Everything you wanted to know about flash synchronisation but were afraid to ask

I thought I understood flash synchronisation, until I opened a big old can of worms trying to hack my camera to get high speed synchronisation out of it. Its been kind of interesting in a geeky way but its also worth knowing if you are going to play with flash. Watch out, because this is going to get technical

In the good old days before Edgerton invented the Electronic flash in 1931, flash photography meant burning chemicals. Initially in the open air, and then more conveniently in sealed flashbulbs. Chemical flash isn't instantaneous, it takes about 10 milliseconds to burn, and there is a peak in its intensity. Consequently flash synchronisation was all about timing the flash to ensure that the shutter was open during the peak of the flash intensity.

Electronic flash was based on exciting molecules of a gas like Xenon, in a tube. The initial output of the flash rises to a peak almost instantly and then falls off over a longer period of time. At full power the flash duration of a flashgun like the Nikon SB25 is 1/500s or 2 milliseconds. But clever circuitry within the flash can drop the output of the flash to zero nearly instantaneously. So its possible at lower powers to get very short flash durations indeed.

The short duration of electronic flash makes it possible to do things like freeze motion, by only lighting the subject with the flash. It also allows you to control your ambient exposure by altering the shutter speed, without affecting your flash exposure; so that you can overpower the sun.

So the flash is the easy bit, now we need to understand how the camera's shutter works. On most cameras, the the shutter has two curtains the first of which is covering the exposed sensor. When the shutter release is pressed, the first curtain exposes the sensor and the second covers it up again. Now the thing that may be hard to grasp is that when we talk about shutter speed, we are not talking about the speed at which the shutter passes over the sensor, we are actually talking about the length of time that a single point on the sensor is exposed to light. So imagine we have set the camera for a 1/200s exposure. We press the shutter release, and the first curtain starts to open, 1/200s later the second curtain starts to close. The animation on the left shows this in action.

So far so good, but now it starts getting tricky. There is a practical limit to how fast your shutter can move across the sensor, too fast and it increases wear and tear. So many cameras have a maximum speed of about 1/200s or thereabouts. This limit means that in order to get exposure times shorter than 1/200s the second curtain has to start closing before the first curtain has finished exposing the sensor, in effect there is a slit moving across the sensor in 1/200s. The smaller the slit, the shorter the exposure. The animation on the left shows that slit moving across the frame

So what does this have to do with flash photography? If the top speed of the curtains is 1/200s and the flash duration is about 1/500s that means that the flash duration is shorter than the time it takes to fully expose the sensor. So you need to fire the flash at the point that both curtains are fully open, otherwise when the flash fires, part of the image will be covered by one of the curtains. The maximum shutter speed at which both curtains will be open long enough for a flash exposure is known as the X-Sync speed.

Now, if you are as dumb as me, I know what your thinking. What if the flash duration is longer than the shutter speed? If the shutter is 1/4000s and the Flash duration is 1/500s then surely if you triggered the flash the instant the shutter opened, then the flash would be burning for the entire time that the slit between the shutters is moving across the sensor and everything would be lit? But if you were paying attention earlier you will have realised that if your x-sync speed is 1/200s then regardless of the exposure time you set on the camera, your shutter moves at a constant 1/200s to move from one end of the frame to the other, so you will still get a shadow from the shutter on the picture.

Some camera manufacturers have flashes that are capable of High Speed Sync, this allows the flash to synchronise at any speed. The way this works is that the flash duration is extended by making the flash fire multiple times at high speed, extending the time the flash is lit, to cover the entire length of the exposure. Effectively turning the flash into a short duration continuous light source. The flash fires a soon as the first curtain starts to open and stays lit until the second curtain has fully closed. Its great, but you will lose lots of power.

So thats as far as I will take you on this geeky journey. Not all cameras work in this way, some have electronic shutters and can sync at any speed, and there are many other kinds of shutters, but this article covers the reality for many of you. I hope it helps.

Do something different

Its been a while since I've posted. I have a few things in the pipeline, an article about flash synchronisation, a review of some ebay barn doors and a compact DIY barn door solution, but while all that is being prepared, I thought I would post something simple. Just like me.

The image on the left is The Monument; a tower commemorating the great fire of London that started in pudding lane. The scaffolding is there for refurbishment and re-gilding of the globe at the top of the tower. At night it looks for all the world like a destroyed tower block from a disaster movie or a fragment of the death star

If you have been reading my blog you may be aware of my lunchtime walks project. I'm attempting to walk every street within a one mile radius of my office. Its been a real eye-opener for me and I highly recommend it to everyone. If you get an hour for lunch use it and get around. One thing I have realised from doing this project is that its not enough to simply walk down every street, there are two directions you can walk in and you will see different things. Not only two directions, but even a change of perspective from one side of the road to the other can reveal overlooked details.

