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07-07-2007, 07:19 AM   #1
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Low light versus Poor light

After my recent wedding shoot I have been thinking about low light, bumping up ISO, etc etc all to try and gather more light. But it occurred to me that it's not just the quantity of light that determines how a photo turns out, it's also the quality.

Low light, for this arguments sake, will be referring to a 'white' or (relatively) full spectrum light source. I'm suggesting the 'poor lighting', under dim tungsten or fluorescent lighting is not only low intensity, but also comprised of light from a limited
segment of spectrum.

The photosites on our sensors have two green, one red, and one blue sensors, each recording analog data from specified range in the light spectrum. Under poor lighting conditions, say dim tungsten, the camera is absorbing yellow-orange light, which is absorbed by the red, and partially by the green sensors. The blue channel is basically left out of the equation, and as such we're missing one third of our colour data...our "colour resolution". If there is a blue object in the frame, it will be very difficult to represent it as true blue in our photographs because there is so little blue light being reflected into the sensor.

So this brings in the question about how white balance works. Does shifting white balance mean that certain parts of the information in the red channel, say, are moved into the blue channel to fill in the 'missing' blue information? Will this 'interpolation' not lead to a 'false blue' representation of an object?

I guess what I'm saying is that low light, where you're capturing a light across a a large part of the visual radiometric spectrum will necessarily result in photos with higher 'colour' resolution (and I would imagine higher spatial resolution due to bayer interpolation as well) as compared to capturing an image where light was captured from a limited part of the spectrum, even if the absolute intensity (amount of light energy being absorbed by the sensor) of both light sources was equal.

A little bit obvious perhaps, but it makes sense, right?

07-07-2007, 08:32 AM   #2
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I'm inclined to agree but I define poor light a little differently. For example, I once photographed a wedding where the bride and groom were illuminated by overhead track lighting, resulting in horrible shadows in the subjects' eye sockets. I couldn't move them and I couldn't use flash. It was definitely poor light and its spectral distribution was the least of my problems. Didn't do much for the facial expressions either.

Richard
07-10-2007, 10:43 PM   #3
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QuoteOriginally posted by Old Timer 56 Quote
I'm inclined to agree but I define poor light a little differently. For example, I once photographed a wedding where the bride and groom were illuminated by overhead track lighting, resulting in horrible shadows in the subjects' eye sockets. I couldn't move them and I couldn't use flash. It was definitely poor light and its spectral distribution was the least of my problems. Didn't do much for the facial expressions either.

Richard
How did you go around the problem? Kept on shooting? or did you try something to compensate?
07-10-2007, 11:03 PM   #4
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QuoteOriginally posted by d.bradley Quote
After my recent wedding shoot I have been thinking about low light, bumping up ISO, etc etc all to try and gather more light. But it occurred to me that it's not just the quantity of light that determines how a photo turns out, it's also the quality.

Low light, for this arguments sake, will be referring to a 'white' or (relatively) full spectrum light source. I'm suggesting the 'poor lighting', under dim tungsten or fluorescent lighting is not only low intensity, but also comprised of light from a limited
segment of spectrum.

The photosites on our sensors have two green, one red, and one blue sensors, each recording analog data from specified range in the light spectrum. Under poor lighting conditions, say dim tungsten, the camera is absorbing yellow-orange light, which is absorbed by the red, and partially by the green sensors. The blue channel is basically left out of the equation, and as such we're missing one third of our colour data...our "colour resolution". If there is a blue object in the frame, it will be very difficult to represent it as true blue in our photographs because there is so little blue light being reflected into the sensor.

So this brings in the question about how white balance works. Does shifting white balance mean that certain parts of the information in the red channel, say, are moved into the blue channel to fill in the 'missing' blue information? Will this 'interpolation' not lead to a 'false blue' representation of an object?

I guess what I'm saying is that low light, where you're capturing a light across a a large part of the visual radiometric spectrum will necessarily result in photos with higher 'colour' resolution (and I would imagine higher spatial resolution due to bayer interpolation as well) as compared to capturing an image where light was captured from a limited part of the spectrum, even if the absolute intensity (amount of light energy being absorbed by the sensor) of both light sources was equal.

A little bit obvious perhaps, but it makes sense, right?
You are totally correct. I never understand anyone who would think about undertaking a professional shoot without control over the lighting, unless its a gig shot on stage where you dont expect the lighting to be natural. For a wedding, unless I was allowed to use flash, I would simply turn the job down. Why try and do a job with your hands tied behind your back - makes no sense to me.

Both the limited spectrum of the ambient light combined with the limited DR and noise of high ISO makes for poor quality results. If someone is paying for the shoot, they deserve the memories as they saw them, not as the camera can manage in such adverse conditions.

07-11-2007, 07:53 AM   #5
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QuoteOriginally posted by AVANT Quote
How did you go around the problem? Kept on shooting? or did you try something to compensate?
I kept shooting. This was many years ago with black & white film. The lab I used tried dodging the eye sockets and this at least produced an acceptable image. The client was happy but I wasn't.

The wedding was on short notice and I had no chance to review the conditions of the shoot beforehand. I don't do weddings anymore (I'm sort of semi-retired now) but if I did, I would probably decline the job. I always preferred to light weddings, especially when shooting color.

Richard

Last edited by Old Timer 56; 07-11-2007 at 07:58 AM. Reason: incomplete post
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