Thursday, 3 April 2008

Sync speed and flash duration

Many off-camera-flash users got excited when they found out they could use their flashes above the camera's normal sync speed if the camera had an electronic shutter. Well now there is a way to get high sync speed even if the camera has a mechanical shutter!

Now I can't claim credit for this, I read about it in the Strobist forum here and here.

First some background:
When a camera with a mechanical shutter is triggered the bottom of the shutter opens and when the exposure has been long enough the top of the shutter closes (it can also open from the bottom up). Up to a cameras top sync speed there is an instant where the entire shutter is open, when you go faster than this speed the shutter will be closing before it is full open, kind of giving a band of light that crossing the shutter plane.

So what does this mean for a flash? Well because a flash duration for most speed lights is approximately 1/1000 seconds long if it goes off when the shutter plane is only part way open it will look like this.

So how can we light the entire shutter plan with one flash when it is never fully open? Well while a flash is very quick, it still has a duration and through a little work around we can make use of this. Now I'm still not entirely sure how it works and don't have the equipment to do all the testing I want but I have my theories. The catch, you need a flash with High Speed Sync (HSS) to trick the camera into syncronising the flash and shutter as if the flash was operating on high speed sync (many little strobes to light the entire shutter plane).

So how did I make it work:
-Take camera - Canon 30D
-Attach flash - 430EX
-Put flash in HSS mode, manual, 1/64 power (so it doesn't contribute to the light much)
-Put Nikon sb-25 on an optical slave, manual, 1/1 power (longest flash duration)
-Start shooting


(click for larger image)

Nikon users have been attaching their flash via their camera's PC sync port, I don't have a cable so I can't try that however I did attach an eBay trigger to the PC sync port and it didn't work. I don't know if this is a difference with the port or just that the eBay triggers aren't great. Drop me a comment if you've had success with a canon.
Edit: Canon users can not use the PC sync port for this so they will have to be content using an optical slave (which actually doesn't trigger all that well from a flash in HSS mode).

Now why I think it works:
Well we are using the flash duration so that during the single "flash" the entire shutter plane is opened as some point, the flash suddenly becomes like a continuous light source. Why does it only work when the camera thinks it is doing HSS, well I think in HSS mode the flash triggers a little earlier and this allows it to overlap better with the shutter.

It isn't without its losses and it ain't perfect but I'm sure it'll come in handy at times. Anyway let me know what your experience with it is and if you have any other ideas on why it works.

UPDATE: I've added some further info in a blog post here.