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A Guest post by Marlene Hielema from www.imagemaven.com.
Several years ago I borrowed my piano teacher’s grande piano and her lovely home for a portrait shoot of an up-and-coming pianist. As I set up my studio strobe lights for the shoot I was inspired by Arnold Newman’s iconic photo of Igor Stravinsky that I had recently seen at a gallery. I wanted to use the piano as a design element, just like Newman did.
The shoot began with my usual warm up roll of film. Just like Newman, I was shooting in black and white. By the end of the session I had shot five rolls of 36-exposure film. I couldn’t wait to get into the darkroom to process it. When I hung my film up to dry, much to my horror, four of the film rolls had thick black bars running across the them. My heart sank.
Somehow when I changed film after my warm-up roll, I must have accidentally caught my thumb on the shutter speed dial and set it on a different shutter speed than what I had started with. In all the excitement I hadn’t noticed.
Black bands like this are caused by the shutter curtain blocking part of your image during the exposure. If you use studio flash or other off-camera flash systems, you might have had this happen to you, and didn’t know the cause. Let me explain.
dSLR cameras use a focal plane shutter which sits right in front of the sensor. A focal plane shutter consists of two curtains that travel from top to bottom that open to reveal the sensor. Think of a wipe transition that you might see in a slide show. In the case of an SLR camera, there are two wipes. One curtain follows the other. The longer your shutter speed is, the more time it takes for the curtains to travel down the sensor.
It takes the average focal plane shutter in today’s cameras about 1/200 second to travel down the height of the sensor. Since a studio strobe (flash) is instantaneous by comparison, you have to make sure your shutter is all the way open when the flash fires. If not, you’ll catch a black bar across your photo. The faster the shutter speed, the more black you’ll get in your photo. See the samples here to compare. (Ed note: Put photo sequence in here in this order 1/200s, 1/250s, 1/320s. 1/500s)
At some point curtain number one is finished travelling down the focal plane, and curtain number two has not yet started, revealing the whole sensor. That is the point in time at which the flash needs to fire, or you will end up with black bars when one of the curtains covers part of the sensor. See the animation below for a demonstration.
In order to avoid this problem you need to shoot at, or below, your flash synchronization speed. For Canons the sync speed is 1/200s. For Nikons, 1/250s.
Changing to a slower shutter speed will not change the exposure because in the studio when you’re using flash, the ambient light is usually so low that it doesn’t register. Of course there are limits. A really long shutter speed such as 1 second or more, will most likely take in the ambient light coming from the modelling lights, and that will affect how your photo looks as well as the white balance.
Your on-camera flash won’t let you set a shutter speed higher than your sync speed. However, some on-camera flash systems have a setting for high speed sync. In other words you can set a faster shutter speed than the normal sync speed. This setting is useful if you’re using flash outdoors because sometimes 1/200s is too slow a shutter speed, and will result in overexposure on a very bright day even at f22.
High speed sync sends several pulses of lower intensity light as the curtains travel across the focal plane, instead of the usual big burst a studio flash makes. Using high speed sync is also useful in situations where you want shallow depth of field, or fill light, if using flash outdoors
If you’re using an older (pre electronic) SLR film camera, the shutter probably travels sideways instead of top to bottom. You can check that by opening the back of the camera, without any film in it, and crank the film advance lever. You’ll see the curtains travel across the focal plane as you reset them. In older sideways shutter cameras the distance the curtains have to travel is longer (the long side of the film instead of the short side) so your sync speed will have to be longer too. For Canons, 1/60s and for Nikons, 1/90s.
Your image is actually upside down and backwards when it hits the sensor or film, but you view it right side up and the right way around because of the prism built into your SLR camera. I thought I’d put these photos in here the right way up for this demonstration to avoid confusion.
I learned this lesson the hard way after blowing my portrait shoot many years ago. In the days of film I didn’t have the luxury of immediate image playback. The importance of flash synchronization is forever burned in my brain.
My creativity is maxed out when I’m helping people solve their technical problems, especially those involving digital photography. I’m not woo woo or a fine artist, but I get a great deal of joy teaching creative people about the technology of digital photography, and watching how that understanding opens the doors to their creativity.
I teach photography and Photoshop online, in the classroom, for corporate workshops and one-to-one. I’m a camera raw evangelist. I’ve worked with kids, artists, engineers, doctors, and designers, and I’ve helped many photographers in their transition from film to digital. Find out more about what I can help you with at www.imagemaven.com and then let’s connect via Twitter and Facebook.