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I am having trouble computing this in my mind. My goal is to photograph a person in front of a bridge, during day time and during night time.

I want the bridge to be in focus as well. So, I would use a smaller aperture.

During day time, if I meter for the person, the bridge is overexposed. So, I meter for the bridge and add fill flash for the person.

During night time, if I meter for the person, the bridge is underexposed. So, I meter for the bridge and add fill flash for the person.

It does not seem to make sense, since the person is backlit during the day and "frontlit" at night. So, how is fill flash working its magic in both cases?

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This sounds more like "I did this and don't understand why it worked out that way" rather than a hypothetical scenario. If that is the case, would you be able to add (upload or link) examples? They don't need to be hi-rez images, but seeing the scene might help us provide an explanation. –  user2719 Aug 14 '12 at 1:31
    
Although I think I understand the 'problem' I can't manage to explain this very well without a whiteboard or diagrams. But one part of it is that you're too close to the person in the foreground so using a longer lens and moving back will help, and another part is that you're probably shooting from the 'wrong' angle or at the 'wrong' time of day. A photograph that needs to emphasise both the foreground and background is fiddly to setup. –  Clara Onager Aug 14 '12 at 8:04
    
@StanRogers Purely hypothetical. I don't have any pictures to share. –  publicRavi Aug 15 '12 at 14:58

1 Answer 1

up vote 2 down vote accepted

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.

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OK, I was confused and confused all of you. In the second case, I use the flash to freeze the person. It is not fill flash in the second case. –  publicRavi Aug 15 '12 at 15:02
1  
Note that the ghosting itself can be a really neat effect when it's present; don't let initial disappointment lead you to deleting a ghosted flash picture until you've had a chance to look at it again with fresh eyes. It may not be the picture you wanted to capture, but there's a chance that it may be better than what you had in mind. –  user2719 Aug 15 '12 at 15:44

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