Shutter & Aperture for portraits.

[jnm]

So I've been playing around with aperture and shutter speeds, most recently this:

15 second exposure, 18mm, f/6.3, flash camera left about 45* set to 1/32 +0.3 power on the rear curtain.

so that's an extreme example of using shutter to expose the background and aperture and flash to control light on the subject.

here's what i want to do with 'normal' portraits, or at least not ones that are 15 seconds. when part of the goal of the picture is to separate the subject from the background would this work to help aide that? i want to get the shutter speed setting right to expose the background so the histogram is in the middle. then i want to take that same shutter speed and add in the flash and adjust the power and aperture so the flash portion (on the subject) is exposing to the right of the histogram.

is that a normal practice? does it end up looking nice, weird, or is it as i suspect a case by case basis?

 

[Big Mike]

Everything is pretty much a case by case basis...but yes, that's pretty much the procedure for outdoor portraits.

Shutter speed has not affect on flash exposure (as long as you keep it under the max sync speed). So you can use the shutter speed to change the ambient exposure without affecting the flash exposure.
Flash power doesn't affect the ambient exposure or background (especially if the background is a lot farther away than the subject), so you can adjust that without really affecting the background exposure.
Aperture and ISO still affect the exposure of both the ambient and flash exposures.

Each photo taken with flash, is actually two exposures, and with these controls, you can adjust either one individually to get the results that you want.

The are some challenges though, for example...
If you are shooting outdoors in bright light you might want to use flash for fill. Since there is a lot of ambient light, your ambient exposure settings might be something like ISO 100, 1/250 and F16. Now since the aperture affects the flash exposure, you would need a lot of flash power to shoot at F16 (or else your working distance will be very limited). Without flash, you could just set a shutter speed of 1/1000 and use an aperture of F8 (or 1/2000 and an aperture of F5.6).
However, the max sync speed of most SLR cameras is something around 1/200.
This problem also applies if you want to shoot with a wide aperture to get a shallow DOF. You want to shoot at F4 or F2.8, but that requires a shutter speed of 1/2000 or 1/4000 because of the ambient light.

One solution, if your flash is capable, is High Speed Sync, HSS (on Canon units anyway). This mode allows you to shoot with flash at any shutter speed, which is great. However, this mode also limits your working distance and eats up your batteries faster.

I posted this tutorial recently...
http://www.thephotoforum.com/forum/beyond-basics/184937-lighting-tutorial.html