A common problem with indoor photography is that the view outside the window is usually blown out. In order to achieve balance while shooting inside, looking out through a window, you have to think of what you are seeing through the view finder as if it were two separate exposures – the outside world (being lit by the sun) and the indoor scene (being lit by your flash).
Until you’ve developed the “knack” for doing it, this is hard enough when the environment is a cozy living room … it’s a bit more daunting when your environment involves a 100′ tunnel, and the time of day is high-noon. However, whether in your living room, or on a spelunking adventure, the steps are the same (although I did have to keep dodging traffic when I was taking the tunnel shots – and I’ve never had that particular problem in my living room … yet, anyway).
The first step is to get a rough idea of how much ambient light I’ve got at my disposal.
The photo above was taken on AV mode with the aperture set to 5.6 – this gave me a good idea of what a “correct” shutter speed should be – if I wanted to over-expose the exterior, I could dial down the shutter speed, if I wanted to under-expose the exterior, I could dial it up (faster shutter = darker image).
The next step was to attempt to get a decent interior exposure using nothing more than a camera-mounted strobe. There was no way to set up additional lights – this is a well-used tunnel and traffic tends to come around that corner pretty fast.
I used a strobe which has an adjustable zoom setting set it to its widest setting (28mm with a diffuser). This is not a particularly strong strobe, but conditions were such that it turned out to be the absolutely perfect light for the job. I shot w/ a 17mm lens, so the lens can see a wider field of view than the light does. What this means is that the “feathered edge” of the light beam is illuminating the areas at the edges of the image. The further down the tunnel the light travels, the beam gets more concentrated. If the beam of light was wider than the field of view of the lens, the rocks along the edges of the photo would have been horribly over exposed.
At f/5.6, I simply could not get enough light out of the strobe to illuminate the tunnel, so I opened the lens to f/2.8 and doubled my shutter speed. This allows the same amount of ambient light into the camera, but since the aperture is twice as wide, twice as much strobe light hits the sensor.
Why does this work?
The strobe only lasts for 1/1,000th of a second – so altering the shutter speed has no effect on the amount of strobe light hitting the sensor. Whether shooting at 1/30th of a second or 1/250th of a second, it makes no difference to the strobe – aperture & ISO are the only factors that effect the amount of strobe getting into the camera body. So I locked down my f-stop and ISO first, then adjusted the shutter speed to control the amount of ambient light in the scene. For more in depth info, check out Strobist.
Once I got the rocks looking fairly decent, I dialed the shutter speed up 2 clicks (2/3 of a stop under what the camera’s TTL metering told me was “correct”) to darken the outside scene and the highlights on the road surface.
The whole process took less than 3 minutes (including time spent dodging traffic and squinting at the TFT screen on the back of the camera).
Give it a try for yourself!