A very interesting technique, that can create stunning “night by day” pictures.
So, why not shooting with high speed sync ?
Imagine that you are asked to do a glamour shoot on the beach, with a shallow Depth of Field (DoF) and focus on the models. The sun is high on the sky casting hard shadows and heavy contrasts. Not at all the most desired light circumstances. Never the less you have to complete the session.
The lighting crew has put up a large diffusion screen that softens the light and keeps the posing models out of the harsh sunlight. Yet the background (beach-ocean) is so bright that it forces you to use a set of flashlights to light the models. The art director reiterates that the pictures must have a very shallow Depth of Field and a dark background.So, you set your camera to the base ISO and the aperture to a small F-number e.g. F2.8 to meet the art directors expectations. The flash units are adjusted and ready to fire. The DSLR flash sync is set to “1/125 second”. The first shot goes off, disaster strikes. The background is way to bright and blown out, and yet the flash light metering was OK.
We have the control over tree parameters that affect the exposure; aperture, ISO and shutter speed.
The aperture cannot be changed without losing the shallow Depth of Field. The ISO is already at it’s base value. The only thing that remains is the shutter speed.
DSLR’s have in general a slow Flash Sync speed. The most common is 1/64 sec and 1/128 sec (sometimes with exceptions for speed lights). That means that the sensor will be exposed for 1/125 sec (shutter fully open). Within that time window the flash must go off . The flash duration is a very short pulse of light and varies depending on the brand. Most mid-range flash units will have a T.5 pulse duration of 1/900 sec or faster.
This means that the flash will correctly expose the model for 1/900 of a second (after adjustment of the flash intensity), however the sensor will be exposed to the ambient light for 1/125 seconds. That is almost seven times the flash duration. The ambient is so bright that it creates a very light background.
The solution is to reduce the flash sync time, in such a s way that the influence of the ambient light is minimized. Hence creating a darker background.
High-end camera’s such as the Phase One and Hasselblad have the ability to operate with high speed flash sync shutter speeds of up to 1/800 sec. If combined with a leaf shutter lens it can go as fast as 1/1600sec. Easy, isn’t it. Just adjust the flash sync time. No need to adjust the flash unit (except if the flash pulse is longer in duration then the flash sync speed of the shutter).
The DSLR Flash Sync is rather limited.
But what if the camera has no high speed flash sync and is limited to 1/125sec or even 1/250sec , like most DSLR’s ?
Note; Flash mono blocks or packs are not the same as speed lights like the Nikon SB900. ( picture below ;Auto FP relates only to use with speed lights)
Don’t despair; there is an easy way out.
The “Neutral Density filter”. The ND filter is a gray smoked glass placed in front of the lens, thereby stopping down the light. ND filters come in fixed or variable densities (Stops). A very useful ND filter is the variable type with a range from 0.5 stops to total darkness.
Fitting the ND filter on the lens will stop down the ambient light during the 1/125s exposure time, while the flash will still provide a correct exposure of the model. Stopping down the ND filter requires more power from the flash. So you will have to crank up the flash intensity.
Now you have the ability to shoot with a standard flash sync (1/125sec), low ISO and a wide-open lens even with very bright ambient light.
Thanks for reading,