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Sync speed and studio lights
Old 05-10-2005, 08:21 PM   #1 (permalink)
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If my lights have a flash duration of 1/500th of a second can I use any shutter speed up to 1/500th of a second (at the metered f stop) or does it even matter. If the flash duration is that short isnt the effective shutter speed 1/500th of a second even if the shutter is set to 1/60th?
Inquiring minds want to know. [img]/ubbthreads/images/graemlins/confused.gif[/img]
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Re: Sync speed and studio lights
Old 05-10-2005, 08:55 PM   #2 (permalink)
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Yes, you can, in principle, use any shutter speed and you will freeze the image in the 1/500th second in which the flash fires. FWIW, many of the Hasselblad style of medium format cameras use a leaf shutter rather than a film plane shutter. Some of the leaf shutters can synchronize to the flash pulse at speeds upto 1/500th, if my (rather faulty) memory serves me well.

However, there's a practical "catch" - it only works IF your shutter is fully open during the flash. And that catch is why your camera has a specified "sync" speed. The pulse of light from the flash does not occur instantly after the shutter release is pressed, nor does the shutter instantly open after the shutter release is pressed. So it's necessary to use a shutter speed that will keep the shutter entirely open during the time the pulse of light occurs - the maximum sync speed. You can shoot at slower speeds, but if you shoot higher, only part of the frame will be exposed.

Assuming that the ambient light level is low, you will be freezing the action at 1/500th second, even though the shutter speed is 1/60th or slower.
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Re: Sync speed and studio lights
Old 05-10-2005, 09:10 PM   #3 (permalink)
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sync speed is defined as the fasted shutter speed at which the sutter is complety open at one point in time. The burst rate (the amount of time that the flash tube is illuminated with energy) is usually in the x/1,000+ range. Because the flash tube is on and off well before the shutter speed can be of any relevance... the shutter speed is ,,, well,,, irrelevant. There will be no difference in exposure whether the shutter is set at 1/250 or 1/15th. The only difference will be the exposure from any ambiant light in the room from either the room lights, OR the modeling lamps in the strobes.

Hope this helps.

-Craig
www.craighphoto.com
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Re: Sync speed and studio lights
Old 05-11-2005, 09:36 AM   #4 (permalink)
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The easiest way to think about this is to imagine that you are taking a photo in a totally dark room. Suppose you put the camera on bulb (the shutter will remain open as long as you hold down the shutter button). Now fire the strobe. If the strobe fires at 1/500th, then if effect, your shutter speed was 1/500th.

Now, set the camera to 1/125th which is a typical sync speed. This means that when the camera is fired, the shutter will be open during all the time interval that is necessary for the burst of flash to be recorded. If you set it too high, then the shutter may close, or not be open when the flash fires.

Now suppose you have a neon sign behind the subject as part of your background and that the strobe's light does not fall on the neon light. So shooting at 1/125th in a dark room, you probably will get very little of the neon light showing. But if you meter at the fstop you are using with the flash the neon light, and set your camera to that shutter speed, you'll not only have a perfectly exposed subject, but also a perfectly exposed neon light.

So the rule is this. The fstop and the speed of the flash determine the exposure for the main subject on which the strobe light falls. The shutter speed determines the exposure of the background at the given fstop.



If shooting outdoors in bright light and you are lighting the subject with flash, but want the background properly exposed, you might need a shutter speed higher than the sync speed. But that's a different sort of challenge.

Cheers,
rfs
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Re: Sync speed and studio lights
Old 05-11-2005, 09:42 AM   #5 (permalink)
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Thanks for the replies, they were very helpful!
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Re: Sync speed and studio lights
Old 05-11-2005, 08:26 PM   #6 (permalink)
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Just a corollary to what's already been said here.
- A leaf shutter is either open or closed.
- A focal plane shutter (on a recent Nikon for instance) has two shutter curtains, a lead and a trail. At very fast shutter speeds (above flash sync), the shutter is never completely open; it's more like a window or slot moving over the film plane as the the trailing curtain chases the lead across.
- a fast maximum shutter sync is useful because of what RFS was talking about. it gives you more flexibility to set the aperture to control the illumination ratio between ambient and strobe.

Practically speaking, I shoot 125th with strobes in the studio when ambient light isn't a factor.
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Re: Sync speed and studio lights
Old 05-14-2005, 12:14 AM   #7 (permalink)
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Thank you for this information. I have been shooting and getting great looking subject, but very dark backgrounds. This is a factor I was unaware of. I usually set at 1/180 and shoot. I havn't had the time to play and figure out what happens if I set it slower. This will be my new thing to toy with.
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