Off-camera flash 2

More on off-camera flash

Syncing your flash with the camera:

When your flash is on the camera, the camera communicates with the flash to make it flash at the right moment: when the shutter is open. When your flash is off-camera you need to sync it, so that it will flash at the right time.

There are several ways of doing this. In the old days we did it with a cable. Then, earlier or later, someone will trip over the cable, and take a flash on a stand with him. The person can get hurt, and so can the flash. A wireless system is better.

You can use a built in system like the Nikon CLS (Creative Lighting System). This has the advantage that you don’t need to buy any new equipment. Also you can control the flash settings from the camera, so you do not need to move to the flash every time you want to change a setting. You can even use ttl light metering, and automatic settings on your camera (like aperture priority A, or shutter speed priority S, or T. Even program P works. The disadvantage of ttl light metering when using multiple flashes is that the output of one flash influences the metering for the other, so when you change one setting, all the other flashes may change their output, and you go crazy trying to find out what every flash is doing. The CLS works with pre-flashes, which make some models blink right at the time you make the photo. This can be overcome by using an infrared filter over the master flash on the camera. (By the way: it took me a while to realize that it only works when the flash on the camera is tipped up!). For using this system refer to the camera and flash manuals. It seems complex at first but pretty soon you will figure out your favorite settings. I rarely use this system for the reasons explained.

optical trigger

Indoors I usually use an optical trigger (or optical combined with radio triggers). They are cheap and robust, less chance of anything going wrong than with more complex systems with batteries and all. Some flashes even have a built in optical trigger, so then you do not need any extra equipment. To trigger the remote flash you need a master flash on the camera. When you use a modern flash, turn off the ttl, set it for manual. If you use ttl, the flash gives off pre-flashes to meter the light output, and the optical trigger will trigger your remote flash on the pre-flash, before you make the photo. It is perfectly possible to use an old fashioned cheap flash for triggering. It is important to not get the light from the master flash in the photo too obtrusively, so I usually point it away from the subject. For this you do need a flash with swivel head. Before I had two good flashes I did use a cheap flash reversed on the camera, so the light would be directed away from the subject. In my (then) living room in Amsterdam with white walls this worked fine. Outdoors or in a really big hall you may have to point the master at the optical trigger for it to see the master, and trigger your remote flash. My optical trigger can easily be overpowered by light outdoors, on a sunny day even in the shade. 

radio trigger

For that reason outdoors I usually use radio triggers. Mine have several channels, so when several photographers use radio triggers in the same space, they can use different channels to avoid triggering each other’s flashes. It also means that you have to make sure your transmitter is set for the same channel as your receiver. Also you need to make sure that both transmitter and receiver have fresh batteries. My radio triggers do not work with rechargeable batteries. On the up side: the power they use is very limited, so your batteries have long life. It is perfectly possible to combine radio triggers with optical. Then you radio triggered flash will trigger the optical triggered flash. All this is fast enough, this equipment is much faster than your shutter.

There is one thing with the shutter, that you need to be aware of: a focal plane shutter (most of our cameras use these) has a flash sync speed. This is the fastest speed at which you can use it with flash.

A focal plane shutter opens one curtain. Then, after the time of your shutter speed it closes with the second curtain. For fast shutter speeds, the second curtain starts closing before the first curtain has opened all the way. So then a slit moves in front of the sensor to expose every part of the sensor for only a very short time.

A modern camera does not allow you to use a faster shutter speed then the flash sync speed when it detects a flash on the camera. But it may not detect your radio trigger, or very old flash. Then you may have set your camera for a very fast shutter speed, and only part of your image will receive flash.

Example of flash sync speed:

In image 1 the subject (woodcarving) is in the shade, while the background is in sunlight. This happens a lot when people sit in the shade because it is too hot in the sun. Then the subject comes out too dark. If you compensate with the exposure, you find that the background will be overexposed and you may even get lens flare around your subject. The solution is to add light to the subject, in this case with a flash. 

In image 2 I added off camera flash, to the right/above the subject. You can see a small shadow on the lower part of the wood carving. The subject and background are correctly exposed. Here the shutter speed is 1/250s, which is the flash sync speed of this camera. 

In image 3 the shutter speed is 1/500, faster than the flash sync speed of this camera. You see that only the top part of the wood carving receives flash, and is correctly exposed, the bottom part receives no flash and is under exposed. 

In image 4 I tilted the camera for a vertical image to show that the shutter turns with the camera. At a shutter speed of 1/500, twice as fast as the flash sync speed, only the left side receives flash and the right side is underexposed.