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04-06-2010, 07:51 AM   #1
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Curious about Photos OF light from a DSLR

This is something I do once in awhile when I get really bored.. I'll take the camera out for a drive at night and shoot time exposures through the windshield to see what kind of pattern develops from the camera movement..



This particular one is done with the K7, DA12-24, 9 sec at f16. 35-40 mph.

What I'm curious about, is why the broken lines?? I'm certain it isn't a problem with the camera or lens, they all do it (I've tried this with every digital I've ever owned).

I've posted it here instead of a photo thread because I'm interested in the operation of the camera that gives that broken line. Is it the camera? or am I looking at the frequency of the power driving the lights?

Though if you're interested http://www.flickr.com/photos/13300089@N08/sets/72157623647813847/..


Any thoughts??



04-06-2010, 07:56 AM   #2
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I've done and noticed the same thing. As a totally uneducated guess I think its the minute flickering or modulations of the lights. and the size/distribution of the dashes is indicative of the speed or frequency that the lights modulate.

Mike
04-06-2010, 08:10 AM   #3
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is this potentially an issue of line width and pixel size giving an indication of flicker?

when you zoom in this is what it looks like to me.
04-06-2010, 08:13 AM   #4
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Mike, thats the same line of thought I had. Not knowing the source of power for the lights shown above but wondering if the camera can catch 60hz flicker when our eyes don't. Guess I can test that with an 110v/60hz indoor lamp versus a battery powered bulb. ....if I get motivated.

04-06-2010, 09:42 AM   #5
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Hello,
The lights probably have a 120Hz frequency (2x the power line). You could check by counting the number of dashes in the line and dividing it by the exposure of the photo.

@ imtheguy,
FYI, If you are going to test this out at home, beware of incandescent bulbs, as the hot filament probably makes the frequency of the power line unobservable.
04-06-2010, 10:25 AM   #6
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It is indeed the frequency of the powergrid. You can easely test it with a flashlite or a candle, that won't flicker and will give a contiues line.
Even car lights will flicker, acspecially led-backlights of cars.
04-06-2010, 12:22 PM   #7
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In my curious youth, I sometimes played with NE-2 neon glow tubes, glass bulbs about 7x15mm with two wires sticking out one end. These cheap little lamps, only drawing 1 watt, were used as indicator lights in appliances and equipment (since replaced by LEDs). Their trigger level is about 85 volts. To use one on 110 VAC line current, put a 1 meg-ohm, 2w resistor in the circuit.

So: take a 2m- or 3m-long plug-in power cord. Solder the glow tube and resistor in series at the bare end. Carefully tape over all exposed wires.
'''resistor
+--XXXXXX------------->
O
+---------------------> power
NE2
Plug into wall socket. Turn off room lights. Swing light around at end of wire. Enjoy the line of orange light. At certain rotational speeds, the line breaks up into dashes or dots. You're seeing the strobe effect.

And that's exactly what causes the dotted line in the photo -- strobing at line frequency. Experimenters out there might want to build this simple rig and try time exposures, eh? It'll be even more fun if you have a little multi-color-filter wheel spinning slowly in front of the camera lens.
04-06-2010, 12:30 PM   #8
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There are any number of light sources in these photos. Halogen street lamps, traffic signals, store signs of all sort including Neon (which I would expect to flicker), headlights and tail lights of moving cars, etc. I don't think it's the camera but it seemed to me that 60 hertz (and 120) was too fast to see by the naked eye. I guess the camera sensor picks it up though. In some cases, I've moved the camera to see if I could build geometric shapes with the light. Probably better to try when somebody Else is driving though

My father use to do this with slide film. I don't remember ever seeing the cycle flicker in those photos.



04-06-2010, 12:47 PM   #9
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QuoteOriginally posted by JeffJS Quote
What I'm curious about, is why the broken lines?? I'm certain it isn't a problem with the camera or lens, they all do it (I've tried this with every digital I've ever owned).
may not initially sound relevant - flashlights.

In LEDs especially there is a technique called PWM (Pulse Width-Modulation) which kind of "fools" the eye into see lower brightness levels by pulsing the light at full intensity - but because the duty cycle is less than 100% we see lower light levels.
Better explanation at:
Pulse-width modulation - Wikipedia, the free encyclopedia.

So taking a a stationary photo of the light source will just reveal the lower intensity if PWM were applied - but moving the camera or the light source - probably would catch this pulsing/strobing.
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