How can I use wide aperture with fill flash?

Question

I was working with an off-camera flash this weekend for some portraits of my son. I was shooting in medium sunlight (early morning, partly cloudy, w/ some shade), and I like the lighting control I got with the flash (it softened the shadows on his face), but it sort of messed up my aperture control.

Using a 30D w/ 50mm F/1.8, I can get a narrow DOF at low apertures, but since the use of the flash constrained my shutter speed, I couldn't use apertures low enough to produce as much bokeh as I'd have liked.

I'm thinking that one solution would be a neutral-density filter to let me use a lower aperture. Would this work, and if so, is this the preferred way to handle this situation?

Answer 1

In your shooting conditions the constraint is that a large aperture requires a very short shutter speed to expose the ambient light appropriately and you can't sync the flash faster than about 1/200 sec on the 30D. A strong ND filter might solve this problem. Alternatively, if your flash supports HSS, you can use that to reduce the exposure time (as little as 1/4000 second, I believe). If neither is available, an expedient solution is to move the location and the background into dark shade: you can still get some nice ambient light but it can be reduced several stops.

Answer 2

As stated already your main options are high-speed sync (if your flash supports it) or an ND filter. There is another extreme option which is available which is not very practical but I include for completeness, and that is to shoot above your max sync speed.

You may need to use the flash in manual mode + mask the extra pins in order to trick the camera into letting you shoot past the max sync speed.

But if your subject doesn't occupy the whole frame you can shoot above the max speed so the flash only illuminates part of the scene. By orienting the camera you can chose which half is illuminated so this might be just enough. For more details read:

http://strobist.blogspot.com/2007/06/hacking-your-cameras-sync-speed-pt-2.html

Also, using Pocketwizard transmitters that have the Hypersync function, the flash firing can be tuned to precisely time the peak output with the shutter opening in order to up the sync speed. By combining two these techniques you could quadruple your shutter speed allowing you to open the aperture at least two stops. See:

http://www.pocketwizard.com/inspirations/technology/hypersync_fpsync/

Answer 3

I would look at reducing the flash power before I would start putting more glass in front of the sensor. This does, of course, depend on your flash, but in many cases you can get as low as 1/64th of the full power on the flash when configured manually.

Beyond that, there are diffusion options available that can reduce the power by a couple of stops and you can find ones made by companies like Stofen or Gary Fong that attach to the flash or other forms of diffusers that are held/mounted off the flash (such as softbox or a shoot-through umbrella).

Finally, using bounce lighting in some form can allow for even more light reduction. Indoors it could be a wall or ceiling, outside a smaller reflector can be used whether hand-held or attached to something. Of course, it could be done with umbrellas as well.

In general, an ND filter becomes more appropriate when there are no other means to reduce the light.

Answer 4

It's crazy expensive, but there is an alternative, and that is high-speed synch. The flash and camera need to support this, and you can do it with both Canon and Nikon systems. Generally the high-end flashes like the SB-800 and 580-EX support this. It's expensive because you need two flashes, a commander flash on the camera which communicates to the off-camera flash, so you need two high-end hotshoe flashes. The commander's role in this setup is to send a pre-flash sequence to the off-camera flash telling it what to do, it doesn't contribute light to the scene. You put the camera in high speed synch mode and the off-camera flash will actually emit several flashes to illuminate the scene as the slit formed by the front and rear shutter curtains move across the sensor.

The two dudes behind TriCoastPhotography Workshops teach this technique for TTL off-camera flash work.