Maximum Shutter Sync Speed for Flash

What is a focal plane shutter?

The SLR cameras, and almost all of the DSLR, have focal plane shutters, covering the film or sensor plane like a curtain. Focal plane shutters have been used in the better 35mm cameras since the first Leica in 1925, because they provide superior timing. This system is great for interchangeable lenses, because it only needs one good shutter in the body, instead of a shutter in every lens. And this is a better shutter, but FP shutters do have the one limitation of maximum shutter sync speed with flash (this page).

A focal plane shutter is two moving shutter curtains in front of the sensor (like roller window blinds). At slower shutter speeds, one curtain opens (to expose the sensor to light from lens), and after a timed duration, the second curtain closes to cover it. That makes the full frame open to the lens for the duration, which allows the instantaneous flash to work. Flash is fast, and it can occur anywhere within this duration, but normal mode is early front curtain sync, and we also have later rear curtain sync for the blur trails with slow shutter speed.

But to implement the faster shutter speeds, the second curtain starts closing before the first is fully open, tracking together to create a narrow timed slit between the two curtains, the open slit moving across the frame (down the frame today, the shorter travel is faster). This narrow slit of course reduces exposure under it.

There are a few good descriptions of focal plane shutters. A very interesting picture (a classic) using a focal plane shutter is this 1912 picture by Jacques Henri Lartigue. The shutter travel motion can cause distortion of the subject. The shutter slit was moving down the picture there (upwards in the camera, but we see it after inversion). The spectators are leaning left because the 4x5 camera was being panned right, following the race car. Their feet were exposed earlier than their heads, at slightly different times, due to the moving slit, and the panned camera. The car was going faster yet, so it leans the other way. This classic picture is responsible for us imagining speeding wheels as being slanted ovals, at least in cartoons. Shutters are much faster today, but the fastest motion can still be an issue (see the FP flash picture of the grinding disk on next page). The narrow open slit can provide a very fast exposure, but it takes much longer for this slit to travel across all of the frame.

The downside for flash is at these faster shutter speeds, the shutter opening (of light to the sensor) is only under this narrow slit. An instantaneous flash then only illuminates what is under that narrow slit at that instant, and the rest of the frame is not exposed to the flash. We can only use flash at shutter speeds slow enough that the entire frame is open at the same time, to let the fast flash expose the full frame area. This is generally a problem, and the camera normally will not let us set shutter speeds faster than maximum sync speed.

What is Maximum Flash Sync Speed?

For focal plane shutters, the fastest shutter speed at which the full frame is all fully open at any one instant (to allow instantaneous flash to fully cover the entire frame), is called the Maximum Flash Sync Shutter Speed. The Nikon spec charts in rear of camera manuals call it Flash Sync Speed, and I often say Shutter Sync Speed, but it is all the same thing (sync of the flash to the fully open shutter). Flash sync is not possible if the focal plane shutter speed is faster than this limit (because some of the frame would be covered by the shutter, and so unexposed, causing a dark band in picture). Some cameras use leaf shutters in each lens, or electronic shutters in some camera sensor chips, and these can sync faster than focal plane shutters. Until up into the late 1980s, focal plane shutter maximum sync speed was 1/60 or 1/80 second. Today, it is 1/200 and 1/250 second, which is pretty good. There is no Minimum speed - any slower shutter speed can always be used too, causing no issue regarding sync.

The duration of electronic flash is extremely brief, and the shutter must be fully open first, so that all areas of the frame are exposed simultaneously. At focal plane shutter speeds faster than maximum sync speed, the shutter is not fully open, and we see a black unexposed band at the top or bottom of our flash pictures (focal plane shutters are vertical travel today). If you rotate your camera to portrait orientation, you will of course see this dark band at one side.

Many modern camera shutters can sync flash at 1/200 second. The Nikon D200/D300, D7000, and D700/D800 cameras have faster shutters which can sync flash at 1/250 second, and (except for the D200), these even include strong hints about 1/320 second working. The 1/320 second is sort of a confusion factor, Nikon still says the specification is 1/250 second, but that they can do 1/320 second, warning that flash range can drop.

The D300/D800 use different shutters with different life expectancies, and note that their focal plane shutters move different directions. Images are always inverted, the lens projects upside down on the sensor, so the actual travel is the opposite direction, but as seen in the picture, the D300 shutter slit is moving down, and the D800 shutter slit is moving up. The illuminated area is the traveling open slit width, fully open over full frame only up to maximum sync speed. The first curtain is synced when it opens, but at speeds faster than maximum sync speed, the second curtain has already started closing, blocking the flash, causing the dark band. I don't know this curtain speed, but my guess is around three milliseconds to travel the frame height (the full frame camera has a 50% greater travel distance than DX). This was 1/32 power SB-800 speedlight flash (fast - it stops the shutter travel). The flash exposure is not affected by shutter speed, where we can see it. However, this is of course regular flash mode, and is NOT FP flash mode. And yes, this example is the difference in the DX and FX view with the same lens. With the same lens, DX is more telephoto, and FX is more wide angle.

