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04/25/2006 04:05:20 PM · #1 |
Hi
I have trouble understanding Flash Sync Speed.
What's the difference between 1/500 and 1/125 sync speed? Does that mean you can't shoot at a shutter speed of faster than the given sync speed?
With the 30D Custom Function 3. Flash Sync Speed in Av mode. (auto or 1/250). When would you use auto and when would you use 1/250?
Thanks for your time. Been having trouble grasping the concept. :D |
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04/25/2006 04:11:48 PM · #2 |
| Using a flash sync of 250 or higher is used mainly for sports. even then it is rarly used due to the lower light output of the flash. I've used it a couple of times in ski photos, and the effect is very cool. Basicly it lets you shoot at a lower iso and higher shutterspeed but still give enough light to show detail in the subject and not blow out the background. |
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04/25/2006 04:22:51 PM · #3 |
If I could explain how a shutter in a camera works this would be easier..but basically it is a slit in a fabric curtain that moves - the slit lets the light fall on the sensor. It takes time for this slit to cross the sensor.
If the slit crosses too fast (shutter not open long enuough) then not all the sensor will get light on it and you will get an image with a black band on it.
So the X-sync, or max shutter speed for flash use, on teh 30D is 1/250 sec. It's best to shoot a step below this - 1/160 or for studio strobes 1/125 - the reason being it takes time for the signal to get from the camera to the strobe, it fires and finished and then the shutter closes. Some studio strobes are ver long duration (1/300 of a sec) and when you have several strobes firing optically from eachother there is a millisecond or so of delay - th eslower shutter assures you of getting all the light captured on the sensor.
Besides, with strobes the shutter speed has no effect on the exposure, only aperture affects exposure.
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04/25/2006 04:25:38 PM · #4 |
The flash sync speed is the fastest shutter setting that gives the camera time to open the shutter, fire the flash, then close the shutter again. The faster the blades of the shutter can move, the shorter shutter time you can use with the flash. If I had the choice, I would rather have a camera that could do 1/500 sync over one that could do 1/250 sync.
For the custom function question. In Av mode with auto set, the camera will meter the scene the way that it would in Av mode with out the flash. This makes it so that the background is correctly exposed and the flash is used for fill on the foreground.
With the flash sync set to 1/250, the shutter will be set to 1/250 and the Aperature will be set to whatever you have Av set to. This probably means that the only light of any consequence in the picture will be from the flash. There's a good chance that the background will be underexposed. |
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04/25/2006 04:26:47 PM · #5 |
Oh, Av mode...
On a canon in Av or Tv mode you pick one setting and the camera picks the other -to expose properly for ambient light. So teh flash works like fill flash UNLESS the total EV is uner 10, then the camera changes to a mode like P mode - the flash is assumed to be the main light and the background may go dark or black.
Now in Av mode you pick the ap, the camera chooses the shutter speed - from 1/250 to 30 seconds. As you might imaging, you can't really handhold at 30 seconds, flash or no flash. So by shifting the shutter speed to only 1/250 you eliminate this issue. In P mode the shutter locks at 1/60 and you can adjust the aperture to you're liking - better IMO than the 1/250 option here, but again, it all depends on what the total EV is in the scene.
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04/25/2006 05:48:52 PM · #6 |
I've always thought of the (focal plane) shutter as two "curtains". The first one opens (say, from bottom to top) to expose the shot, then the second one closes in the same direction (i.e. bottom to top). If the sync speed is 1/250, then any shot at 1/250 or slower will have the sensor completely exposed--that is, the first curtain will have opened, and the second one will not yet close. Note that these curtains don't move faster with faster sync speeds or move slower with slower sync speeds.
When the shutter speed is greater than the sync speed, the second (closing) shutter will start moving before the first shutter stops moving. This means that the botom of the sensor is exposed at a different time than the top of the sensor. The flash is very fast (much faster than 1/250), and if you could use a standard flash with a fast shutter speed, you may see where the gap between the curtains was.
The auto part of Av mode allows the camera to properly expose for the background and for the flash. The rule about handholding -- shutter time of less than 1/(lens length * crop factor) applies here, so if you have too long of an exposure time, you should use manual mode.
I'd use the 1/250 when the ambient lighting was bad, so that I could overpower the ambient lights with a flash. For example, some fluorescent lights have a high green component, while the flash is close to sunlight. If you mix them, you get noticebly bad results, so you would set the shutter to be as fast as possible and set the aperature so that you underexpose by two or three stops, and let the flash contribute most of the light. Unfortunatly, the background is generally greatly underexposed.
Check out //photoworkshop.com/canon/ In the lower right hand corner, view the "EOS Speedlight Tips" in the "Tips and Techniques" box.
Message edited by author 2006-04-25 17:52:34. |
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