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PostPosted: Sun Feb 28, 2010 8:44 pm 
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Earlier today I tried using the 5D as a wildlife cam, and initially I got some odd exposures which at the moment I can't explain. As a sanity check, I was going to test it against the 7D at home under controlled conditions.

I set up a simple test - put the 50mm f/1.8 on the 5D. Pointed the combo at a random leaflet lit by mains powered continuous lights. Set aperture priority mode to f/5.6, manually vary ISO through range taking a shot at each. When I scrolled through the 1st set I noticed the apparent brightness wasn't consistent. I repeated this, and got a different set of variations. On the histogram, there was a variation of about 1/10th of the whole range between the extreme shots. This is visible on the image as a variation in brightness.

The exposure set by the camera was consistent, with the shutter speed varying shutter proportionately to ISO. It did not fluctuate even 1/3 stop from 1/25 at ISO100 or equivalent. Taking 3 shots at 1/400s, I got the bigger variation. Taking 3 shots at 1/25, there was insignificant variation.

Does that mean my "continuous" lights are flickering? I haven't looked at them on computer yet, to see if there is any banding from the shutter movement if this is the case. Otherwise, is there any other mechanism? Guess now to do part 2: repeating it on 7D and see if it still happens.

Edit: Done same on 7D, it metered exactly the same, and also exhibited the brightness variation. On average, it is about 1/10th of a histogram width brighter though, but I do have different picture style setting applied which may contribute to that.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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PostPosted: Sun Feb 28, 2010 10:26 pm 
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Fluorescent lights flicker at 60 hertz so you will see flickering at 1/60th and above. I can't speak for halogen lights or tungsten but I presume they will also exhibit the effect (albeit less or more apparent). It's the way our lightbulbs work, I guess.

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PostPosted: Mon Mar 01, 2010 12:09 am 
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Yes and no. I did wonder if they flicker at 100 Hz (double mains frequency due to two halves of sine cycle), but I was still seeing a small variation at 1/25s. Modern types *should* operate on much higher frequencies.

I did try a test shot at 1/8000s and that was completely even. The flash sync speed of 1/250 implies a similar shutter travel time, which would be 1/10th of an assumed 100 Hz cycle if there was one.

I think I need an alternate test for slower flicker - done in past by long exposure and moving the test source across the field of view over it, revealing a pulsing pattern. E.g. try doing a long exposure zoom night city scene, you can see this on street lamps. But I need to do it at home with a bright source. Might be harder...

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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