Flash Exposure


 

Right Image: Canon EOS D30, Tokina ATX 20-35mm f-2.8 zoom,  Bogen Tripod and ball head.
Multi-segment metering on aperture priority, 1/125 sec at f-5.6. Flash & autofocus.

As I've mentioned before in other topics, I try to avoid using flash and work with natural light whenever possible. The main problem with flash is the way it illuminates the subject which looks unreal. Flash photos look the same. There are times however, that flash must be used to properly expose your subjects. In low light or when working indoors for instance, you may need flash to freeze action or to create correct color balance under artificial light. Fill-in flash can also be used to remove shadows in day light photography. One thing many people don't realize is that using flash does not always guarantee proper exposure. Go to any sporting event and you'll see many people photographing from far in the stands with flash thinking they can illuminate the whole court and all the players. It doesn't work that way. Flash units have limited powers. They can only illuminate your subjects up to a certain distance. You must be within flash range to properly expose your subject. All TTL modern flash units automatically calculate flash exposure, but you must make sure you are within flash range. Manufacturers provide information for their units known as Guide Number or GN. GN is used to calculate the correct aperture at different distances between flash and the subject. Guide Number = Aperture x Flash to subject distance. GN is usually given at ISO 100 by the manufacturer and must be recalculated for different ISO films. If GN for a flash unit is 80 at ISO 100 and you are 10 feet from your subject then f-8 should provide correct exposure. 80/10=8. How do you know exactly how far you are from the subject? Focus and note the distance on your lens focusing mark. Keep in mind that flash to subject distance is important not camera to subject. If your flash is mounted on the camera, then both are in the same distance. If you use your flash off camera and closer, then your distance obviously becomes closer.

If you switch to a different ISO film, you must recalculate GN. You can use this formula: GN of the new film= GN of known film x the square root of the known ISO/new ISO. Unless you are a mathematician, you might feel a little dizzy trying to figure out the new GN using the formula. The best way is to drop the zero from the known GN, in this case 80, and you have the number 8 which is f-8 in the f-stop series. To calculate GN for ISO 200 for example, which is one stop faster than ISO 100 and needs one stop less light, in this case f-11. Add the zero to 11 that you dropped earlier, and you have 110 which is the GN for ISO 200 with your flash. Many flash units display the range at different f-stops on the back of the flash and you should always check to see if you are within range. Why do you need to know about GN if your high tech flash automatically calculates this for you? I believe you should always know what your equipment are doing. Camera meters for example, calculate exposure but they are not right all the time. If you rely on your camera all the time, you'll never know what went wrong. The more you know how your equipment work the more control you have over your photography.

Now that you know about flash distance and coverage, you still need to know if your flash will give correct exposure even if you are within flash range. TTL flash like all other metering systems have been calibrated to give middletone results. Middletone is neither white nor black, neither light nor dark. It is half way in between (see Exposure for more detail). If your subject is middletone, no compensation will be needed. For non middletone subjects you must use exposure compensation dial to get correct exposure. Changing f-stop alone will not work. The flash will compensate for the change. If your subject is lighter than middletone dial the +, or -- if your subject is darker than middletone. You should work in 1/3, 1/2, or 1 stops for maximum control. If your subject is light blue, dial in +1/2 or +1 stop of compensation. If it is dark blue, -1/2 or -1 should give proper exposure. How much compensation depends how light or dark you want the final result to be.

Working with flash is no different from any other accessory. You must know exactly how flash works to get good results. Placing your flash on top of the camera and releasing the shutter does not guarantee good results. Know when to use or not to use flash and when to trust your TTL flash and when to override it.