Flash photography is very exciting topic to discuss about and confusing too at the same time. It offers variety of techniques to produce excellent photographs but you also have to learn its technical term and understand those techniques. We discussed earlier about different types of flash sync modes and when to use it. Today we are going to discuss about Auto FP High Speed Sync mode. But before jumping right into the topic, let’s discuss what the flash sync speed is and how it works.
What is Flash Sync Speed?
Flash sync speed represents the maximum, or fastest, shutter speed any camera can use with flash, period. Different camera offers different flash sync speed and over the time, flash sync speed has been increased due to advanced technology and digital world. Nikon D90 has sync speed of 1/200th sec whereas D7000, D300s and D700 offer 1/250th sec or 1/300th sec. Similarly Nikon D3X and D3S have sync speed of 1/250th sec. To make it really simple, let’s put it in this way. When you are using built-in flash unit or external flash unit, you can’t use your shutter speed faster than your camera flash sync speed. It is as simple as that. But if you need to use flash light beyond your flash sync speed, you need to use your camera feature called Auto FP High-speed sync provided that your camera supports this feature. This situation may arise when you are shooting under bright day light condition and you have to use your shutter speed faster than flash sync speed to compensate exposure but you still need flash light as a fill light for shadows.
Now after this short briefing about flash sync speed, let’s get into the point. FP stands for focal plane and it represents the type of shutter used in DSLRs camera. A focal plane shutter is two precisely timed curtains that move vertically between lens and camera sensor allowing or disallowing the lights hitting the sensor. The reason to use two precisely timed shutter curtains is to increase the performance of shutter speed. So the shutter speed is determined by timing between the start of the first curtain opening and the start of the second curtain closing. This is important to understand that entire sensor will be exposed (the first curtain completely opens before the second curtain begins to close) to the light up to the shutter speed that matches flash sync speed and that’s called maximum normal flash sync speed which we talked bit earlier.
If you are using faster shutter speed than normal flash sync speed, second curtain (also called rear curtain) starts closing before first curtain (also called front curtain) is completely opened thus never exposing the entire sensor to the light at any one time. And at really high shutter speeds, this result in very narrow 'slit' of light that travels across the sensor. In this case if there is not enough ambient light, you will get dark area at the bottom of your picture. This is because when camera takes picture, image is recorded upside down on the focal plane and as second curtain already started closing before first curtain completely opens, sensor will not get enough light to brighten top part of the image which is actually bottom area of the subject. Now to overcome this issue, Nikon developed a technique called Auto FP High Speed Sync Mode. Auto means camera will automatically use high speed sync whenever it’s needed or otherwise it will use normal standard sync speed. When you are using very fast shutter speed like 1/8000th sec, you will also have to consider the power of flash unit as flash power is stretched more as you increase shutter speed resulting more narrow slits and it may not be able to brighten your subject area.
Here is what Nikon says about Auto FP High Speed Sync Mode:
Auto FP High Speed Sync is a flash mode used for fill-flash photography under brightly lit conditions. It will fill in and open up shadowed areas in order to portray the greatest detail in subjects. It's also ideal when using wide aperture lenses, and because it allows fast shutter speeds up to fastest shutter speeds on compatible Nikon D-SLRs. It is often used for action-stopping sports photography. And for portraits, you can open up your lenses to their full aperture in order to isolate your subject against a blurred background without overexposing the image.
When Auto FP High Speed Sync is selected, the flash will fire for the duration of the shutter curtain's travel, thus syncing with the camera's shutter speed when that speed is set higher than the camera's normal sync speed.