Hockey rink pictures and lighting

Discussion in 'Photography' started by JME, Feb 28, 2006.

  1. JME

    JME Guest

    I was shooting a hockey game this evening, and I ran into an interesting
    problem.....
    As I was shooting in manual exposure mode F 2.8, 1/250 I though all was
    going well until I took as look at a few images.
    I took a sequence of images on a breakaway, the histogram showed variation
    in exposure, and on reviewing the images, to my surprise, some were bang on
    for exposure, some were off by 1-2 stops, and then there were the images
    that seemed to be half and half. One part of the image looked good and
    another part of the same image looked under exposed.
    I suspect it has to do with the lighting in the building and the fact that
    they were using sodium vapour lights. I suspect that as this building is
    probably three phase, and the lighting would have been wired utilizing the
    three phase. I could assume that each row would be on a different phase and
    at 1/250 of a second I might have been getting lighting that was uneven due
    to the lights being on opposing phases..Anyone have any experience with this
    kind of thing. Slower shutter speed seemed to fix it..
    Any info would be interesting...
    Jason
     
    JME, Feb 28, 2006
    #1
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  2. JME

    Pat Guest

    I can't imagine it being related to having 3-phase power in the
    building -- not unless the power load is really screwed up and there is
    a voltage drop on 1 leg, but they would notice that when they had to
    keep replacing burned-out equipment. Attributing the problem to
    3-phase would be the same as attributing a similar problem anyplace
    else to the fact that they have 220 and that half of the lights are on
    one leg and the other half are on the other.

    If you were using a flash, that could explain the problem but you
    didn't say you were. Zooming in could also cause it -- as you zoom in
    you will lose light, generally.

    It could be that the light is really blotchy and some areas are quite a
    bit brighter than others. I've seen a lot of that at my kid's box
    lacrosse, which is sometimes played in hocker rinks. It could also be
    a glare issue, where glare from other lights is helping to illuminate
    certain areas.

    Otherwise, I'm not sure but it would be interesting if you could post a
    sequence of pictures somewhere.

    Good luck.
     
    Pat, Mar 2, 2006
    #2
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  3. Except that on 220 there is only 1 phase, and they would all be the
    same. If the lights used 120, then yes there would be two phases
    180 degrees out. But something like a hockey rink isn't going to
    be lit using 120v, and wouldn't likely have single phase either.
     
    Floyd L. Davidson, Mar 2, 2006
    #3
  4. JME

    Pat Guest

    In any case, I don't think 3 phase power is screwing up the pictures.
    With three phase or 120/220 in your house, if you overloaded one leg
    enough to make it noticeable on the lighting, it would show up in other
    ways. For example, if you had an uneven load it your house and some of
    your lights were bright and some were dim, you might notice the dim
    lights but you would also notice other things too -- the the
    refrigerator or microwave or TV would be on the fritz, depending what
    was on that side of the 220.

    If you had a 3 phase lighting problem, you would notice that as
    dimming/flickers when the HVAC came on or someone used the elevator.
    It wouldn't be just some lights being bright and some being dim. In
    fact, it would be more likely that someone just didn't turn on all of
    the lights.
     
    Pat, Mar 2, 2006
    #4
  5. I think the point was that with some types of lighting the lamps
    actually flicker at power line frequency. Fluorescent lamps are
    one example. Taking pictures of a TV screen is another example.

    If two sets of lamps illuminate different parts of a scene, and
    are on different phases, the flicker will be at different times.
    With a 1/30th shutter speed that will not be noticed, because
    the entire flicker cycle will be exposed. But as shutter speeds
    increase above 1/60th of a second there will be less and less of
    each flicker cycle exposed. In that case it might well be
    possible to get some very strange lighting effects that vary
    from one exposure to another.

    That said, I have no idea whether that is actually what the OP
    was seeing or not. Without looking at some of the images it is
    hard to say. The point is only that his questions about 3 phase
    power and shutter speed were indeed well taken.
     
    Floyd L. Davidson, Mar 3, 2006
    #5
  6. JME

    Colin D Guest

    The lights are most probably gas discharge, high-pressure sodium or
    mercury or similar, and the might well show 'flicker', as opposed to
    incandescent lamps where the thermal inertia of the filament tends to
    smooth the light output, though not completely.

    A simple test is to wave your hand in front of a lamp as you are looking
    at it. If the silhouette of your hand shows pronounced flicker or
    strobe effect, then the lamps might be your problem. You can do this in
    front of your monitor to see the effect.

    Colin D.
     
    Colin D, Mar 3, 2006
    #6
  7. JME

    Pat Guest

    I don't follow your thought.

    Assume it is flicker and let's make some simplifing assumption. Assume
    you had a flickering light on a 110 circuit, such as an incandecent
    light or a fluorescent lamp (which I'm not sure if they flicker or
    not). In either case, it flickers at 60 cycles per second. The OP
    said he was taking the picture at 1/250th, which we all have done. So
    the picture takes place during 24% of one cycle (60/250). Therefore,
    if you sat in your living room, you would except roughly one-fourth of
    your pictures to be properly exposed. But that's not the real world
    because the lights flicker, but they're not strobes. The elements in
    the lights can't cool off that quickly.

