Understanding Nikon's Auto FP High-Speed Flash Sync Mode

Flash sync speed is the shutter speed at which your popup or external flash unit can be used to take a picture without the camera's shutter curtains getting in the way of light from the flash. Most cameras are limited to a maximum flash sync speed of 1/250 of a second (1/250s) shutter speed. Anything faster will cause a dark banding effect on the picture because the secondary shutter curtain is partially covering the imaging sensor when the flash fires.

The Custom Setting Menu > Bracketing/flash > Flash sync speed setting lets you select a flash synchronization speed from 1/60s to 1/250s. Or, if you prefer, you can use the two Auto FP high-speed sync modes on many Nikon DSLR cameras—1/250 s (Auto FP) or 1/320 s (Auto FP).

Auto FP high-speed sync mode allows you to use shutter speeds faster than 1/250s. Many do not know whether they should use the Auto FP high-speed sync mode at all times or only when needed. Some do not fully understand what this special camera setting is supposed to accomplish. This article may help.

The Auto FP high-speed sync modes are available only with certain external Speedlights, not with the built-in pop-up Speedlight. Currently, the five Nikon Speedlights that can be used with your Nikon in Auto FP high-speed sync modes are as follows:

• SB-900
• SB-800
• SB-700
• SB-600
• SB-R200

Auto FP high-speed sync enables the use of fill flash even in bright daylight with wide aperture settings. It allows you to set your camera to the highest shutter speed, up to 1/4000s or 1/8000s with some Nikons, and still use the external flash unit to fill in shadows. There are two upcoming subsections in this article, Auto FP High-Speed Sync Review and Special Shutter Speed Setting X + Flash Sync Speed, that provide a detailed discussion on how the Auto FP high-speed sync system works.

Here are the menu screens used to select a Flash Sync Speed:

Figure 1 – Flash sync speed choices

The following are your Flash sync speed choices in most Nikon DSLRs (figure 1):

• 1/320 s (Auto FP)
• 1/250 s (Auto FP)
• 1/250 s
• 1/200 s
• 1/160 s
• 1/125 s
• 1/100 s
• 1/80 s
• 1/60 s

When you’re using Auto FP high-speed sync mode, the output of your flash is reduced, but it doesn’t cut off the frame for exposures using a shutter speed higher than the normal flash sync speed (X-sync). Why? Let’s review.


Auto FP High-Speed Sync Review

In a normal flash situation, with shutter speeds of 1/250s and slower, the entire shutter is fully open and the flash can fire a single burst of light to expose the subject. It works like this: There are two shutter curtains in your camera. The first shutter curtain opens to expose the sensor to your subject, the flash fires to provide light for the correct exposure, then the second shutter curtain closes. For a very brief period of time, the entire sensor is uncovered. The flash fires during the time when the sensor is fully uncovered.

However, when your camera’s shutter speed is faster than 1/250s, the shutter curtains are never fully open for the flash to expose the entire subject in one burst of light. This is because at fast shutter speeds the first shutter curtain starts opening and the second shutter curtain quickly follows. In effect, a slit of light scans across the surface of your sensor, exposing the subject. If the flash fired normally, the width of that slit between the shutter curtains would get a flash of light, but the rest of the sensor would be blocked by the curtains. A band of the image would be correctly exposed, and everything else would be underexposed.

What happens to your external Nikon Speedlight to allow it to follow that slit of light moving across the sensor? It changes into a pulsing strobe unit instead of a normal flash unit. Have you ever danced under a strobe light? A strobe works by firing a series of light pulses. Similarly, when your camera’s shutter speed is so high that the Speedlight cannot fire a single burst of light for a correct exposure, it can use its Auto FP high-speed sync mode and fire a series of light bursts as the slit between the shutter curtains travels in front of the image sensor. The Speedlight can fire thousands of bursts per second. To a photographer or subject it looks like one flash of light, even though it is hundreds or thousands of bursts of light, one right after the other.

When the camera is in Auto FP high-speed sync mode, you’ll see something like this on your Speedlight’s LCD monitor: TTL FP or TTL BL FP. The FP designation tells you that the camera and Speedlight are ready for you to use any shutter speed you’d like and still get a good exposure, even with wide-open apertures!

You can safely leave your camera set to 1/320 s (Auto FP) or 1/250 s (Auto FP) all the time since the Auto FP high-speed sync mode does not kick in until you raise the shutter speed above the maximum setting of 1/250 s. With slower shutter speeds, the flash works in normal mode and does not waste any power by pulsing the output.

This pulsing of light reduces the maximum output of your flash significantly but allows you to use any shutter speed while still firing your external Speedlight. The higher the shutter speed, the lower the flash output. In effect, your camera is depending on you to have enough ambient light to offset the loss in power. I’ve found that even my powerful SB-900 Speedlight can provide only enough power to light a subject to about 8 feet (2.4 m) when I use a 1/8000 second shutter speed. With shutter speeds that fast, there needs to be enough ambient light to help the flash light the subject, unless you are very close to the subject.

However, now you can use wide apertures to isolate your subject in direct sunlight—which requires fast shutter speeds. The flash will adjust and provide great fill light if you use Auto FP high-speed sync mode.

Note: If your flash fires at full power in normal modes, the flash indicator will blink in the Viewfinder to let you know that all available flash power has been expended, and you need to check to see if the image is underexposed. When the camera is firing in Auto FP high-speed sync mode, that doesn’t happen. You won’t get a warning in the Viewfinder if the image does not have enough light. Check the histogram often when using Auto FP high-speed sync mode.


Special Shutter Speed Setting X + Flash Sync Speed

When using exposure modes Manual (M) or Shutter priority auto (S), you can turn the shutter speed all the way down to 30 seconds, then to bulb. There is one more setting below bulb, named X + Flash sync speed. This special setting allows you to set the camera to a known shutter speed and shoot away. You will see X 250 if Custom Setting e1 Flash sync speed is set to 1/250s. Whatever Flash sync speed you select will show up after the X. If you selected a Flash sync speed of 1/125s, then X 125 will show up as the next setting below bulb. Selecting a Flash sync speed of 1/60s means that X 60 will show up below bulb, etc.

The shutter speed will not vary from your chosen setting. The camera will adjust the aperture and flash when in Shutter priority auto (S) mode, or you can adjust the aperture while the flash controls exposure in Manual (M) mode.

This special X-Sync mode is not available in Aperture priority auto (A) or Programmed auto (P) modes since the camera controls the shutter speed in those two settings. You’ll use this setting primarily when you are shooting in Manual (M) or Shutter priority auto (S) mode and want to use a known X-Sync speed.

My Recommendation: I leave my camera set to 1/320 s (Auto FP) (as shown in figure 1, image 3) all the time. The camera works just like it normally would until one of my settings increases the shutter speed to faster than 1/250s, at which time it starts pulsing the light to match the travel of the shutter curtains. Once again, you won’t be able to detect this high-frequency strobe effect since it happens so fast it seems like a single burst of light.

Remember that the flash loses significant power (or reach) at faster shutter speeds since it is forced to work so hard. Be sure you experiment with this setting to get the best results. You can use a big aperture, like f/1.8, to create a very shallow depth of field in direct bright sunlight since you can use very fast shutter speeds. This will allow you to make images that many other cameras simply cannot create. Learn to balance the flash and ambient light in Auto FP high-speed sync mode. All this technical talk will make sense when you see the results. Pretty cool stuff!

Keep on capturing time...
Darrell Young
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