and significant light fall off at angles over 10-15 degrees
    away from incident. Losses of 15% and more, not 2.6%...

    Your description of the experiment raised some other issues, too, eg:
    But hang on - the lens *does* in fact restrict the light very
    effectively to the image circle of that lens - how was your light
    source collimated (which is not the same as a 'point source')? Could
    there have been reflections from the chamber, focusing screen, etc? To
    do this properly you would surely want a thin 'tube' of light.
    This concerns me, because that description shows you have a significant
    amount of light hitting the sides of the mirror chamber, the focussing
    screen, etc and bouncing around onto the sensor- this would not happen
    (as much) with a lens, because there is effectively nothing much
    outside the image circle.
    OK, but what angle *is* that?
    I'm not sure I am convinced of that. I don't have a DSLR in front of
    me, but when a wide angle lens is fitted and focused to infinity, and
    the *opposite* side of the rear element is used as a potential light
    source for an opposite-edge-located sensor, I would have thought it
    would be quite a bit steeper angle than you could achieve without the
    lens. But without some much more precise detail, i can't really argue
    this.
    Like I said, it sounds like there is a lot of light bouncing around!!!
    This is a rather worrying statement, and as I stated above, it suggests
    that the sensors may be getting illumination from areas other than your
    point source.
    Could we see the images and exif data? I don't usually work in 'PS
    levels'! I would imagine by their nature they are not ridiculously
    large files. It would also be interesting to see what the 'unexposed
    areas' actually look like.. As was stated above though, RAW data would
    be more interesting.
    That's where I disagree. I don't think this is a simple test at all!!
    Categorically? Hmm. I used to work in the sciences, and I have heard
    people get very excited before.... and then realise that somewhere in
    their data there is a significant 'whoops' (or several..)...

    And it flies in the face of the figures I have seen, eg Figure 5 on
    that kodak spec sheet I linked to. I'll try to find another set of
    data for Canon or Sony to see if that is just an unusual/poor sensor
    design.

    Please don't get me wrong, Kennedy - I appreciate the effort you have
    gone to and it's a fascinating topic, but be open to suggestions to
    improve the experiment, and don't dissmiss the naysayers. Sometimes,
    naysayers are very useful people to have around! I'm happy to have my
    arguments shredded, and if they are wrong I'll give up gracefully and
    congratulate the 'winners', *but I want links and references and
    verifiable data*! Too many people here with 'opinions'... Me
    included!

    And I have to say that the more research I do into this (yes, I have no
    life!), the more conviced I am that there *is* an issue with light
    incidence. How much, I don't know, and I am frustrated by the fact
    that it should be fairly easy to test *properly*, using the same wide
    lens on a full frame Canon-DSLR and film SLR. But has anyone done it?
    - if they have, I can't find it.

    PS There's another interesting read here...

    http://www.swissarmyfork.com/digital_lens_faq.htm

    Scroll down to "3) Do we really need digital lenses?" It's an old
    document, and I'm not suggesting that Mr Wisniewski is a foremost
    authority, but it sums up my argument pretty well, and his numbers are
    backed up by what I read from the manufacturers and designers of
    sensors. In simple terms, they *are* sensitive to angle of incidence
    where film is not, and from what I have seen and read, this *can* be a
    problem that is worse on digital than on film.

    Just depends on the lens and the sensor. (o: Like I said, I'd *love*
    to see the specs of the Canon CMOS, but I haven't found it yet - and
    I'm not sure, if I was Canon, that I would want it to be 'out
    there'....

    In the same way that Olympus want to push their telecentric approach to
    help boost the 4/3 system, Canon and Nikon and maybe Sony might be
    biased in exactly the reverse way...

    (O:

    PS - More links:
    http://www.dalsa.com/dc/documents/I..._Whitepaper_Digital_Cinema_00218-00_03-70.pdf
    Check pages 7 thru 9. Before dismissing it, find out who 'Dalsa' are..

    http://oemagazine.com/FromTheMagazine/feb02/detectors.html
    "Microlensed imagers ... show a strong sensitivity dependence on
    incident photon wavelength and angle."
    OK, it's an old article, but the basic CCD and CMOS design hasn't
    changed much. And again, find out who James Janesick is...
     
    mark.thomas.7, Mar 17, 2006
    #71
  12. You don't understand this, do you. I didn't *want* to "know the
    difference between using film and digital"!
    Because that "something else entirely" *is* what I wanted to know!

    That "something else entirely" was precisely how much the sensitivity to
    incident light angle the digital sensor (which you and many others
    continually cite without any reference quantifying it) actually was. So
    I set out to measure that - not the difference between film and digital,
    but to quantify how big this effect that *you* were claiming dominated
    corner light fall-off on the 5D actually was. Quantify the cause, not
    the symptom.

    The *implication* of this being zero is that there is no difference
    between film and digital sensors *because* that is the only difference
    people like you ever do cite. Over to you Batman, come up with some new
    excuses why digital full frame sensors should be different to film,
    since your original justification is now in flames!
     
    Kennedy McEwen, Mar 17, 2006
    #72
  13. But if his test is repeatable, he does have point.

    ahem. repeatability is *not* the only criteria for proof of a
    postulation..!! I reckon I would probably get similar results if I
    repeated the test, but I am not at all convinced that this test is
    valid. See above posts for details.
     
    mark.thomas.7, Mar 17, 2006
    #73
  14. Kennedy McEwen

    w.beckley Guest

    One problem that I see from such a test is that film itself is an
    unreliable medium for this kind of test. Which film stock would you use
    for the test? How would you expose the test? How would you process the
    test? How would you print the test? Each stage of this could change the
    outcome of your test, to exagerate or hide exposure variations in the
    corners, and it would be hard to find a neutral happy medium.