    Second, I'm not an electrician but I know a little about electricity.
    I wonder about whether one would use a 3-phase circuit for lighting.
    3-phase is generally restricted to things with large electric motors --
    something about it being difficult to start a large electric motor
    under load if it stops in a certain positioning. It is would be there
    for the HVAC and the elevator, but not the rest of the lighting.

    But let's see the pictures. It might be something simple he zoomed in
    as the skater moved away from him or there was a spotlight on the
    skater. Or, more likely, the corners were dark.
     
    Pat, Mar 3, 2006
    #7
  8. JME

    JME Guest

    Nice to see some conversation..
    I have yet to see this problem in another rink or building so I was very
    interested in find out what was going on.
    The image sequences were for example break away to the net. A sequence taken
    at high speed (8FPS) would have 2-3 under exposed images. Now when I look
    back a the images from the day I notice that a lot of single shots were a
    bit under but a pick me up in PS got around that. It really only stood out
    in a series of images..
    As an electrician(Commercial-Industrial),I have installed open frame motors
    and if you looked at the rotor under the HPS it would seem to be
    stationary, if you looked at the same motor with an incandescent trouble
    light it was very obviously spinning.
    I went back to the rink, to see what was up and sure enough three phase
    lighting panel, all the way down the panel the lights were labelled ROW
    1--ROW 2--ROW 3 etc . Check on of the other rinks in the area and the lights
    were all jumbled up.
    panel was marked Breaker 1 Light 3-5-9 Breaker 2 Light 2-4-8 etc. I have
    never done any lighting in rinks as an electrician so I asked a few of the
    other contractors around and only one other guy had done lighting. Sure
    enough the lighting layout is done to allow for VIDEO and Photography
    balanced lighting, taking in to consideration light quenching...
    I contacted CANON and they e-mailed me and said I should avoid shooting
    1/120-1/500 of a second as they would be the most problematic shutter.
    Having said all of this all of the images were usable, just some were a bit
    under exposed..
     
    JME, Mar 3, 2006
    #8
  9. You have to make the *right* assumptions... ;-)
    Try it! Incandecent lamps don't flicker. You'll have to try it
    with something that does.

    The easiest way to really get an idea what happens is to take
    pictures of a TV screen. Try that at different shutter speeds
    and see what your results are...
    Understanding this requires some fairly basic knowledge of power
    generation and distribution.
    That is not optional! Industrial power is 3 phase. Residential
    power is single phase. Any building with industrial power
    necessarily has 3-phase power for lighting. Of course whether
    there are two different phases used for the lighting in any
    given location would be another thing...
    The whole building is going to be supplied either with 3-phase
    power or with single phase power, never a combination of both.

    It's fairly easy to determine to (at least in the US), as the
    "220" will be very close to 208 VAC for 3 phase power, while
    with residential single phase power the "220" will almost
    certainly be closer to 236VAC.

    The suggestion was that the massive requirements to light a
    hockey rink would necessarily require load balancing, and hence
    different banks of lights are going to be powered from different
    phases. That means the flicker will not be at the same instant.
    All possible, except he said certain areas varied (if I remember right).
     
    Floyd L. Davidson, Mar 3, 2006
    #9
  10. Hey, wait a minute [email protected]#$%^&*, *you* are exactly the person who
    should be telling everyone else how this happens? Why were *you*
    asking *us*? ;-)
    Hmmm... tells us something about the difference between good
    systems engineering and merely engineering, eh? (That isn't
    something I would have thought of... but I don't design power
    system either.)
    Thanks for the *detailed* and authoritative comments!
     
    Floyd L. Davidson, Mar 3, 2006
    #10
  11. JME

    JME Guest

    I was asking to see if there was another possibility that I had not thought
    of...If I had put up my experiences first no one would have bothered to
    offer an opinion..I like to look at all of the option.
     
    JME, Mar 3, 2006
    #11
  12. JME

    Pat Guest

    To paraphrase someone like Jerry McGuire, "show me the pictures".

    The examples everyone uses of a flicker is a TV where you can see the
    lines if you photograph them. Right. Also, if you move your fingers
    back and forth in front of it, you can see the flicker as your fingers
    "jump" due to the flicker. Right, again.

    So, if the lights were "strobing" and that was the problem, how come
    you don't have a strobe effect when you see the kids with your bare
    eyes?
     
    Pat, Mar 3, 2006
    #12
  13. JME

    JME Guest

    Because a shutter speed of 1/250 of a second is much quicker than humans
    possible perception.
    Please see following taken from

    http://en.wikipedia.org/wiki/Flicker_fusion_threshold

    Like all psychophysical thresholds, the flicker fusion threshold is a
    statistical rather than an absolute quantity. There is a range of
    frequencies within which flicker sometimes will be seen and sometimes will
    not be seen, and the threshold is the frequency at which flicker is detected
    on 50% of trials.

