The arch of the tunnel provided a frame within the photo frame, and the pool of light hitting the street by the mouth of the tunnel could be used to highlight a subject that would hopefully wander into the scene, walking past the mouth of the passage. I had a few technical points to consider. They mainly came down to three:
- How to make sure the subject was in focus.
- How to properly expose for the subject.
- How to freeze the motion of the subject.
To make sure my subject was in focus I had to set my aperture so that anyone that walked down the street across frame would be in focus. I set the camera on autofocus and aimed at a point on the ground about half way between the two sides of the street, then I switched to manual focus to keep that focal distance fixed. I then set the aperture to f4.0, estimating that that would give me enough focal range to get anyone walking down the street in focus.
Now that I had the aperture locked to f4.0, I had to figure out what shutter speed to set to get the proper exposure. To do that I metered for the wall at the other side of the street (the one with the little sign framed in red), which exposed properly at a speed of 1/250s. I then brought the speed down to 1/200s. My reasoning was that the tone of the wall seemed a bit darker than the average skin tone, but not too far off. So by metering for the wall and then compensating with a slight shutter speed decrease, which would let in less light, I would probably be in the right zone. I also knew that exposing for the lighter area of the scene would make the inside of the tunnel go very dark and I would most likely lose detail there. But I didn't care about that as the tunnel was going to be used as a framing device, and the lack of detail there would actually help bring the eye to the lit street, where my subject was going to be.
Since I intended to take a picture of someone walking across frame I needed to check that my shutter speed would be high enough to freeze a walking person. I estimated that my current shutter speed of 1/200s should be enough to do that.
As usual, I had started out with the lowest ISO setting I could get with my camera, which is ISO 200. Now, if after estimating the shutter speed for a correct exposure I had ended up with a speed too slow to freeze the subject in motion (say, 1/80s), I would not have decreased the aperture to compensate for that. Doing so would have decreased the depth of field, and would have run the risk of getting the subject out of focus. So what I would have done instead in a case like that would be to set my shutter speed to 1/200s and then increase the ISO to what would be needed to get the subject properly exposed.
Sometimes taking the ISO past a certain limit (depending on the camera) may introduce noise in the image. But I'll take a noisy image over a blurry one any day.
I should also clarify that even after estimating each of these parameters in my head, I still took some test shots of people walking across frame. I looked at them in the camera display, checking exposure, focus and sharpness until I was satisfied. In other words, if you have the chance to do some tests before taking the important shots, do it, now matter how experienced you are.
So I was set and ready. All that was needed now was for a good subject to be kind enough to step into frame.
I was there for about 15 minutes and took multiple shots, some of which worked better than others. In the end this is the one I liked the most: