Wednesday, December 11, 2013

Introduction about Flash Sync Speed

Brief Intro 

This blog is for photographer who are trying to use off-camera flash but not able to shoot at higher shutter speed .... why ????? ... Lets find out. 

Note: being a Nikonian, i am using Nikon camera D7100 and my SB900 flash for the demonstration of flash sync speed. But the theory will remain the same for other camera manufacturers.

What is flash sync speed? 

Answer is simple ..... Sync speed is the fastest shutter speed you can use with your flash. But before we jump into further detail we need to understand how shutter speed work. 

Shutter opening is basically composed of  2 curtains. They are just like curtains that we have in our home window & they open and close vertically (from left to right or vice versa). But flash curtain works horizontally like UPSIDE DOWN and they have a name " First/ Front Curtain" & "Second/Rear Curtain". 

When we press shutter release button ... say at the speed of 1/160 of second, the First/Front curtain "lift up" and expose the sensor to the light coming from the lens all the way. The camera wait by "1/160" of the second then "lift up" the Second/Rear Curtain to block the light to the sensor and then whole system "RESETS" by "lifting down" both curtain at the same time and ready to take another image. Note that while both curtain are coming down during the reset, both curtain are over lapping onto each other so there is not exposing over the sensor.

The same scenario happen when we use external off-camera flash. What additional thing happens is  when shutter opens by 1/160 of a second, the flash fires in between to expose the subject quickly before Second/Rear curtain closes. Thats where the term "Flash Sync Speed" comes into the picture. 

Every Camera manufacturer always mention camera flash sync speed in the camera data sheet which mean that camera will be "in synchronise" with the flash to certain flash speed which is in general maximum allowable upto 1/200 (for Nikon is 1/250 sec). So question is, what happens when we increase the shutter speed beyond 1/250 ?????? .... well ones we cross this threshold we will start having "black bar" into the image :) .... now whats that black bar really are ??? 

let see some example first.

I am using cheap "Phottix" radio trigger for illustration purpose who work perfectly fine by the camera flash sync speed range (not beyond that) 

Lets start shooting

ISO: 200, f/1.8, 1/160, 35mm

ISO: 200, f/1.8, 1/250, 35mm

So far everything looks normal ... now watch what happens when we cross the camera sync speed of 1/250.

ISO: 200, f/1.8, 1/320, 35mm

ISO: 200, f/1.8, 1/400, 35mm

ISO: 200, f/1.8, 1/500, 35mm

ISO: 200, f/1.8, 1/640, 35mm

So what exactly going on here ????? 

What happen actually is, as we go for higher shutter speed, in time domain, the duration in which shutter curtains suppose to open & close is getting less and less .... so flash light is not able to get time and power to expose the subject. 

As soon as First/Front curtain start opening for the flash exposure, the Second/Rear Curtain dont WAIT for the first/front curtain to be fully open and start closing the shutter opening right behind it so flash only see the portion of the image as the Second/Rear curtain is blocking it. So as much as higher shutter speed we go, the QUICKER the Second/Rear curtain closes and didn't gave a chance to flash to fire properly. 

So how we over come this problem. The solution is simple :) .... The good thing about Nikon is, you can adjust and over this problem by enabling the feature "High Speed Sync" ... that what they called NIKON CREATIVE LIGHT SYSTEM (CLS). Also note that not all Nikon camera can support this high speed sync feature. Only the following can support this high speed sync ...

1- Nikon D90
2- Nikon D300s
3- Nikon D7000
4- Nikon D7100

5- Nikon D700
6- Nikon D750
7- Nikon D800

8- Nikon D800E
9- Nikon D810

10- Nikon D4
11- Nikon D4s

Here how you will enable the high speed sync in Nikon D7100 ....

Go to menu then Bracketing/flash

From there go to "Flash Sync Speed" (menu E1)

By default, your camera was selected "1/250s" as mentioned below

All you need is to select one menu above of it which is "1/250 s (Auto FP)" .... (Auto FP stands for Focal Plane)

Thats it .... You are done. Now you can shoot using external flash upto 1/8000 of a shutter speed easily. Here are some examples ....

ISO: 200, f/1.8, 1/1000, 35mm

ISO: 200, f/1.8, 1/2000, 35mm

ISO: 200, f/1.8, 1/4000, 35mm

ISO: 200, f/1.8, 1/8000, 35mm

Now what exactly going on and how flash is NOW working by just enabling one menu inside the camera system.

What happening now is, as the shutter speed goes higher, instead of one big flash, the flash start sending small burst of multiple flash exposure one after another as soon First/Front curtain start opening from DOWN the frame towards UP.... so even is Second/Rear curtain follows quickly to close the exposure towards UP & try to block the flash, the flash already fired small burst from down all the way UP to expose the subject over the sensor along with the First/Front curtain.

But this is not the end, as you go with high shutter speed and trying to use the flash with it, you need to open up your aperture or use little higher ISO then usual because, as more higher speed you go, the flash fires small burst of light in between of the shutter curtains opening  but those small burst are not in much power as compared to one big fire in normal sync speed. The reason for low power is the limitation of the battery inside the flash and they can't quickly recycle to fire quickly those small burst of light. Thats why the new Nikon SB900 & SB910 have small connector at the flash front (below of the infrared sensor window) so we can connect external battery pack for "more juice".

If you look closely at image where i had an exposure at the shutter speed of 1/1000 of sec, it looks under expose because flash was firing small burst of light ... not one big burst. So i had to increase the flash power. Here how i increase the flash power as i increased the shutter speed,

Shutter Speed               Flash Power
1/1000                                 1/128
1/2000                                  1/64
1/4000                                  1/16
1/8000                                   1/4

So as you can see, you need to push more power through flash as you go to higher shutter speed to get the proper flash exposure. 

I hope above explanation will make you understand the Pro's and Cons of the flash sync speed. In Canon, i believe there is no built in system to go higher sync speed like Nikon so they need high speed sync trigger .... the cheapest and best i found is from Yongnuo model YN-622N (N stand for Nikon), its simple AWESOME. It not only support higher sync speed but also iTTL / eTTL function :). You really don't need expensive Pocket Wizard solution. The best thing about Yongnou high speed trigger is, you can put another SB700/900/910 in commander mode or SU-800 to trigger & adjust the power of the remote flash independently along with the support of group.

Enjoy learning .... Happy Clicking :)

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