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Photography Question 
Anna K. Masewicz
 

Light streaks


 
 
Is it possible to have a light streak acoss your photo if the picture is taken in a low light situation at an f/6 1 sec exposure w/400 speed film. I own a cannon eos rebel xsi. We were taking pictures of the kids next to their pumpkins last night and one of the shots has a huge light strak going up from the pumpkin through the middle of the picture. Ididn't know if 400 speed film would catch that big of a light streak at a second exposure or not. My son and everything else by the way is in focus but the pumpkin and the streak.


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10/27/2008 12:07:22 PM

 
Alan N. Marcus   Hi Anna,

Likely you combined the built-in flash with ambient light. Even if no flash was employed the below explanation is valid.

In a low light situation, the shutter lingers in the open position. Since light is is accumulative the cameras chip logic held the shutter open to compensate for the low ambient light. In this case the shutter lingered open for 1 full second. 1/15 is the longest one can generally hand hold a camera and this lengthy exposure takes practice. Generally hand-held exposures should be 1/60 and faster.

During the 1 second shutter action you moved the camera. Illuminates from the lighted pumpkin and other shinny objects continued to image as the camera was moved. The streak you see follows the movement of the camera.

Using a tripod or monopod or perching the camera would have mitigated camera movement but not subject movement.

Alan Marcus (marginal technical gobbledygook)
alanmaxiemarcus@att.net


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10/27/2008 1:33:10 PM

 
Anna K. Masewicz  
 
 
Thanks so much Alan,

one last question though, Why isn't everything else in the picture be blured as much as the pumpmkin

Sincerely,
Anna


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10/27/2008 7:09:55 PM

 
Alan N. Marcus   Hi Anna,

Both film and digital camera have a marvelous attribute, they have the ability to accumulate light. In astronomy, the norm is to expose for hours, in this way dim objects, never seen by human eyes are recorded. We are talking about a time exposure. Down-to-earth, we use time exposures to photograph landscapes at night under dark skies. Modern digitals have a shutter that can be set to automatic or manual. The adjustable shutter serves two main purposes.

1. The exposure time is set short enough to capture movement without undue blur. The blur can be due to subject movement or camera movement or a combination of both. Generally we can hand-hold at 1/15 second or shorter. 1/60 is considered about right for subjects that are stationary coupled with reasonably steady hands.

Now when we employ flash the situation is a little different. Modern cameras utilize a gas tube filled with xenon gas. A battery coupled with a capacitor builds up high voltage that is discharged into the tube when the shutter is pressed. The result is a blitz of light with duration of about 1/500 second or shorter. If the camera is operated in a poorly lit surrounding, the flash becomes the major contributor supplying light for the exposure. Such a short duration will arrest both subject and camera motion (true regardless of shutter speed). Try shooting an electric fan; you will be surprised, the image will reveal the fan blades frozen in place.

2. The exposure time can be used as an adjunct to aperture or ISO. In other words, exposure time plays a pivotal roll as a control over exposure. Exposure time can be set short under bright conditions. Exposure time can be set long to capture subjects in dim light. One can photograph black cats in coal mines if a faint light is present and the cat can be persuaded to stay still for a time.

In your case: The room was illumined, the pumpkin was illumined, the flash fired. The flash alone was sufficient to record the vista. Due to the dim ambient light, ordinary objects failed to record in the 1 second allotted time. If the exposure was longer, say 5 seconds they would have recorded. Meanwhile the pumpkin and other self illuminated objects were bright enough to record. They recorded as streaks during the 1 second exposure. The streaks were due to camera movement or subject movement or a combination of both. Again, ordinary objects recorded during the flash, however there was insufficient light energy for them to re-record during the 1 second the shutter remained open.

Alan Marcus (marginal technical gobbledygook)
alanmaxinemarcus@att.net


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10/27/2008 8:19:01 PM

 
Anna K. Masewicz   Alan thanks so much for being so detailed. It is very much appriciated. I took photography in high school and in college which was before digital photography, and all I could recollect were the fundamentals so I was having a hard time wrapping my brain around this one. Again thanks so much you explained everything perfectly.

Sincerely,
Anna Masewicz


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10/28/2008 8:35:39 PM

 
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