I thought I would make my own contribution to helping folks understand their flashes better, and perhaps to help them understand why their images aren't turning out the way they expected them to. I will assume you understand the basic stuff like aperture/shutter speed / ISO relationships, and what they mean in terms of measuring light. Flash Photography: Basic Rules of Thumb All of these "rules" are applicable whether you are using a strobe, flashgun. Continuous lighting is a bit "simpler animal" but I won't cover that here. 1. Flash photography is essentially TWO exposures in one. You are exposing for both the ambient lighting (more in a moment) and for your flash's exposure. The flash fires during the time that the shutter opens and closes. For the duration that the shutter is open, the ambient lighting and flash output will contribute to expose your image. 2. Flash effectiveness is affected by distance. If you've ever heard of inverse square law, this rule applies here. Essentially, what this means is that the further away your subject is, the more light falloff your flash will have. For example - if your subject is 10 feet away from you, then you move your subject another 10 feet away (20 feet in total), your flash is forced to attempt to cover an area FOUR times as larger as the original 10 foot distance. 3. Shutter speed has no bearing on flash exposure. It does, however, affect ambient exposure. The entire duration of the flash illumination is in effect when the shutter is open to when it closes, so keeping the shutter open for a longer amount of time will not be beneficial for flash photography. It is, however, beneficial for allowing more ambient impact on your photographs. Shutter speed is one way of managing your ambient light impact on your photography. 4. In camera metering meters ambient and flash output independently. When you're in aperture priority, shutter priority or program modes, it will attempt to properly expose for the ambient light by adjusting f-stop values or shutter values. If you're using manual mode, the in-house meter in your viewfinder will only measure ambient light - since that is all there is to measure. Hence why a lot of photographers use manual mode for weddings - they use the in-house meter for metering ambient light, then set the flash for the proper illumination amount (one stop under, one stop over, whatever is needed depending on distance and lighting situations). 5. Every camera will have a maximum sync speed. Read your camera's manual, it should state what your max sync speed is - generally most cameras have a max sync speed of 1/200th or less. Some can go as high as 1/500th, and some even more. Guide Numbers: What do they mean? Now that we got some of the "rules" out of the way, let's explore flash power. All flashes have some way of quantifying their power, generally in the form of a guide number (GN). Depending on where you are located in the world, guide numbers will be in measurements of feet or meters. Generally in the US, guide numbers will be advertisted in FEET. Conversion factor from meters to feet (if your flashgun's in meters) - multiply the GN by 3.3. Again, read your manual to determine the flash output measurement. I will assume you understand what F-stops, shutter speeds, and ISO mean in quantifying power. Bigger numbers (e.g. F22) generally mean more power, and smaller numbers (e.g. F4) generally mean less power. Distance also plays a role here, which I will illustrate in the next paragraph. Guide number's formula is as follows: GN = Aperture X Distance With using the above formula and some simple algebra, we can also conclude the following statements are also true: Aperture = GN / Distance Distance = GN / Aperture Distance is the placement of the flash from your subject. Aperture is your F-stop, and GN is the power output of the flash itself. The way we use the guide number is as follows: 1. Determine the proper aperture for flash illumination at a set distance from the subject. Example: Your flash has a guide number of 160, in feet. Your flash is 20 feet away from your subject. To determine the proper exposure needed for FLASH illumination (not ambient) you would calculate the following: Aperture = 160 / 20 Aperture = F8 F8 is the Flash's measurement of output - remember that this is the flash's measurement, not the ambient. 2. To determine where to place your flash to get the desired amount of power. Let's say you want to use F-8 as the amount of your flash output, but don't know how far away to place it. Example: Flash has guide number of 160 - in feet. You want to have a flash output of F-8 to expose your image. You need to know how far away you need to place your flash to get F-8 output. Distance = GN / Aperture Distance = 160/8 Distance = 20 You would need to place your flash at 20 feet away to get the proper illumination for F8. All in all, seems pretty simple yes? This is only for flash output measurement, this has nothing to do with ambient measurement, which is a completely different thing altogether. If you want to balance the two, I'll give you a short example here. Using my light meter set at ISO 100, I take an incident ambient light reading of my background or environment that I'm in (indoors). The light meter tells me at ISO 100, the background is properly exposed at 1/125th of a second, at F11. I want to have my flash be fill light for my ambient lighting, about one stop under. I know the guide number of my 580 EX II is 190, and I want to get the flash at F-8 (one stop under F11), so I need to know how far away to place the flash to get the F8 illumination from it. Distance = 190/8 Distance = 23.75 feet. I can place my flash about 24 feet away from my subject, and achieve that amount of illumination. All in all, seems pretty simple, yes? There are a few caveats you need to remember, but the above is pretty much the way Guide numbers work. The caveats: 1. ISO does have a bearing on how guide numbers are interpreted. Most Guide numbers are at ISO 100, but some wacky manufacturers set their ISO at 25 or 50 just to throw you off. Make sure you see what you're getting at the guide number specifications. 2. Increasing your ISO increases your Guide number, by a factor of 1.4 for every stop increase in ISO. 100-200-400-800... increases by a factor of 1.4 each time you go up. Keep that in mind if you want to increase your flash's range and it's effective power. 3. Adding modifiers like softboxes, telephoto reflectors, scrims, grid spots, and the like will have an impact on your flash's power. Generally, most modifiers have a negative impact (usually about 1 to 2 stops less) on your flash's power. Factor that into your calculations. I hope I didn't make this too long to read, and that some people will benefit from this. Good luck!