Monday, 11 February 2008

Correcting light for complex ambient sources

I mentioned in my previous entry that when I checked the ambient light, it was pretty similar to tungsten light, meaning I only needed to correct with a CTO gel, but that sometimes you can require an additional fluorescent gel to correct for certain types of mercury vapour lamp. I think for some people it goes a little against the grain to add more than one gel, so hopefully this example will help a little.

So, in this example, I was photgraphing a large factory building which had high powered mercury vapour lamps high up. There was the added complexity of daylight coming in through windows, but we shall try to ignore that for now.

So taking a daylight balanced photo to check the light, I was presented with the following image

Now, at a glance as you walk in to this building, you'd think "close to tungsten" because there is a distinct orange look to the light. On taking the quick shot above, you'd be forgiven for thinking your thoughts were confirmed, but switching to tungsten, you get this image

A pretty nasty green colour cast! So is it closer to fluorescent? Well, taking a shot in fluorescent white balance mode, you get this image

A rather warm tone in fluorescent and a rather green tone in tungsten? That's the joy of some mercury vapour lamps! So if we take a custom white balance, which is basically a mix of the tungsten white balance and fluorescent white balance, you get the following image.

This is the "correct" white balance, in that the grey areas are indeed grey where they are lit only by the mercury vapour lamps. So to correct your daylight balanced flash, you'd need to have a combination of a CTO gel to correct for the tungsten component, and a plusgreen gel to correct for the fluorescent component. Varying the strengths of these gels, you'd be able to get a pretty accurate white balance match to the mercury vapour lamps.

Out of interest, for the shot these examples are cropped from, I wasn't shooting with flash at all (they just represented a good image to use for the examples). I actually found a white balance close to fluorescent was the most pleasing on the eye. The reason for this is that a combination white balance leaves daylight very cold blue and rather unappealing, and that wasn't the atmosphere I was looking for with this shot. So it isn't always purely about what is "correct", but also what looks good for the intended atmosphere you want to show.

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