    The flicker fusion threshold is proportional to the amount of modulation; if
    brightness is constant, a small flicker will have a much lower threshold
    frequency than a large flicker. Also it varies with brightness (it is higher
    for brighter lights) and with location on the retina where the light falls:
    the rod cells have a faster response than the cone cells, so flicker can be
    seen in peripheral vision at higher frequencies than in foveal vision. The
    flicker fusion threshold is also higher for a fatigued observer.

    The flicker fusion threshold also varies between species. Pigeons have been
    shown to have higher threshold than humans, and the same is probably true of
    all birds. Many mammals have a higher proportion of rods in their retinae
    than humans do, and it is likely that they would also have higher flicker
    fusion thresholds.

    [edit]
    Importance
    Flicker fusion is important in all technologies for presenting moving
    images, nearly all of which depend on presenting a rapid succession of
    static images (e.g. the frames in a cinema film, TV show, or a digital video
    file). If the frame rate falls below the flicker fusion threshold for the
    given viewing conditions, flicker will be apparent to the observer, and
    movements of objects on the film will appear jerky. For the purposes of
    presenting moving images, the human flicker fusion threshold is usually
    taken as 16 hertz (Hz). In actual practice, movies are recorded at 24 frames
    per second, and TV cameras operate at 25 or 30 frames per second, depending
    on the TV system used. Even though motion may seem to be continuous at 25 or
    30 fps, the brightness may still seem to flicker objectionably. By showing
    each frame twice in cinema projection (48 Hz), and using interlace in
    television (50 or 60 Hz), a reasonable margin for error or unusual viewing
    conditions is achieved in minimising subjective flicker effects.

    Flicker is also important in the field of domestic lighting, where noticable
    flicker can be caused by varying loads and can be very disturbing to
    electricity customers. Most electricity providers have maximum flicker
    limits that they try to meet for domestic customers.

    [edit]
    Subjectivity of flicker
    Modern computer CRT displays usually have a vertical scan rate well over 60
    Hz, and can thus be considered flicker-free. (Other display technologies do
    not flicker noticeably so the frame rate is less important.) Some people,
    particularly autistics, can still notice a flicker in CRTs or even in
    fluorescent lights; a few find fluorsecent lights uncomfortable for this
    reason.

    In some cases, it is possible to indirectly detect flicker at rates well
    beyond 60 Hz in the case of high-speed motion, via the stroboscopic effect.
    Fast-moving flickering objects zooming across view (either by object motion,
    or by eye motion such as rolling eyes), can cause a dotted or multicolored
    blur instead of a continuous blur. A common example of this phenomenon is
    the DLP Rainbow Effect. Some special effects like such as certain kinds of
    electronic glowsticks used at raves, have the appearance of a solid color
    when motionless, but produce a multicolored or dotted blur when waved about
    in motion. Some particularly sensitive people claim to be able to see the
    flicker of office fluorescent lighting or street lighting, which occur at
    100 or 120 Hz. Other times, people can indirectly detect the prescence of a
    high-speed flicker via the mere existence of a wagon-wheel effect.

    [edit]
    See also
     
    JME, Mar 3, 2006
    #13
  14. JME

    Colin D Guest

    Not quite. For a sine-wave alternating waveform, there are two peaks
    and two zero crossings per cycle, so for a frequency of 60 Hz, the
    'flicker' occurs at 120 Hz.

    Therefore,
    In gas discharge lamps, the 'element' is an electrically excited gas,
    and the light output follows the 60 Hz mains frequency excursions,
    unlike incandescent or filament lamps where the thermal inertia of the
    filament keeps it glowing between cycles. This is why strobe
    applications like turntable speed indicators look much clearer under
    fluorescent or neon lamps than they do under incandescent.
    Yes, but in large buildings with considerable power requirements,
    3-phase circuits are used to balance the load on the mains supply.
    Dragging multi-kilowatts of power from one phase only creates an
    imbalance on the supply, which can damage transformers and upset voltage
    regulation on other phases. If a building has, say, 40 lamps in 5 rows
    of eight maybe, on a single phase, and each lamp draws 1500 watts,
    that's a 60-kilowatt load, and at 110 volts, a current of 545 amps.
    That's a massive imbalance on the power supply. It gets worse on
    outdoor lighting of playing fields, where the load can run to hundreds
    of kw.Colin D.
     
    Colin D, Mar 4, 2006
    #14
  15. JME

    Colin D Guest

    <another snip>

    Residential power is single phase for an individual home, or sometimes
    2-phase for cookers in older homes. But a street of houses has the
    dwellings on successive phases, so the overall load on the supply is
    more or less balanced. Most people cook at the same time, turn heaters
    etc. on about the same time, so the load presented by a street is pretty
    well balanced most of the time. And combining many streets means that
    statistically, the supply load is well balanced.

    Colin D.
     
    Colin D, Mar 4, 2006
    #15
  16. JME

    Colin D Guest

    because the effect is only apparent if the moving figure is silhouetted
    by the lamps. Overhead lighting can't do this.

    Colin D.
     
    Colin D, Mar 4, 2006
    #16
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