How do you underexpose background while the subject is flashed and exposed correctly?

[zack_musarsa]

I want to use my radio slave and make the background underexposed by

one stop while using a flash to expose my subject correctly.

 

If i use my lightmeter and the ambient is 1/250 f5.6 Then what should

i set my aperture to and what aperture should my light meter give me

with flash?

 

Thanks alot.

[tim_norman1]

using your example...

 

Under exposing the ambient light in the room by 1 stop would mean you up the f stop on your camera to f8 and make the flash put out enough light to correctly expose the subject at f8.

 

OR

 

If you can flash sync at higher than 1/250. Then Up the shutter speed to 1/500 leaving the fstop at 5.6 and then make the flash putout enough light to correctly expose the subject at f5.6.

 

If your camera can do 1/3 f-stops and shutter speeds then you can have even more range of achieving the same thing. (drop the shutter speed by 2/3, raise the fstop by 1/3 match fstop with flash....etc.etc.etc.)

 

I use this techniuqe quite often on portraits and the such. Usually doing 1, 2 or 3 stops under exposure for ambient lighting. It's really a neat look for outdoor portraits at times especially at sunset or sunrise. It allows the colors to be more saturated.

 

Hope that helps.

[zack_musarsa]

Ok cool, i understand. Thanks for your help.

[steve_levine]

This is an amusing question.Usually the novice has no problem attaining what you are seeking(dark backgrounds), but cant figure out how to make the background bright!

[jbq]

Zack: assuming that both the subject and background get 1/250 f/5.6 of ambient light (that's EV 13, i.e. very bright overcast assuming ISO 100):

 

You'll want to expose at 1/250 f/8 so that the background gets underexposed by a stop. Without flash our subject would also get underexposed by a stop, i.e. receiving half the light. Set the flash for the power that would match f/5.6.

 

Let's put it another way:

 

If you shot at 1/250s f/5.6 without flash everything would be properly exposed by ambient light.

 

If you shot at 1/250s f/5.6 in totaly blackness there'd be a specific flash setting that would properly expose the subject.

 

If you shot with ambient light at 1/250s f/5.6 with the same flash setting the background would be properly exposed and the subject would be overexposed by 1 stop.

 

If you stop down the lens to f/8, all else being equal, the background will be underexposed by 1 stop and the subject will be properly exposed.

[matt_lew]

Jean is on the right track. But The flash would actually have to be a f8.0 exposure not a f5.6. You could also do like what some of the others are saying and expose at 1/500 at f5.6 and the flash at f5.6 but you would most likely have to be shooting with a leaf shutter because of the sync rate. You might also have to at 1/250th depending on your camera. In that case youd have to do something like 1/125 at f11 with the flash at f11.

 

So basically all you have to do is take the ambient reading making sure that the shutter is no faster than the sync rate and close down the shutter or appeture and expose the flash for the same appeture. Hope this makes sense and helps you out.

 

Matt