What is maximum flash sync speed? What does it really mean? When and why it is important for flash photography? To answer all these questions, let’s start by looking at the basic camera component related to flash photography.
Most SLR cameras have a focal plane shutter. It consists of two shutter curtains (typically called 1st/2nd or front/rear curtain). When the shutter release button is pressed, one curtain will travel across the sensor and the second curtain follows the first one after a period of time. Even though the entire sensor area is not exposed at once due to the limited curtain travel speed, but on average the entire sensor area is exposed for the same amount of time. This time is the shutter speed.
Depending on the exposure time, there may or may not be a moment when the entire sensor area is fully exposed to the light. At short exposure time, a slit formed by the two shutter curtains will travel across the sensor, exposing the sensor by scanning through it. By reducing the slit width, the shutter can effectively produce a shorter exposure time otherwise impossible due to the limit of the mechanical shutter movement. The image below illustrates the shutter operations at long (top row) and short (bottom row) exposures.
Maximum flash sync speed
The maximum flash sync speed is the fastest shutter speed at which the entire image sensor can be illuminated by flash pulse. Most modern digital SLR cameras have maximum flash sync speed of ~1/250 of a second. A few camera models (e.g. Nikon D70(s), D50) have maximum flash sync speed as high as 1/500s.
Since the shutter travels at finite speed, the time during which the entire sensor is exposed at maximum sync speed is:
maximum sync speed - 1st curtain travel time
This duration is the upper limit of the flash pulse duration. Flash duration various greatly depending on the power output. For typical shoe mount flashes, the longest flash duration is about 1/1000s. If the flash duration exceeds the limit, a dark band will be observed at the edge of the photo. It is caused by the closing of the 2nd curtain before the flash has extinguished.
From the brief description of the focal plane shutter operation above, it is easy to understand that a single flash pulse cannot completely light the whole frame when the shutter speed is faster than the travel speed of the 1st curtain.
Practical implications of the maximum flash sync speed
So how and when will the maximum flash sync speed affect your flash photography? Let’s look at the following two typical shooting situations involving flash.
The picture shown on the left is where the flash is used as the main light source for exposure and the ambient light level is negligible. In this case, shutter speed does not affect the exposure of the main subject, only the ambient. The reason is that the flash pulse is very short. The flash output can be delivered in ~1/1000 sec to 1/10000 sec, a much shorter time than the exposure time. That’s why in dim light situations, you could be shooting at 1/60sec with flash and were able to freeze the motion.
The picture shown on the right is where the flash is used as a complementary light source to fill the shadows (fill light) in outdoor shooting. In a sunny day, the shutter speed is ~1/100s at f/16 @ ISO100 based on the “sunny f/16″ rule. For many shooting situations, it is desirable to shoot at wider apertures to have a nice background isolation. So 1/200s at f/11, 1/400s at f/8, 1/800s at f/5.6, 1/1600s at f/4, … oh wait, this may already far exceed the maximum flash sync speed of your camera!
When shooting outdoors with fill flash, definitely pay attention to the shutter speed on the viewfinder info screen. If you shoot in aperture priority (or Av) mode, the shutter speed is typically capped at the maximum flash sync speed by the camera. There should be some indication that you will over-expose. Even if the camera does not over-expose, the shutter speed of ~1/250s may not be sufficient to freeze motion in day light. What can be done?
If shutter speed is not a concern for freezing motion, you may be able to use lowest ISO setting and stop down the lens to bring down the shutter speed in order to avoid over-exposure. If this is not enough or not desirable (for background isolation with wide aperture), you can use neutral density filters to cut down the light.
If you need to maintain high shutter speed above the maximum flash sync speed of your camera, the typical option is to use high speed flash sync.
High speed flash sync
Some camera/flash combinations (typically high end ones) support high speed flash sync. Different manufacturers have different names for it: focal plane flash, FP flash, FP mode, high speed sync, etc. The basic working principle is the same. Instead of using single flash pulse, the flash unit sends out very fast repeated pulses as the shutter curtain slit travels across the sensor. On average, different sensor areas will receive the same amount of flash exposure. In theory, this can be done with a constant flash light source during the whole exposure time. This is however not practical because the flash units do not emit constant light intensity during a single discharge cycle, causing uneven exposure across the frame.
High speed flash sync is not available on all cameras/flash combinations. You cannot get this capability if any of your flash or camera does not support it. Another issue with that is the greatly reduced effective flash power (guide number, or GN) because of the pulsing outputs of the light and blocking by the shutter curtains.
Another option that may work for you
Some cameras have combined mechanical shutter and electronic shutter. These cameras can sync at much higher shutter speed when they are fooled into thinking there is no compatible flash connected. This can be achieved by using external PC cord, or attaching an old incompatible non-TTL flash unit, or covering certain contact pins on the shoe-mount flash. Example cameras are Nikon D1(X,H), D70(s), etc. There are obvious benefits of this approach: no need to purchase more expensive camera/flash, higher effective GN compared to FP flash. On the other hand, there might be something not so good about them that prevent the manufacturers marketing these undocumented features.
Keywords: Flash photography, flash sync speed, FP mode, high speed flash sync