Metering ambient / subject in a hurry

I've been reading Strobist and various others and can't find an answer to this.

If I am balancing ambient background at sunset with a bride in the foreground

how do I know what power to have the flash at? Especially if I am in a hurry.

 

I read that if I expose for the background the flash will light the foreground.

 

So this is what I am thinking will work quickly and accurately. Am I right?

 

- decide aperture for the depth of field I want (say f5.6) and set camera to

manual mode f5.6, changing shutter speed using the "needle" in the viewfinder

to tell me where I am until I am correctly set for exposing the background.

 

- change to AV mode and spotmetering. Change Av mode aperture to f5.6 and

spotmeter the brides face. Dial f5.6 and the shutter speed reading I get into

the flash

 

- change camera back to manual mode and take the shot

 

Then camera and flash are set to depth of field I want, the camera is set to

expose the background and the flash is set to expose the bride? Right?

I'm not a flash guru, so I'll read other answers with interest, but my impression is that (for a manual flash) you would set the aperture for a proper flash exposure and then set the shutter speed for the proper ambient exposure at that aperture.

 

If you have a smart flash and camera combination, it may be as simple as setting the camera to a 'long' flash exposure, where it first exposes for the flash and then keeps the shutter open to properly expose the background.

Thanks Robert, but in the example I've given how would I measure the aperture for a correct flash exposure? That's my problem - how do I calculate what the flash exposure would be?

Bob,

 

After setting your camera for the ambient exposure you then set your flash power output to what level you think may work. Say 1/4 power. Take a shot. Look at the image and determine if this is what works for you. If not then adjust the flash output +/- depending on what you want. You can also use a meter to determine the light difference between the ambient and the flash power. Usually the output from the flash should be about 2 stops above ambient. It all depends on the look you're trying to achieve.

 

-Sherman

You could use a flash meter. It only takes a second.

In a hurry? Try it with the bride's sister in the same location the day before and see what works. The "scientific" approach would involve measuring both ambient and flash, then calculating a range of different proportions so you could predict what ratio of ambient to flash gets the color you want, then measure the same factors at the shoot and set your exposure to get the ideal balance. One takes a quick field trip and some chimping the day before, refined by evaluating onscreen the night before the wedding. The other could take weeks!

 

Or years. If you do your tests at ten minutes to sundown on a day in April, will that information be of use at ten minutes to sundown the next January?

 

As you stop down and/or cut the power of the strobe (to reduce the 5000K influence) and extend the exposure time (to increase the golden glow) you obviously increase the effect of the warming light. But as you approach sundown, the color of the ambient light is changing pretty fast. The only thing you can really do here is do some experimenting around the same time of day at the same season to get a general feel for things, then bracket like a crazed weasel on D-Day.

 

Van

It really depends on the system you're using. For instance, when I use a D200 and an SB800, it's as easy as pointing the camera and taking the shot. It just seems to work every time (although I prefer it underexposed 1/4-1/2 stop most of the time).

 

With an ordinary, non-dedicated auto-flash with various settings (a Vivitar 283/285, for instance) and a autoexposure camera (and assuming an unlit subject is going to be underexposed when the background looks good), the fastest method I know is to shoot in AV mode with an averaging metering pattern and set the flash for the same aperture you're using on the camera. It's not as precise as metering the subject and background separately, but it's quick and doesn't take as much brainpower. It works well with negative film, since a neg will compensate on the high end a bit, but for slides or, more likely, digital, you'd probably need to back down a 1/4 stop or so to keep from blowing the highlights.

I see you have spot metering capability on your camera so here is the easy way of doing this. Set the camera meter to spot. Meter a part of the sunset sky that is strongly colored or about the same VALUE as middle gray. Note the settings in view of the aperture you want (for DOF). If that is f5.6, then note what the shutter speed would be at f5.6. These become the settings you put on the camera. After doing a test (I'll get to the flash part next), you can manipulate the shutter speed to make the sky more intense in color (make the shutter speed faster) or less intense (make the shutter speed slower). Note that you need to stay at or below whatever highest sync speed your camera can produce or use high speed sync, if your subject distance allows. Just as the sun sets or right after, you still may need to use ISO 100 since the sky is still bright.

 

For the flash, if you trust your automatic flash metering, whatever it may be, dial in a compensation or not (probably plus comp for backlighting) and fire a test. Adjust accordingly. Or, put the flash in manual flash mode. Use the flash's calculator to find the partial power setting you need. Estimate the distance between the flash and the subject. Find that distance on the flash's calculator or dial down or up to find it. That's it.

Thank you Nadine. So I can have the camera in manual, set for the sunset exposure, and the flash in auto (I do trust it) and as long as I ride the compensation on the flash I can get something I want. That's the simplest explanation I've ever read!

Bob--I do this all the time at my wedding shoots. Keep in mind my statement about sync speed limits and high speed sync. You don't want to be wondering why plus compensating your flash doesn't seem to brighten the flash exposure. Here is another trick--you can manually narrow the flash beam if you are photographing just the couple, to have 'more' flash reach.

http://strobist.blogspot.com/2006/06/free-custom-backdrops-delivered-daily.html

It's under on assignment.