Question on lighting and shutter speed.why half my picture is missing when 2x shutter

[koentao]

I'm trying to do a white background by using 3 remote flashes. Two at the background, turning up the power to make my gray cloth white... while my third light firing at 45 degree in front of the subject... In this case a pillow.

While playing around with different shutter speed, one thing that struck interesting to me (which I cannot figure out) is why I cannot go beyond 1/200 sec on my shutter speed.

Here are two pictures to demonstrate my problem:

Although at 1/400 sec the picture looks like it would be properly exposed, but half my picture is missing. Why is that? at 1/320sec approximately a third of my picture is gone. And this seems to be related directly to my shutter speed. In the pictures above, the shutter speed was the only variable that changed, the ISO and Aperture stayed the same.

I got a feeling the answer to what's going on is a stupid one, but I can't figure it out!

 

[quiddity]

is your hand on the lens :0

 

[dxqcanada]

You have exceeded the camera's flash sync speed.
The shutter to moving too fast. By the time the flash is fired the shutter is already partially closed, so that part of the image never gets exposed.

 

[480sparky]

You're not letting the shutter open completely when the flash fires. It's called Synch Speed. You're setting the shutter speed so high that by the time the first curtain opens and crosses over the sensor, the second curtain has already started across.

If you jump up to 1/800, you'll get more black & less subject.

 

[koentao]

Any way to compensate for it besides slowing down my shutter speed?

 

[480sparky]

Any way to compensate for it besides slowing down my shutter speed?

Nope. You've exceeded the mechanical limit of your shutter.

 

[Overread]

To help here is a bit more detail:

A shutter isn't one part, but two parts which slide over the sensor. When the shutter button is pressed the first shutter curtain flips up, this exposes the sensor to light, the second curtain then comes up following the first, thus closing the shutter off to the light. At speeds below your flash sync speed the second curtain only flips up after the whole of the sensor has been exposed to the light (even if for only a fraction of a second). After the flash sync speed (typically around 1/200sec or 1/250sec on some cameras) the second curtain starts to rise before the first has cleared the sensor; at very fast speeds only a narrow strip of the sensor might be exposed at any one time.

Now flash light itself only appears for a very tiny fraction of a second; as a result if the flash is used beyond the sync speed only the tiny portion of the sensor open to the light at that point gets the flashlight, the rest is covered by the shutter curtains and only gets the normal light exposed to it. Thus you get the black bar (because the ambient isn't strong enough to expose the scene) or even a black strip if you go fast enough with the shutter.


Now of course this means that 1/200sec (or 1/250sec) is your max shutter speed that you can use with flash, however there are a few considerations:

1) Some flashes have a highspeed mode where they allow faster shutter speeds to be used because instead of a single burst of light the flash puts out a fast series of mini-bursts. The result however is that the flashes power is significantly reduced and as such this mode is only any good for fill-flash- when the flash is the lesser light source and is only touching up the shadows/foreground lighting.

2) Action - if you're shooting action and you want that crisp exposure, but also need flash the best way is to have the flash dominate the scene - that is without flash the exposure would be black (fully underexposed). Now the scene is controlled totally by the flash light itself, and because its only there for a fraction of a second it acts like a super fast shutter speed on the exposure (in fact flash is faster than the camera can work the shutter; so when you see many of those bullet speed type shots they are done this way; with flash light providing the splitsecond exposure rather than super fast shutters).

 

[Village Idiot]

Any way to compensate for it besides slowing down my shutter speed?

Possibly. What light(s) are you using? Find out what your x sync is from the manual. Most APS-C cameras are 1/250 max and Canon's FF DSLRs are 1/200. Pocket wizards make a set of triggers that will let you toy with the sync to get a couple more 1/3 stops out of it. You can also zoom out and up your shutter speed and crop the portion of the photo you wanted in the frame that was correctly exposed.

And why do you need a higher shutter speed?

Overread explained about the flash being able to stop action, but if you're trying to kill off the ambient light you can either close your aperture and turn up your flash or get an ND filter and turn up your flash.

 

[joealcantar]

Any way to compensate for it besides slowing down my shutter speed?

-
Yep, like someone mentioned we have to know what kind of lights you are using. 1/250 is a very fast shutter for studio work, what is the need for this? most of the time you see this need outside when you have a very bright day and you are trying to overpower the light somehow.
-
Shoot well, Joe

 

[KmH]

To help here is a bit more detail:

A shutter isn't one part, but two parts which slide over the sensor. When the shutter button is pressed the first shutter curtain flips up, this exposes the sensor to light, the second curtain then comes up following the first, thus closing the shutter off to the light. At speeds below your flash sync speed the second curtain only flips up after the whole of the sensor has been exposed to the light (even if for only a fraction of a second). After the flash sync speed (typically around 1/200sec or 1/250sec on some cameras) the second curtain starts to rise before the first has cleared the sensor; at very fast speeds only a narrow strip of the sensor might be exposed at any one time.

The shutter curtains actually move down, rather than up. That way, at least when the camera is held in horizontal orientation, gravity can assist shutter curtain movement.
In the image it appears that the curtains are moving upwards, because the lens projects the image onto the image sensor, upside down.

The camera flash x-sync speed is the maximum shutter speed that has both shutter curtains fully open. At any faster shutter speed, one or both shutter curtains cover some part of the image sensor.
In effect the opening in the shutter curtains becomes a slit, and the width of the slit narrows as shutter speed gets faster.

From the image the OP posted I can tell the camera was set to sync the flash on the front/first shutter curtain, which causes the flash unit to fire at the instant the front curtin is fully open, and the black bar is the rear/second shutter curtain blocking part of the image sensor. If the flash sync was set to the rear curtain, the top part of the photo would have the black bar. (Nikon calls the shutter curtains front and rear. Canon calls them first and second.)

As mentioned by Overread many DSLR cameras and flash units can be set up to do flash at faster than the flash x-sync speed. Nikon calls it FP-sync, Canon calls it High Speed Sync (HSS). The width of the shutter curtain slit determines how many 'mini-bursts' the flash unit has to fire during an exposure. The smaller the width of the slit, the more times the flash has to fire during a single exposure.

 

[Edsport]

Any way to compensate for it besides slowing down my shutter speed?

Yes, you can get a flash that has high speed sync capabilities...

 

[Robin Usagani]

Op... Change your shutter speed to 1 second, I bet you virtually get the same image. Why 1 second? Just so you learn that flash exposure isn't affected by shutterspeed unless you exceeded the sync speed.

 

[Village Idiot]

Op... Change your shutter speed to 1 second, I bet you virtually get the same image. Why 1 second? Just so you learn that flash exposure isn't affected by shutterspeed unless you exceeded the sync speed.

Unless there's enough ambient in the room to drastically affect the exposure. That's almost 9 stops. 1/500 to 1/1 is 9 stops, so 1/400 would be about 8 2/3 stops. That's a lot of light.

 

[gsgary]

What F stop were you using because shutter speed is not important if you are trying to get a white background, but you need to learn the basics of flash first

 

[Robin Usagani]

True, but considering half of the image is pretty much black and the end of the room is black, I take my chance

Op... Change your shutter speed to 1 second, I bet you virtually get the same image. Why 1 second? Just so you learn that flash exposure isn't affected by shutterspeed unless you exceeded the sync speed.

Unless there's enough ambient in the room to drastically affect the exposure. That's almost 9 stops. 1/500 to 1/1 is 9 stops, so 1/400 would be about 8 2/3 stops. That's a lot of light.