This tutorial is about flash photography, not flash the program. Just to make it clear.

It's been my understanding that using a flash in photography is something that is often viewed as hard or confusing. Many people leave their flash on automatic and hope for the best. With modern digital cameras with various types of TTL (through the lens) flash metering, auto flash gives pretty good results, but still, the ability to use a flash in manual mode is something that is a valuable addition to any photographer's toolkit.

If you ever use off camera flash, either by a sync cord or by optical slave, you'll need to work your flash manually, because TTL metered flash needs two way communication between the camera and the flash, and sync/optical slave only has one way communication. Same goes for studio strobe lights. So you'll be able to use the skills in this tutorial for everything from using a hotshoe mounted flash in manual mode to using studio strobes. You can use this technique for both on camera and off camera flash.

Hence this tutorial. I won't go into too much mathematical stuff, and the little there is will be easy enough to understand.

All you will need is an SLR camera (digital is best because it lets you see the results immediately) and a flash that operates in manual mode; that is, you can manually adjust the output of the flash without it being controlled by the camera. You'll also need some way to connect the flash to the camera so the flash will fire at the correct time. The most common methods are: placing the flash on the camera's hotshoe, using an optical slave, using a radio slave, using the flash's built in wireless system (both canon and Nikon flashes have this, but I don't cover the set up in this tutorial) and using sync cords.

Step One
Connect the camera to the flash. See the section "Getting the Camera to communicate with the Flash" for various methods of getting the camera to communicate with the flash. Don't turn the flash on yet.

Step Two
There are two exposures that we are going to be using; the ambient light exposure (which is what you expose for when you don't use a flash) and the flash exposure. Luckily we can do both of these seperately. The first exposure we are going to take care of is ambient light. Using the camera in Manual Exposure Mode (M on the mode dial), set your shutter speed and aperture to whatever settings you want to have to expose for the background (that is, anything that won't be lit by the flash). Don't worry at the moment if your subject is under-exposed. Take a photo (without firing the flash, leave it turned off for now) and adjust your exposure if needed. If you aren't familiar with using manual mode on your camera, have a read of these two tutorials [link] and [link] .

Step Three
Now that we have the exposure for the background, we'll set the flash exposure. Turn on the flash and position it where you want. If you'll be using bounce flash, angle the flash head to bounce the light where you want it. Turn on the flash and set it to some power, be it full power or half power, it doesn't really matter, because we'll be adjusting it. Take a photo with the flash firing. The background exposure will stay the same, but the flash will be illuminating the subject. But, unless you're rather lucky, chances are the flash power isn't set correctly. It will be either too bright or too dark. No worries, just adjust your flash's power, increasing the power if it is too dark, or decreasing it if it is too bright. Keep taking test shots until you get the right flash exposure.

Step Four
Take your pictures! The settings you have with this technique will work provided the ambient light levels stay the same and the distance between the flash and the subject stays the same (if you are using off camera flash then you can change the distance between the camera and the subject, but if the flash is mounted in the camera's hotshoe then you can't alter the camera distance because it alters the flash distance as well). If the lighting conditions change or the flash - subject distance changes, you'll need to adjust your settings. If you adjust for ambient lighting by altering your shutter speed, then you won't need to alter your flash exposure, but if you change your aperture to correct for alterations in the ambient light, then you will need to correct the flash exposure, as aperture alters both ambient and flash, wheras shutter speed alters only the ambient.

Getting the Camera to communicate with the Flash.

There are several different ways to get the flash to fire when you take the photo. You can attach the flash directly to the camera's hotshoe. Or, if you are using off camera flash, you can attach an optical slave device to the flash (this is a small attachment that detects the flash from the camera's built in flash and automatically fires the flash gun to which it is attached), or use a radio slave, which works in a similar way, but uses radio signals instead of light. Pocket Wizards are the industry standard in radio slaves. The other option is to connect the flash to the camera via a synch cord. A sync cord is probably the easiest option, as they are readily available and many cameras and flashes have sync ports.

An additional note: If you plan on using an optical slaved off camera flash to be triggered by the built in flash of your SLR, you'll have to put the SLR's built in flash into manual mode if you can. The reason for this is that in TTL modes the flash often fires twice. The first time is a metering preflash that the camera uses to figure out how strong the actual flash should be. The second time is the actual illuminating flash. These two flashes fire so quickly together that they look like a single flash. An optical slave will be triggered by the metering preflash and won't be able to fire again fast enough to actually light the photo. Putting the camera's built in flash to manual mode means it will only fire once (there's no need for metering because you are telling it how brightly to fire), so all you have to do is turn it down to minimum. If your camera's built in flash doesn't have a manual mode, then you'll have to fire the preflash manually (on Canon SLRs, you can do this by pressing the * button) and then waiting for the optical slave to recharge, but an easier way is to fire the preflash, then turn on the slaved flash.

