What Is Flash Shutter Speed?

This post is a part of our Q&A section. If you want to submit your question, please use the form in the Contact page.

Dinesh (Mumbai, India) asked : I read your post about flash sync speed and got confused about flash sync speed and flash shutter speed. I went through Nikon D90 manual but couldn’t be satisfied with the explanation they gave. Can you please tell me the difference between flash sync speed and flash shutter speed?

If you already read my post about flash sync speed as you have mentioned in your question, I will skip that part and jump right into your question about what is flash shutter speed and when should we use it?

Flash shutter speed represents the minimum (slowest) shutter speed your camera will use when the flash is set to normal sync mode. Sometimes it’s also referred as a “studio sync” speed. Many photographers especially studio photographers have used sync speed of 1/60th of sec which has become somewhat standard. However, some photographers like to shoot in a different shutter speed. If you would like to eliminate as much ambient light as possible, you should go with the higher value such as 1/200th of sec or 1/250th of sec shutter speed. But if you want to capture ambient light (using rear curtain slow sync), you can start with 1/60th of sec and decrease the speed as you need.

There is no standard or correct sync speed, but if you are shooting under normal light inside studio or outdoor, default flash shutter speed, which is 1/60th of sec can be best choice and then you can manipulate your speed depending upon whether you want to capture the ambient light or avoid it. And if you want to change the default value in your camera settings, it is fairly an easy setup. If you are using Nikon D90, you have to go to CSM (Custom Setting Menu) and go to the Bracketing/flash and choose option e1. But if you are using D200, D300, D700 and other higher D-series like D2 or D3, you have to navigate through CSM and go to option e2.

Auto FP High Speed Sync Mode

Flash photography is a very exciting topic and confusing too at the same time. It offers variety of techniques to produce excellent photographs, but also requires you to learn its technical terms, understand those techniques and apply them properly. Earlier, we discussed about different types of flash sync modes and when should we use them. Today, we are going to discuss about another flash technique/mode, Auto FP High Speed Sync mode. But before jumping right into the topic, let’s discuss about what is the flash sync speed and how does it work?

What is Flash Sync Speed?

Flash sync speed represents the fastest shutter speed any camera can use with a flash. Different camera offers different flash sync speed, and over the time, flash sync speed has been increased significantly due to advancement of technology in digital world. Nikon D90 has a sync speed of 1/200th sec, whereas D7000, D300s and D700 offer 1/250th sec to 1/300th sec. Similarly, Nikon D3X and D3S have a sync speed of 1/250th sec. When you are using a built-in flash unit or an external flash unit, you can’t use a shutter speed faster than the camera’s flash sync speed. It is as simple as that. If you need to use a flash light beyond the flash sync speed, you need to use a feature called Auto FP High-speed sync. This situation may arise when you are shooting in a bright daylight and need to use a flash as a catch light or fill light for the shadows. When you use flash in such condition, you would need to use a shutter speed faster than the flash sync speed to compensate the exposure from the flash light.

After this brief intro to flash sync speed, let’s get into today’s topic, Auto FP High Speed Sync. 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 the lens and the 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 the shutter speed. The shutter speed is determined by the timing between the start of the first curtain opening and the start of the second curtain closing. This is important to understand that the entire sensor will be exposed to the light (the condition where the first curtain completely opens before the second curtain begins to close) for the duration of the shutter speed that matches the flash sync speed, also called maximum normal flash sync speed.

