Wednesday, July 8, 2015

Nailing the Off-Camera Flash exposure

When lighting with flash, you can independently control the main subject and the ambient exposure. The first is achieved by adjusting the camera diaphragm, and the second with the exposure time. Fine.

The camera sync speed sets the limit where you can get the most power out of your flash, 1/250 in my case. You can resort to use a ND filter to keep your shutter below this level. You can also use a ND filter to keep a wide aperture even in very bright conditions.

With my ISO at 100, I knew a few configurations that work well, but now my base ISO is 64, and not always I’m in a safe light zone, so I’ve put together a simple spreadsheet to calculate this things for me, both camera and flash in manual mode.
First, we need to enter two basic parameters, your camera ISO and sync speed. Mine are 64 and 1/250.
Now, you may be in a Sunny 16 situation, or even f/22 at the beach. You can perfect the math if you make a reading with the light meter of the required aperture, setting the time equal to your ISO. Say we’re at Sunny 16.
Choose your aperture. I want to shoot at  f/1.4. You want to maintain the camera shutter speed below the sync speed, so a ND filter is required. I will use a ND6, and with this combination, my shutter speed comes down to 1/128. 
With these settings, I could be shooting without flash, and the camera exposure is compensating for the ND. 
Now we want to add some flash. Since the flash is only affected by the aperture, and I'm shooting below the sync speed, all I need is to throw flash light as to read f/1.4 plus 6 stops of the ND filter, i.e., f/11.
I also want to underexpose the ambient light by one full stop, but I want the flash to nail the exposure for the main subject. Then I only need to modify the shutter speed, if I'm within the comfort zone of the sync speed.
The spreadsheet also includes a table that is updated when you modify your settings.  It will indicate for the ambient light level what shutter speed do you need for a few ND combinations. In green, those safe values below the sync speed, and the yellow entries are those combinations that give your calculated shutter speed.

For my ISO 64, there are a few useful results and easy to remember combinations:
  • To shoot at f/1.4, I need the ND 6, like it or not,
  • f/4 goes well with a ND 3,
  • f/5.6 is a good companion for a CP filter.
If you feel like trying this, the table is available as follows:

Happy OCF summer!