Some additional notes for Canon users: First, if you are a Canon user, there is an Off Camera Shoe Cord which you can use to have the flash off camera, however, it is rather short (as it was designed simply to attach a speedlite to a bracket beside the camera) and it is rather expensive. However, it does maintain E-TTL metering between the flash and camera. Second, using an optical slave attachment on a Canon flash might cause some problems. I've tried it, and I can only get one flash out of the slaved flash (I used a 580EX with an optical slave unit attached). After it fires once, I had to turn the flash off and then turn it back on again. Apparently this is a common probl;em with Canon flashes.

As such, I will advise you to use the flash mounted in the hotshoe or mounted off camera via a sync cord. However, if you are using Canon flashes (They seem to get all the bad luck! ) you'll need some adapters, as the popular Digital Rebel cameras (the 300D, 350D, 400D and the new 450D) don't have sync ports. You'll need to buy a sync port adapter that plugs into the camera's hotshoe. Also, Canon flashes don't have sync ports (except for the new top end 580EX mk II - note the original 580EX doesn't have one). However, you can get plugs that slot onto the shoe of the flash that will take a sync cord as well.

Optical slaves aren't my first choice because they need a direct line of sight, don't work well outside on sunny days and if you have two optically slaved flashes in different directions (such as one flash on each side of the camera) it is hard to work.

Effects of Shutter Speed on Flash
Shutter speed has little effect on flash strength. The reason for this is that the burst of light from the flash is much briefer than the shutter, somewhere around 1/5000 of a second (although the length of the flash depends on the power; increasing the flash power actually increases the duration, hence decreasing flash power just means a shorter burst of light, so it doesn't effect the distance the light reaches). The only exception to this is the camera's X-sync speed. This is the speed at which the flash will no longer work correctly with the camera.

The reason for this is that the shutter on an SLR is actually made up of two curtains, like the curtains in the theater that go up and down, not the curtains you have at home that go side to side. When you press the shutter release, the first curtain drops down, letting light through to hit the sensor (or film). Then, after a set period of time (determined by your shutter speed), the second curtain drops down to cover the sensor again. Then both curtains reset themselves, ready for the next photo. The shutter speed is the length of time that the second curtain waits before dropping down after the first curtain drops. However, if you use a fast shutter speed, the second curtain will start to drop down before the first curtain has reached the bottom, meaning that the sensor will never be completely exposed all at once. In effect, only a slit between the two curtains is exposed, and this slit travels down the sensor.

When you fire the flash, the flash actually fires as soon as the first curtain has dropped completely. However, if you are using a fast shutter speed, then the second curtain will have already started moving, meaning that the second curtain is covering a part of the sensor. This covered part won't be exposed by the flash.

The X-sync varies depending on the camera, but is often around 1/200 or 1/250. If the flash is mounted in the hotshoe of the camera it will prevent you from setting a shutter speed beyond the X-sync, but if you are using a sync cord you'll need to be aware of it.

But other than that, shutter speed has no effect on flash.

Effects of Aperture on Flash
Unlike shutter speed, the aperture value does have an effect on flash exposure. Adjusting the aperture will alter the flash exposure just as if it was ambient exposure. This does mean that you can use aperture and shutter speed alone to adjust flash exposure. If you increase aperture by 1 stop and decrease shutter speed by 1 stop, the ambient exposure will remain the same, but the flash exposure will increase 1 stop (read this tutorial for more details [link] particularly the secontion "Using Manual Mode, Aperture Priority, Shutter Priority and Program AE). However, you will be altering depth of field and the movement as well, so this may not be an option for you.

Effects of ISO on Flash
This is pretty simple. If you double the ISO, multiply the flash's guide number by 1.4. If you halve the ISO, multiply the flash's guide number by 0.7.

Effects of Distance on Flash
This involves the Inverse Square Law. Doubling the flash - subject distance doesn't give you half the light intensity, it actually quarters it. The reason is that if you double the distance, the area lit by the flash will be twice as high AND twice as wide. If you imagine that you have a square with sides of one meter (an area of one square meter), then twice as high and twice as high will give you a square with sides of two meters - with a total area of four square meters. In order to get half the light intensity, you have to move the flash only 1.4 times as far, not twice as far. (In other words, if the flash is 1 meter from the subject, you'd have to move the flash 1.4 meters away to get half the intensity.) To get twice the flash intensity, move the flash to 0.7 times the distance; if the flash is at 1 meter, moving it to 70 centimeters will double the light intensity on the subject. This, incidently, is the reason why doubling the ISO only gives and increase of 1.4 times the flash intensity.

FINALLY
One of the best articles about flash of all sorts can be found over at Photonotes. The article there is primarily based on the Canon EOS flash system, but a lot of what it contains will apply to all flash systems. You can read it here: [link]