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04-04-2014, 04:04 PM   #1
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A question about white balance

I am using gphoto2 to take a series of photos of a culture in my lab. The camera is an Olympus C-700 (very old, but I had it and wanted to put it to use).

The culture is lit by two 13 W LED lights, continuously outputing about 800 lumens each. The frames are being taken every five minutes.
The image is mostly a white (ish) background, with some gray concentric circles, and a slowly growing yellowish-green fungus. The camera exposure is automatic, as is the white balance. The on-camera flash is not used, so the camera must think it has enough light.

I've noticed that successive frames have slightly different appearances - the white balance or perhaps the exposure seems to shift about. I haven't looked in detail at any histograms (my first experiment is still running) and I don't have an example image to show right now. Sorry.

Why would there be visual differences in a sequence of images taken of the same subject, lit by (presumably) constant lighting? Is it necessary to run the lights off a constant voltage transformer, do you think? Or is it simply that I'm asking more from my poor old C-700 than it's really capable of? Not really eager to dedicate my K10D to the lab, but I suppose I could try a sequence with it...

Thanks in advance.

04-04-2014, 04:18 PM - 1 Like   #2
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are there windows in the lab? Perhaps daylight is casting some light enough to confuse the meter in your camera.
04-04-2014, 04:31 PM - 1 Like   #3
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Why not just use manual white balance and exposure? Your light source is known (color temp) and won't vary in intensity I don't see why you'd leave it in auto.
Your time lapse will look more fluid if it doesn't varies from shot to shot. No matter how good your camera is, it may calculate exposure and WB slightly differently between each shots.
04-04-2014, 07:06 PM   #4
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windows

QuoteOriginally posted by carrrlangas Quote
are there windows in the lab? Perhaps daylight is casting some light enough to confuse the meter in your camera.
Yes, and while the lab is about 15 m x 10 m and the camera system is approximately 15 m from the windows, that might be an issue. I've "locked" the room lights on because I found (quickly) that there was a huge difference if someone turned the lights off during the run.

I CAN try blocking the windows if other things don't work.

Thanks.

---------- Post added 04-04-2014 at 10:10 PM ----------

QuoteOriginally posted by fgaudet Quote
Why not just use manual white balance and exposure? Your light source is known (color temp) and won't vary in intensity I don't see why you'd leave it in auto.
Your time lapse will look more fluid if it doesn't varies from shot to shot. No matter how good your camera is, it may calculate exposure and WB slightly differently between each shots.
Yeah, this is one of the things I'm considering. I chose "auto" on the initial runs out of laziness, but I'm going to set the exposure and balance manually for the next trial. It seemed to me that the issue was most likely the camera, not the actual color temperature/intensity of the lights. But I wanted to get some other ideas too.

Thanks.

04-04-2014, 08:00 PM   #5
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Most LED lights have a poor color balance; the best LEDs are merely OK. The color spectrum graph usually shows horribly large spikes and valleys, compared to a traditional tungsten light which has a perfectly smooth (though warm) curve. So even if you can get it to be consistent from shot to shot, don't expect good color balance.

LED lights can also be subject to various types of flicker, so at normal to fast shutter speeds one frame may be taken at a different point in the "flicker cycle" than the next, producing a noticeably different color balance in back to back shots. I experienced a similar problem a couple of weeks ago shooting in a gym, although I believe most of or all of the lights were non-LED. Nevertheless, some of the overhead lights clearly had very different color temperatures, which one could easily see just by looking at them. The the color temperature in my photos (even though I'd set a fixed, custom white balance using an Expodisc) would vary significantly depending on exactly when the 1/640s shutter was fired.

Try using some type of halogen or similar light if possible - even the basic ones like the the "Halogenas" made for conventional bulb sockets.
04-05-2014, 08:48 AM   #6
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Thanks - I didn't realize that

QuoteOriginally posted by DSims Quote
Most LED lights have a poor color balance; the best LEDs are merely OK. The color spectrum graph usually shows horribly large spikes and valleys, compared to a traditional tungsten light which has a perfectly smooth (though warm) curve. So even if you can get it to be consistent from shot to shot, don't expect good color balance.
I knew this, but it shouldn't cause shot-to-shot variation.

QuoteOriginally posted by DSims Quote
LED lights can also be subject to various types of flicker, so at normal to fast shutter speeds one frame may be taken at a different point in the "flicker cycle" than the next, producing a noticeably different color balance in back to back shots. I experienced a similar problem a couple of weeks ago shooting in a gym, although I believe most of or all of the lights were non-LED. Nevertheless, some of the overhead lights clearly had very different color temperatures, which one could easily see just by looking at them. The the color temperature in my photos (even though I'd set a fixed, custom white balance using an Expodisc) would vary significantly depending on exactly when the 1/640s shutter was fired.
THIS, on the other hand, I didn't know. Thanks.

QuoteOriginally posted by DSims Quote
Try using some type of halogen or similar light if possible - even the basic ones like the the "Halogenas" made for conventional bulb sockets.
No can do. Too much heat, dries the agar within 24 hours. Our culture is sitting uncovered in a sterile (well, sanitized) environment. Otherwise condensation on the lid obscures the imaging. I may have to resort to a xenon flash.
04-24-2014, 06:47 PM   #7
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I think your approach is wrong, you are running this shoot like a scientist who wants to record his results, therefore you get good science and bad photography.


Try to run it like a photographer who wants to create a scientific shoot. The challenge here is then to still do good science.


Your problem is your controlling nothing and then hoping to capture some record shots. Its not surprising theyr not the best quality.


Run the experiment like a photoshoot. Control everything, make a light tent with controlled lighting so theres no window light and no striplight and no led light and no incandescent and no halogen, - other than the lighting you light the experiment with.


When your in total control of the light source, its intensity and colour temperature, only then can you expect consistent results shot to shot.


Lighting will be a challenge, solve it.


Photography is not about controlling a camera, its not about controlling a subject, its about controlling light. so control it.
04-25-2014, 04:47 AM   #8
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QuoteQuote:
Run the experiment like a photoshoot. Control everything, make a light tent with controlled lighting so theres no window light and no striplight and no led light and no incandescent and no halogen, - other than the lighting you light the experiment with.

When your in total control of the light source, its intensity and colour temperature, only then can you expect consistent results shot to shot.
I thought I had. The LED lights are sufficiently intense and close to the plate that the window light IS insignificant; the room lights are on continuously, so there is no variation, there is "no striplight and no incandescent and no halogen - other than the lighting you light the experiment with."

Actually, we've been using a dark background on recent trials and we're getting much better consistency in the exposures and color balance. Previously we used a white background; the media is a light tan, so the whole frame was pretty much high key. Now it averages to a medium gray, which I think is being handled better by the camera.

Thanks anyway.

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