Maximum shutter speed for Tv mode when using flash


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Nov 27, 2005
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#1
Hi,

I am a DSLR newbie who just got 500D and a Canon 55-250 IS lens.
When I am using Tv mode with the in-built flash, the maximum speed I can set is 1/200.
Is this as-designed?
but when I did not trigger the flash, I can go to much faster shutter speed.

A bit strange to me, because I expect the opposite to happen. :think:
I thought with flash, I should be able to take the shot at higher speed since exposure is there.
And without flash, since exposure is not sufficient, maybe the camera set the maximum to 1/200.
No?
I am not sure, just guessing. :confused:
Can anyone please explain to me? :think:

all the above observations are indoor, have not tried outdoor yet.
or did I get a lemon lens? :(
 

shrsrn

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Sep 9, 2007
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#2
this has something to do with flash-sync. despite 'more exposure', too fast a shutter speed and the flash will not be able to fire fast enough for the shot to be nicely exposed.
 

grey_fox

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Jun 12, 2006
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#3
Read the manual, it should be listed in there.
 

J-Chan

Senior Member
Sep 21, 2005
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#4
Thats just the physical limitation of a focal plane shutter. There are 2 shutter curtains that go past the sensor when you click the shutter button, the 1st moves out of the way to expose the sensor and the 2nd comes in at the end of the exposure. There comes a point where just as the 1st curtain fully reveals the sensor, the 2nd must move into place to get your shutter speed. Typically this is known as the flash sync speed, the fastest shutter speed in which a single burst of flash can fully expose the sensor. Any shutter speed faster than that the 2 curtains form a slit going past the sensor.

[vid]mnIwb2IB8xU[/vid]

Even though you technically can still sync with the flash at these shutter speeds, your flash would not fully expose the sensor as most of the light is blocked by the shutter curtains. So most camera manufacturers put a limit to which you can set your shutter speed with the built in flash on, typically 1/200th or 1/250th..

To get proper flash exposure for any faster shutter speeds, the flash has to have some high-speed sync mode, which most built-in flashes don't have.
 

kurtlim

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Apr 30, 2003
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#5
As said aboved, its how the Focal Plane Shutter work. to use higher shutter speed than the Flash sync, a Flash with High Speed FP mode is required.

to save the time for others:

As the shutter is released, the metal blades fall down(or go up), exposing the sensor. At the precise moment, a second set of blades falls down(or go up) to block the sensor. At faster shutter speeds, the second set of blades (curtain) closely trails the first such that, in effect, a slit of light moves across the sensor.

Modern Metal Vertical Focal Plane Shutter
This helps explain why digital SLRs have a maximum flash sync speed of about 1/250 second or less. At faster shutter speeds, the blades block off the sensor before the flash is able to fully expose the sensor. In other words, the first curtain opens, fully exposing the sensor. Just before the second curtain descends, the flash must fire while the entire sensor is exposed. Otherwise, a portion of the sensor is covered and produces a dark band across the image.

like this:

Courtesy of Photography in Malaysia

Most modern flash units have a mode called High Speed Sync (FP Flash) that allows shutter speeds faster than the camera’s top sync speed of 1/125 to 1/250. It works by pulsing the flash to emit a series of flashes as the small slit travels across the sensor. Less expensive flashes like the Canon 420EX have a set pulse rate while more expensive units like the 580EX can be set to specific pulse rates.
[vid]t_sz-cDkd4k&hl[/vid]
 

Nov 27, 2005
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#6
Thank you all for the replies.
I have indeed learnt something new today, i.e. the shutter curtain.
Now I understand about the limitation of shutter speed when in-built flash is used.
 

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