How can fill flash work for a person with a bright background by day and a dark background at night?

Question

I want to photograph a person in front of a bridge in both daytime and nighttime, and I’d like both the person and the bridge to be clearly visible. My first thought is to use a smaller aperture so the bridge stays sharp too.

In the daytime, if I expose for the person, the bridge becomes too bright, so it seems like I should expose for the bridge and use flash on the person.

At night, if I expose for the person, the bridge becomes too dark, so again it seems like I should expose for the bridge and use flash on the person.

What confuses me is that in the daytime the subject is effectively backlit, while at night the subject may be more front-lit relative to the scene. How can fill flash help in both situations, and what changes between day and night when balancing flash with the ambient exposure?

user5270 · Accepted Answer

This may not produce the results you hope for. Here's why. In a typical lighting situation where you can control the output of the lights, in daylight you would meter for the bridge using a spot meter and set the lights for the same setting on the person using an incident meter. That way, both are properly exposed.

At night, you can do exactly the same thing, but you will have a problem keeping the person standing in one place long enough to make the exposure. Let's take an example.

In the daylight, using "sunny 16", let's assume your ambient light (bridge) exposure setting is about f:16 at 1/125 (ISO 100). I'm just throwing numbers around for the sake of the discussion. So you then fire your flash a few times, adjusting the settings until you have a well balanced exposure (or meter it). Now you are ready to go at an acceptable shutter speed and aperture.

At night, you might wind up with a setting of f:16 at 20s (ISO 100). That way, you have sufficient depth of field, but can correctly expose the bridge. Mind you, I haven't seen this bridge. The crux of your problem is setting the flash so it outputs exactly the correct amount of light for an f:16 exposure (which, coincidentally, is the same setting as you used during the daylight shot). If your flash fires at the start of the shot (1st curtain sync), the intensity and short duration of the flash will freeze the person but leave the shutter open long enough for the remaining correct exposure of the bridge. The trick is having the person stand pretty darn still for the remainder of the exposure so as not to show "ghosts".

In both cases, be aware that the primary determining factors in flash exposure are aperture and flash output. Shutter speed is far less of a factor and serves only to modulate the influence of ambient light sources. The higher the shutter speed, the less the ambient light will affect the exposure. Obviously, at night you have no choice. You have to expose for the night image of the bridge so you can expect a long shutter speed. During the day, you can change things around so if you prefer

It's this ghosting I'm referring to when I say it might not work out well. I would classify the first case -- daylight -- as fill. The night image, not so much. At night the flash would be the primary light source. Just picking a nit here.

How can fill flash work for a person with a bright background by day and a dark background at night?

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