Anytime you see a problem like this, just reduce your shutter speed, so it does not exceed maximum flash sync speed (in your camera's specs). Sync is a shutter situation. The electronic flash unit is not a factor, but sometimes radio triggers add a delay which worsens maximum sync problems (to diagnose, try flash without and without the radio trigger. Also it may just be time for new batteries in the radio trigger). FWIW, if this were a FX camera shooting DX crop, that might crop a narrow dark band off, so it might get away with a slightly faster shutter speed than if shooting full frame.

This is the sync problem, and Maximum Sync Speed is about the shutter being fully open over the full frame. The camera will not allow you to choose these fast shutter speeds if it knows about the flash (if flash is on the hot shoe for communication), so this example used a PC sync cord (flash held slightly above the hot shoe), to prevent communication, to fool the camera into allowing the shutter speed. But FWIW, a DX crop exposed on a full frame camera may crop this dark band off, so it might get away with a slightly faster shutter speed than if shooting full frame.

Even on these fast shutters, sync at 1/320 second is of course skating close to the dangerous edge. The Nikon specs clearly say Maximum Sync is still 1/250 second on these shutters, but provides a mode to allow 1/320, saying it might suffer reduced range. There have been occasions I have seen a little darkening at maximum flash power at the top on D300 at 1/320 second, which probably never shows outside in the sun (where you might want it). Indoor or studio flash is not a speed problem (at normal ISO and apertures, when the shutter is not contributing ambient, and because the flash is faster), so I just set maximum sync speed and forget it.

The only way to get around this sync problem with a faster shutter is to use a longer burning flash that stays fully illuminated for the full shutter travel time - more like continuous light. Auto FP flash is one such solution (next page).

The Problem with Fill Flash in Sunlight

The shutter speed limit of Maximum Sync Speed is really no big deal for flash indoors. The flash is fast itself, so in dim ambient light, it matters less what shutter speed is, the flash is faster. The dim ambient is too dim to blur motion seen by a slow shutter. But this maximum sync speed becomes a bigger problem when using fill flash in bright sun (because overwhelming bright sun cannot be ignored). The speedlight is very fast in dim light with no ambient to blur anything, but the bright sun is continuous, not fast.

The Sunny 16 Rule says at ISO 200, exposure in bright sunlight is 1/200 second at f/16. Bright sun does not vary, so this is the norm we expect. And we pretty much have to correctly expose the daylight scene - we cannot ignore the sun like we can ignore dim indoor illumination. Continuous light (like sunlight) has no shutter sync requirement, but when we add a flash for fill, now there is one.

With flash in sunlight, we cannot use faster equivalent exposures, like 1/400 second at f/11, or 1/800 at f/8 - because the focal plane shutter's maximum sync speed is 1/200 or 1/250. That means, if using flash in bright sun at ISO 200, the camera exposure WILL NECESSARILY BE around 1/200 second at f/16 (Sunny 16, but cannot exceed sync speed). The f/16 requires a lot of flash power, but otherwise this works fine, unless you just craved to use f/2.8 out there. Camera P mode knows all about this, and has ability to set both shutter speed AND aperture, and so it is a good choice for fill flash in bright sun. But if you use camera A mode, and set f/4 out there without thinking, the camera will just fuss warning HI at you, until you set near f/16 so it can work in bright sun (at the limited sync speed).

Wishful thinking, but our dream is that if somehow we could increase shutter speed, we could open the aperture, for reduced depth of field, or to an equivalent exposure for daylight which lets the regular flash mode work at lower power level, without affecting either exposure. Or, a faster shutter speed could help the flash to "overpower" the sun, reducing the sun without affecting the flash - if we could, but we cannot (in these bright sun cases.). We are up against the maximum shutter sync speed wall. FWIW, using lower ISO, or using a Neutral Density filter, can allow a wider aperture in that case, but that's all it does (maximum shutter sync speed is still enforced). But these affect both the flash and the sun equally, so these do not change the balance between flash power and sun. We can only use more flash power to affect the balance, and then exposing the higher flash power might end up at f/32, which does decrease the sun that needs f/16, but we still need Maximum shutter speed sync. Flash in bright sun is a special case.

Not the same thing, but Auto FP mode does offer one way to increase shutter speed with flash, for this bright sun case. (next)

Auto FP flash is a big subject, continued on next page.