One of the most challenging things for many beginners in photography involves shooting in a very dark location with a flash.

Cameras will pick the lowest ISO when you use a flash as their default. So what happens is then the background is black, and the subject is well exposed at best. However, in the case of this wedding photo, the couple is walking out into the dark, being backlit.

Nikon D5, Sigma 24-105mm f/4 DG OS HSM Art Lens, ISO 10000, ƒ/5.6, 1/8 – Nikon SB-900

The trick is to set the camera as if you were shooting without a flash, using a high ISO. Here I set the camera to ISO 10000. Then the flash was set to fire on TTL slow shutter sync. Slow shutter sync means the flash fires immediately at extremely short duration, and then the shutter stays open longer.

Nikon SB-900 Flash Duration

1/880 sec. at M 1/1 (full) output
1/1100 sec. at M 1/2 output
1/2550 sec. at M 1/4 output
1/5000 sec. at M 1/8 output
1/10000 sec. at M 1/16 output
1/20000 sec. at M 1/32 output
1/35700 sec. at M 1/64 output
1/38500 sec. at M 1/128 output 

With my choice of ISO 10000, the flash on TTL was probably firing at 1/16 power or less. Shooting at 1/8 shutter speed created the blur of the couple; the flash froze them at about 1/10000.

Nikon D5, Sigma 24-105mm f/4 DG OS HSM Art Lens, ISO 10000, ƒ/5.6, 1/5 – Nikon SB-900

The slower shutter speeds help open up the background at low ISO. However, the slower shutter speed also introduces motion. So we have moved from 1/10 to 1/8 and finally shot at 1/5 to get the background to show up.

If you want to be sure you show where your subject is for reference, then use slow shutter speed sync with the ISO set to where you could make photos without the flash.