Flash sync speed?

Discussion in 'Digital Discussion & Q&A' started by D-50, Oct 24, 2007.

  1. D-50

    D-50 TPF Noob!

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    So Imstill working on using my sb600 off camera. I've done somereading on flash sync speed but its confusing to me, I did not go to photography school so technical talk goes over my head. Can anyone explain this to me in layman's terms.
     
  2. Sw1tchFX

    Sw1tchFX TPF Noob!

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    what dont' you understand? the flash sync speed is the fastest shutter speed you can use without the shutter curtain covering up part of the frame.
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    That's right.

    Picture it this way...your camera has a shutter that opens and closes for the exposure. The shutter is made up of two curtains...one opens up and one opens down. The trouble is that they don't or can't both open at exactly the same time...although they will both be open for the same length of time.

    So during the exposure, there will be a time when only one of the curtains is open...it may be a very small fraction of a second, but the flash is also only a very small fraction of a second...so if the flash fires when only one of the curtains is open...it will only illuminate part of the film/sensor.

    The max sync speed is the fastest shutter speed at which the camera can guaranty that both curtains will be open when the flash fires.
     
  4. D-50

    D-50 TPF Noob!

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    Thank you for that info a follow up question is how do I determine what sync speed to use,or whatthe max sync speed is? I have a D200 and an SB 600 speedlight. In my menu there is a sync speed menu that has options such as 1/60th 1/100th etc. Say Im shooting at night with a shutter of 1/60 should I set my sync to 1/60, does the camera do this automatically?
     
  5. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    I'm not sure...that sounds like something specific to Nikon or that camera.

    I think the sync speed you are seeing in the camera's menu is the 'default' that you can set it to. If I'm correct, this would make the camera use a shutter speed of 1/60 (or whatever) when you have the flash on and it's in an auto mode.

    The max sync speed is determined by the design of the camera (should be easy enough to find out). So all you really have to know, is that when using flash, you need to keep the shutter speed under that speed. Some cameras will prevent you from setting a faster speed, while the flash is attached and turned on.

    Now, it should be noted that shutter speed has no affect on the exposure from the flash. In a completely dark situation, you would get the same flash exposure at 1/15, as you would at 1/250. The flash fires much faster than that so the shutter speed isn't important. The aperture and the power of the flash, are the two things that will control the exposure from the flash.

    However, we aren't always in completely dark situations. There is often some sort of ambient light. The exposure from the ambient light will be affected by the shutter speed (and the aperture) just like on any shot without flash. So basically, every flash shot is really a double exposure...one flash exposure and one ambient exposure.

    Now because the shutter speed doesn't effect the flash exposure, you can use the shutter speed to independently control the ambient exposure when using flash. The longer the shutter speed, the more ambient exposure you get. Using a longer shutter speed with flash, is often called 'dragging the shutter'. This is handy when your subject is fairly close to you but the background is not. A dark wedding reception hall, for example. The flash will light up the people in front of you just fine...but the flash can't light up the background as well. If you use a fast shutter speed, they will look like they are in a cave because the background will be very dark. If, however, you use a slower shutter speed (and there is some ambient light) then the background will get more exposure and they won't look like they are in a cave. The flash exposure will not have changed because the aperture and flash power are still the same.

    The way I do this is to shoot in manual mode when in this situation. You set the aperture based on the DOF that you want, the flash (if it's a dedicated TTL flash) will adjust it's power to suit the aperture. Then you can set the shutter speed to get the amount of ambient exposure you want.

    It should be noted that if you use a long shutter speed and there is ample ambient light...you will still get motion blur like you would without flash. Sometimes this looks good in combination with the flash exposure...but sometimes not. You need to compromise between how much ambient exposure you want the shutter speed you can get away with.

    One more variable....ISO. The higher you set the ISO, the more ambient exposure you can get for a given shutter speed & aperture. It also lets the flash use less power, so you get faster recycle times and longer lasting batteries. For this reason, I will usually use ISO 400 or even 800 when using flash.
     
  6. LaFoto

    LaFoto Just Corinna in real life Staff Member Supporting Member

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    Oh, thank you so much for the information, Mike, for, despite the fact that I have all the books, also I never really understood what I was reading, but I begin to understand it now.

    So what I put into broad print in the quotation from your text seems to be the explanation for the problem I asked about in_this_thread ! Thanks!
     
  7. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Yes Corinna, I think that is exactly what happened there.

    In that situation, you still need to watch your shutter speed...and don't be afraid to crank up the ISO in order to get a faster speed.
     
  8. Helen B

    Helen B TPF Noob!

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    The D200 has a normal sync speed of 1/250 - ie at 1/250 the first curtain is completely open before the second curtain begins to close. Any electronic flash can be sychronised at 1/250 or longer. With Nikon SB600 and SB800 flash guns there is an 'Auto FP' mode that allows synchronisation at higher speeds with the D200, but at the cost of flash power. Metz flash guns have a similar HSS mode - high speed sync.

    Best,
    Helen
     

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