    Shooting negative / print film, for example, would mask vignette and
    other optical defects more than, say, Velvia would. Which one is our
    benchmark for what "film" is?

    Kennedy's test, which is very sound in its approach, not only removes
    the variables of photochemical finishing, it also removes the variable
    of the lens, whose inherent vignette is going to make it difficult to
    imperically measure any exposure variation introduced by either medium
    independent of the lens.

    Which comes to the final reason a film-to-digital comparison would be a
    waste: how do you imperically measure either of these in a way that is
    relevant to the other? Print both of them? How? Scan the
    slides/negatives? How? Printing and scanning both change the outcome as
    much as everything else.

    There are simply too many variables to make a worthwhile test by
    shooting the two media against one another. Empirical testing is only
    useful if there is one variable at play. And in this test, there is:
    the angle of incidence of the light striking the sensor.

    Will
     
    w.beckley, Mar 17, 2006
    #74
  15. Kennedy,

    Thanks for that interesting test - it has provoked a lot of discussion!

    One factor I have not seen mentioned is the light spectrum. I am guessing
    that people are seeing light fall-off in blue skies with wide-angle
    lenses. It would be interesting to see if the fall-off differed between
    the red and blue ends of the spectrum.

    I would also suggest measuring with RAW rather than JPEG data, as the
    differences you are seeing are rather small.

    Cheers,
    David
     
    David J Taylor, Mar 17, 2006
    #75
  16. Kennedy McEwen

    Andrew Haley Guest

    It is showing around 5%.

    (73.80 / 71.82) ^ 2.2 = 1.06.

    Andrew.
     
    Andrew Haley, Mar 17, 2006
    #76
  17. ..good, trivial, and robust..

    Oh, really? You seem to be carefully avoiding some posts above that
    show how non-trivial and non-robust this experiment is.

    And please post just ONE SINGLE link to one of the 'people' who know
    sensor design and state that incident angle is irrelevant. I've posted
    several that state the exact reverse, including a graph from a sensor
    manufacturer... It appears their method must have used a slightly
    different methodology, or perhaps Canon CMOS sensors are just
    completely different to other sensors..?

    And to go back to basics (feel free to argue these point by point):

    1. The problem is 'light fall off on DSLR's' - yes or no?

    2. Hands up those who shoot their DSLR without a lens?

    3. If the lens is kept the same, and you use it on a film camera (eg a
    Canon SLR) and then a digital camera (eg a Canon FF DSLR) at the same
    exposure, same focus setting, same scene, same lighting... You could
    simply look at the images and compare. Too tricky? (And yes, of
    course you would use transparency film to avoid processing issues)

    Can you *please* explain how a 'better' test is to remove the lens
    altogether, use a non-collimated light source, don't tell us what
    angles were achieved, and not even know what angles *might* be expected
    from a typical problematic wide angle lens....? Sheeesh.

    If you can't get that, there is no hope for you, and you need *not*
    attend any more of my science lectures.......

    Your comments about 'we can only estimate the effect' and the 'host of
    other problems' are incomprehensible..

    The simplest way to identify the effect is by looking at, and
    measuring, images that *show the problem*. And doesn't the problem
    (allegedly) occur when you use a wide angle lens???? By pulling off
    that lens and running this experiment you have *added* a whole new set
    of variables and substituted something that is NOT the same. As I and
    others have stated, a lens has an image circle - the light is
    effectively constrained to that area - a point light source is not
    collimated and is quite different - so even silly things like the small
    amount of reflected light off the walls of the mirror box *could* be
    relevant... But you can't know because you have removed the lens......

    sigh.

    I give up.
     
    mark.thomas.7, Mar 17, 2006
    #77
  18. Can you give a message ID of a post that summarizes your position?

    I did not find anything in the 'above posts' that clearly explains why
    his experiment is the wrong way of measuring the sensor's response to
    light with a maximum angle of incidence.
     
    Philip Homburg, Mar 17, 2006
    #78
  19. [...]

    you did not use coherent source of light, thus your test proves nothing.
    And, BTW. how is it that sensor vendors document much bigger light
    falloff than the one you "detected"? Do you even have idea what's the
    actual mechanism of this light fallof, so that you know what to measure?


    B.
     
    Bronek Kozicki, Mar 17, 2006
    #79
  20. Yes, you can do this qualitatively. Print film has similar
    response curves to digital cameras. I've shown that on my web site.
    Your f/# and shutter arguments are irrelevant. What you want to
    test is the relative fall of from center to corner. It
    matters not what the exposure time is. It does matter what the
    f/stop is, but most decent systems should reproduce to a
    fraction of an f/stop with the same lens. And if you do the
    test with the lens wide open, the stop is not moving.

    In my QE testing, I specifically changed aperture, and of course
    one must change exposure. Normalizing aperture, the plot
    is quite linear, showing that both shutter and aperture
    are predictable and reproducible to a fraction of a stop.
    See Figure 3 at:
    http://www.clarkvision.com/imagedetail/evaluation-1d2

    Roger
     
    Roger N. Clark (change username to rnclark), Mar 17, 2006
    #80
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