Another observation is that people tend to stick to the same routes from one place to another. If the pavements were softer you would see ruts along all the main roads. The crazy thing is that parallel to these routes, the streets are often empty of people.

So if you walk anywhere at all, don't get into a rut, at the very least cross the road, better still take a detour down a side street, explore your world and you will find more photographic opportunities.

Peanuts!

Before I invested in my collection of Elinchrom Skyports I bought some Wein peanuts to use as optical triggers for my flash. At £15 for a pack of three from The Flash Centre it was quite a bargain or so I thought at the time. But I really didn't get on with them. They really weren't very easy to attach to my SB28 flash guns and the connection to the PC cord was quite unreliable so I soon gave up on them.

David Hobby at strobist.com recommends the Nikon SB26 because it has a built in optical trigger, but they are as rare as hens teeth. In fact they are rarer than hens teeth because if I do a search on ebay for Hens Teeth I get 5 hits and for the Nikon SB26 I get none. So an SB28 plus an optical trigger is a much more viable option, at least here in the UK.

So fast forward to last night. I was experimenting with High Speed sync and dug out the peanuts because I wanted to see if I could sync with the HSS optically. (Its a long story) I realised that a recently purchased flash mount was ideal for attaching a wein peanut. There are no dangling cables and it seems that the connection it is a lot more stable. Which makes them more of an option to use with my other flashguns.

The flash mount is a Sonia all purpose hot shoe, three piece kit, that I got from Colin's Foto Electrico on ebay for £9.99. Its a solid piece of kit, which is a shame because I wanted to open one up and mod it. It has a hot shoe with two female PC connectors and one male, plus a test button. It also has a cold shoe with a tripod mount. They supply it with a little male-male pc cable as well.

The Wein peanut fits nicely in one of the sockets and the whole thing looks rather spiffy all joined together, like it was meant to be there.

Storage on location

Storage prices have changed a lot over the last few years and I have changed the way that I manage my storage on location. Previously I had used a two card strategy where I would shoot with one compact flash card while the other was being copied to a mass storage device. But with plummeting flash memory prices is that still a reasonable policy?

Several years ago I bought myself a Vosonic V-MP3H portable hard drive it was about the size of a late eighties mobile phone (Think house brick) It had a small colour screen and a compact flash slot. The idea was that I could have a single device onto which I could copy my CF cards and use as an mp3 player. At the time it cost about £250 and it worked out a great deal cheaper than buying enough compact flash cards to last me a family holiday. It had its problems, notably loading images in over 5 seconds, but the neat thing about it was that I was able to leave it copying a card and then on the screen, check that it had actually copied the images, usually by looking at the first and last image.

It worked great for a year or so and then it started getting a little grouchy and unreliable, I lost some images and decided that I had to get another device. The next purchase was an Archos Gmini 400, which was a really lovely machine. Review times were a lot faster, and I actually managed to get videos on it and it was small and neat looking. It had all the benefits of the vosonic but with go faster stripes. But tragedy struck, the drive fell on a marble floor on holiday and all the images became unreadable. I did manage to recover the images by dropping the drive on the floor again, but I couldn't trust it after that and started looking for another device.

The cost of this sort of device just seemed to keep going up instead of down and I was getting fed up of buying a new one every couple of years, so I opted for a JOBO Giga One, which was purely a device for copying images to disk. I was very nervous because there was no proof that the images had copied other than the disk space indicator reducing and the thing would go into power save mode after copying the card, which mean that quite often I wouldn't see that re-assuring 100% done message. Eventually disaster struck. I was at a friends wedding and I had left a card copying. The device ran out of juice part way through the copy and shut off, but I didn't realise because it usually powered down before I got a chance to see the 100% message. Consequently I lost some images which I would much rather I hadn't.

Fast forward to today. I can now get a 4GB card for as little as £13 or a Sandisk Extreme III for £18, so I have bought 4 4GB cards. Combined with the two 2GB cards I already had, thats as much storage as I have on my Giga One for a fraction of the price. The cards will be my digital negative. I'll still use the Giga One for backup, in case something happens to the cards, but from now on the cards will not be erased until the images are safely home.

I'll be really happy when someone starts selling a small battery powered device that will allow me to duplicate my CF cards, or better still a camera with two slots and CF mirroring.

How do you manage your storage on location?