When you are using a faster shutter speed than the normal flash sync speed, the second curtain (also called rear curtain) starts closing before the first curtain (also called front curtain) is completely opened; thus never exposing the entire sensor to the light at any one time. If you keep increasing the shutter speed higher and higher, this will result in a very narrow 'slit' of light that travels across the sensor. And in this case, if there is not enough ambient light, you will get a dark area at the bottom of the picture. This is because when the camera takes a picture, image is recorded upside down on the focal plane, and as the second curtain already started closing before the first curtain completely opens, sensor will not get enough light to brighten the top part of the image which is actually a bottom area of the subject. To overcome this issue, Nikon developed a technique called Auto FP High Speed Sync Mode (not available in all models of DSLR). Auto means camera will automatically use high speed sync whenever it’s needed, or otherwise it will use normal standard sync speed. In a normal flash sync mode, the flash is fired at once with its full power. But in case of FP High speed sync mode, flash is divided into burst of flashes and fires continuously throughout the travel time of small window which travels across the camera sensor. And because of that reason, when you are using very fast shutter speed like 1/8000th sec for example, you will have to consider the power of the flash unit as the flash power is stretched more for narrower slits and it may not be able to brighten the picture or subject area.

Different camera has a different menu option to setup Auto FP High Speed Sync mode. If you are using Nikon D90, you can go to the CSM menu and go into the option e5 to set Auto FP ON, and if you are using D700, you can go to e1 to enable High speed sync mode.

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.

The loss of a flash power is the trade-off when you are using a flash in a High speed sync mode.

How To Use AE-L/AF-L Button Correctly?

This post is a part of our Q&A section. If you want to submit your question, please use the form in the Contact page.

Darren (London, UK) asked : What is the function of AE-L/AF-L button which is on the right side of the viewfinder on my Nikon D60. I read the camera manual but couldn’t grasp the concept of it. Can you write about its feature or may be how to use it and when to use it?

I wrote a blog on how to set AE-L/AF-L button on Nikon D90 and how does it work a while ago. Nikon’s all DSLRs including D60 share same theory about how it works but it depends on the situation when do you want to use it. Basically AE-L/AF-L stands for Auto Exposure Lock/Auto Focus Lock but you can use AE-L/AF-L button for many other different functions. You can even use this button to lock Flash value which we discussed in a earlier blog. Camera menu option and the button position on the camera may vary depending on the camera model, but you will find it under the CSM menu and probably you have to go under the Control option. If you want to see the details on how to set AE-L/AF-L button in your camera, you may want to check the blog post on How to set AE-L/AF-L button on Nikon D90. In this article, I am going to demonstrate the effect of Auto Exposure Lock feature with the help of couple photographs I took. Once you grasp the concept of it, you can apply the technique in similar situations.

Before applying Auto Exposure Lock (AE-L)

My camera was in Aperture Priority mode and the metering system was in Matrix metering mode. That means camera metered the exposure based on the overall scene inside the frame. The white snow background gave the sense of a bright situation to the camera metering system and it calculated the Shutter speed accordingly. But when I took the picture, the main subject came out underexposed because the camera metering system couldn’t figured it out that the main subject was not as bright as majority of the frame.

After applying Auto Exposure Lock (AE-L)

To overcome this issue, I thought Auto Exposure Lock (AE-L) feature would be the best choice in this situation. Before activating the feature, I zoomed in little bit so that the brighter area in the frame will get reduced and the camera metering system won’t be fooled. After that I focused on the main subject and press the AE-L/AF-L button to lock the exposure at the current values. At this point, the camera metering system calculated the exposure values based on the relatively darker area because I eliminated the most of the snow by zooming into the subject. I then zoomed out (recomposed the shot) and took a picture with the locked exposure. This gave me a nicely lit subject and well exposed background. If you are not careful enough with the framing, you might end up getting some overexposed areas in the frame as well.

[Updated on 4/30/2019] Someone might argue that why don't we use spot metering instead of matrix metering and lock the focus on the subject’s face and meter the exposure. By doing so, camera is going to meter the light based on the spot where the focus is locked on and not the entire frame, which will then properly exposed the subject. Yes, it might work in some cases but not a good choice in every situation, and that is the exact reason I chose this picture to demonstrate the effect. If I use spot metering in this particular scenario, it will properly expose the subject but will also overexpose the snow area way too much than I would like it be.