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The key part of determining any flash exposure is the lens aperture, but shutter speed plays an important role too. The duration of the flash output from electronic flash units is quite short 1/1000th of a second or even faster is not uncommon. The exposure is made while the shutter is open, but the actual shutter speed will be slower because there is a maximum synchronization speed that varies with each camera and is the time during which the shutter opens, the flash fires, and the shutter closes.
What happens when you shoot at a higher shutter speed than your flash will synch with your camera? You only get part of a picture! What part is missing depends on which way the shutter travels and how much you getif anythingis determined by the shutter speed you selected. Nowadays, some SLRs have a high-speed synch mode that works with specific speedlights. Check your users manual and see if your camera offers this feature.
And don’t think because youre experienced that you can’t get it wrong. The above image was made with a Pentax 6×7 and 75 f/4.5 SMC lens with a flash exposure was 1/60 sec at f/11. But clearly I needed to shoot it at the 6x7s correct synch speed of 1/30 sec. Nevertheless, the large negative and fine grain of the Ektar 100 roll film allowed more than acceptable images and prints to be made from a cropped negative. But it would have been so much better if I had access to the entire negative.
Besides matching the cameras correct sync setting, lower shutter speeds allow more of the ambient light to influence overall exposure, mostly the background – because the aperture you select determines the main subjects exposure. Using a slow shutter speed can open up the background allowing more ambient light to affect the exposure and show more separation between subject and backdrop. But beware the color temperature of any artificial lights that are part of that ambient light. Depending on how bright they may be, using slower shutter speeds can add unwanted color that can pollute skin tones in your shot. The solution: Increase shutter speed but not too much. On the other hand, warmer light sources can add a pleasant warmth to the photographs with makes the cameras LCD preview screen your new best friend.
Joe Farace is author of Studio Lighting Anywhere a new book thats available from Amazon.con and your friendly neighborhood book or camera store.