If you realize it's time for you to learn off-camera flash, you will quickly discover that there is a number of available options. Your camera's manual will show you a few flash models recommended by the manufacturer and probably give you plenty of reasons why you should buy them. When you check the price tags on these, they're usually extremely high, in the hundreds of dollars. If you're anything like me, you'll look at the costs and think, "That's way too much, can't I do this at a lower cost?" The answer is yes, you can! With the right tools and a little knowledge, it is possible to use flash off camera for a fraction of the cost you might think you need to pay.
Some flashes are built to be optical slaves specifically. I have found that they are often of lower quality, usually lack some features, such as swivel/tilt head, and typically are not powerful enough to be truly useful in the long run. They may be easier to use, but aren't worth the convenience due to the lack of features. The chances are that once you decide you want to try off-camera flash, you probably want to move onto more advanced photography techniques. These flashes are very likely to disappoint.
Look for fully manual flash models. I recommend getting ones that have a swivel & tilt head, to maximize the number of angles possible from your flash. You will also want a model that has adjustable power settings. This will help remedy overexposure should it be a problem. I generally recommend finding a flash with a guide number of 100 feet or higher, because if you decide you want to try bouncing the flash off of walls, ceilings, or umbrellas, you'll want a bright one. There is significant light loss when you bounce the flash, so a powerful one is highly desirable.
It is very important to note, that if you want to use a flash strictly off camera, the kind of camera it is designed for is completely irrelevant. In fact, you'll find the best deals on flashes that were designed for fully manual film cameras. These are a very viable option, and for a low cost, they often have all the features you need! For example, most of my flashes were designed for a fully manual Minolta film camera, and they are used as off-camera flashes with a Nikon digital camera. Once you decide on a flash, you'll need another piece of equipment – an optical slave adapter.
An optical slave adapter is just a small piece of equipment that slides onto the bottom of the flash. You slip it on as if you were putting your flash into a hot shoe. There are a number of these available from many suppliers, and each has its pros and cons. If you do a web search for "optical slave hot shoe adapter", you'll find what you need. All the ones I've seen have an opening in the bottom, so they can easily attach to a tripod or light stand with a proper adapter.
Many of the inexpensive manual flash units feature an auto mode, as well. With them, you will want to switch the flash to manual mode in order to work with an optical slave.
To use an optical slave adapter, you need something to trigger it. You can either use the built-in pop-up flash of your camera or a second manual flash. To be able to use the pop-up flash, your camera must be able to set it to manual power. You will need to refer to your camera owner's manual for how to do this. On my Nikon, it can be set to anything from full power, down to 1/128 power. Unfortunately, not all cameras are made that way. If you are unlucky, and your camera's pop-up cannot be switched to manual mode, your only option to trigger the slaves is to put another manual flash into the hot shoe of your camera.
I typically set my built-in flash to either 1/16 or 1/32 power. This usually is enough power to trip the off-camera flashes without competing with them. Also, make sure that any red eye reduction function is turned off, as these can fire the pop-up flash multiple times, causing the off-camera flash to go off too early.
Please note, I almost always use off-camera flash indoors. So, any outdoor photography done with off-camera flash may require that the on-camera flash power be adjusted accordingly, likely to a more powerful setting.
Once you have your optical slave adapter, manual flash, and your on-camera flash (built-in or hot-shoe) adjusted, you're ready to go! Just remember that if the off-camera flash seems too bright, you can flip the switch or dial on the flash to adjust the power accordingly. Alternatively, you can move the flash farther away. There are also many accessories and modifiers that can be purchased if you decide you want to go down that road in the future. These accessories include, but are by no means limited to umbrellas, softboxes, and snoots.
If you have any questions or comments, please post them below.