Why does using the flash mess up the white balance?

Question

When I take a photo without flash, I get reasonable white-balance:

But when I turn on the flash, everything becomes yellow, which is not how the scene looks to the naked eye:

Why does this happen? Why doesn't the camera analyze the scene, decide the color temperature to use, then fire the flash and take the shot, and then adjust the color in the resulting photo to match the previously calculated color?

I've done this in Lightroom by noting down the white balance settings of the photo without flash and then applying them to the photo with the flash, to produce the following (good) result:

Is there a reason why the camera can't do this? Or is it just another case of cameras not being smart enough?

This is on the Sony NEX-5R, with the following settings:

• White balance: auto (The camera does offer a "flash" white-balance mode, but I didn't use it, thinking that the camera already knows that the flash is connected and enabled, so I shouldn't have to tell it once again. Am I wrong?)
• Flash mode: slow sync
• Flash exposure compensation: -2
• Focus: manual focus
• Aperture: F3.2
• ISO: 200
• Shutter speed: 5s
• Exposure compensation: 0
• Mode: aperture priority

Answer 1

One thing to realise is that in your example it is actually impossible for the camera to make the whole picture look "right" when you're using the flash.

White balance will make a particular colour of light look white in the end image. However, when you have two different colour lights e.g. post sunset sunlight (probably mixed with yellow street lights) and flash, the camera can only pick one of those colours to be "white". That is why either the ambient light looks yellow and the area lit by the flash looks "white", or the ambient light looks "white" and the area lit by the flash looks blue. (I believe that flashes tend to be fairly close to daylight in colour temperature so you don't really notice the effect with fill flash in the day time).

When you use the flash on the camera it assumes that you have something important that you are lighting with the flash and switches to its flash white balance setting. This makes sure that your subject will look good, but may make the background look odd.

You can overcome this colour clash by:

• Overpowering any ambient light with flash - only really indoors or with big strobes.
• Or you can gel (put a coloured filter over) your flash to match the ambient light. In this case you'd want a yellowy filter on the flash.

As this is a very common problem when using flash indoors with artificial lighting there are even standard colours of flash gel available for tungsten and fluorescent (and probably other) lights. (The first hit on google for flash gels has a reasonable example: flashgels.co.uk)

It's also worth noting that if you're using flash under non-daylight lighting then gelling your flash can save you a lot of work later on - fixing these different colours in photoshop can be a nightmare.

Answer 2

The biggest problem with white balance is not inacuracy of cameras, rather the wide range of colour that the human brain automatically "corrects" to white.

How is the camera to know what artistic effect you are looking to create when it is in auto white balance?

The colour temperature shifted because you used the flash, so the camera assumed that it should take the flash setting for its white balance. If you had a foreground subject in the photo, mainly illuminated by flash, the colour would have been correct for the subject.

If you look at the railway ballast in the "corrected" photo, you can see that it has taken on a blue tinge as a result in the change of white balance

Answer 3

Yes, it is in a sense the camera not being smart enough. It probably tries to guess the color temperature of the light source(s), which light the main subject. In the case of flash, it has an easy job - it knows the color temperature of the flash. So, it probably just assumes that your main subject is going to be lit by this. We are still quite far from properly simulating what the human visual system does, so cameras use some simple heuristics / rules of thumb.