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long exposure photography

S

Special K

Diamond Member
I just started messing around with long exposure photography yesterday and have a question - why does increasing the exposure time cause the picture to appear brighter? Doesn't the camera just record what it sees? If the amount of light in the room isn't increasing during a long exposure shot, why is it increased in the picture?

I thought what happens in a long exposure shot is the camera "averages" what it sees over the entire time the shutter is open. For example, if I set the shutter speed to 10 seconds, and I am standing in the picture for half of that time, motionless, and then quickly leave the picture, I will get a picture where I am transparent. It's like the camera averaged the time I was in the picture with the time I wasn't. However, the light in the picture will be much greater that what was actually present, and that is what I don't understand.

BTW I am using an SD300 and cannot adjust the aperture size.

 
the longer the exposure, the more light you let in, hence the brighter image.
 
The longer the shutter stays open, the longer the light is on the sensor, so the brighter the picture will appear.
 
The amount of light picked up by the camera sensor/film is a function of time. The longer the exposure, the greater the amount of light that will enter the lens and be captured.
 
Originally posted by: aphex
the longer the exposure, the more light you let in, hence the brighter image.
Click to expand...

Well this may sound like a dumb question then, but then why don't we all go blind after a few seconds? Light is constantly being let into our eyes. Shouldn't the image around us keep getting brighter and brighter?

 
The camera would've tried to average the picture by closing down the aperture, but at some point it can't stop down enought and the picture will be overly bright.

At that point you would have to start adding neutral density filters to keep the picture from blowing out.
 
Originally posted by: daniel1113
The amount of light picked up by the camera sensor/film is a function of time. The longer the exposure, the greater the amount of light that will enter the lens and be captured.
Click to expand...

So it's basically like the camera is integrating what it sees over time? If the sensor reads a light intensity value of X at a certain point, and you leave the shutter open for a period of time, it will keep sampling that value as X, and just keep adding the result to the total?

 
Originally posted by: Special K
Originally posted by: aphex
the longer the exposure, the more light you let in, hence the brighter image.
Click to expand...

Well this may sound like a dumb question then, but then why don't we all go blind after a few seconds? Light is constantly being let into our eyes. Shouldn't the image around us keep getting brighter and brighter?
Click to expand...

Unrelated to this, but have you ever gone into a really dark room and flashed the flash. If you don't move your eyes you'll get a black and white image appearing after a second. Then if slowly fades away. Something to do with the cones or rods, which ever one isn't used for colour.
 
Originally posted by: Special K
Originally posted by: aphex
the longer the exposure, the more light you let in, hence the brighter image.
Click to expand...

Well this may sound like a dumb question then, but then why don't we all go blind after a few seconds? Light is constantly being let into our eyes. Shouldn't the image around us keep getting brighter and brighter?
Click to expand...

think of a camera as adding all the light it sees together where as the human eye does not. if that makes sense.
 
Originally posted by: alien42
Originally posted by: Special K
Originally posted by: aphex
the longer the exposure, the more light you let in, hence the brighter image.
Click to expand...

Well this may sound like a dumb question then, but then why don't we all go blind after a few seconds? Light is constantly being let into our eyes. Shouldn't the image around us keep getting brighter and brighter?
Click to expand...

think of a camera as adding all the light it sees together where as the human eye does not. if that makes sense.
Click to expand...

So then why doesn't an extremely fast shutter speed reproduce a scene exactly as the human eye would see it? Wouldn't an extremely fast shutter speed (given the right aperture size) be analgous to the human eye (in that it is just sampling the scene instead of adding all the light together)?

 
Originally posted by: Bootprint
Originally posted by: Special K
Originally posted by: aphex
the longer the exposure, the more light you let in, hence the brighter image.
Click to expand...

Well this may sound like a dumb question then, but then why don't we all go blind after a few seconds? Light is constantly being let into our eyes. Shouldn't the image around us keep getting brighter and brighter?
Click to expand...

Unrelated to this, but have you ever gone into a really dark room and flashed the flash. If you don't move your eyes you'll get a black and white image appearing after a second. Then if slowly fades away. Something to do with the cones or rods, which ever one isn't used for colour.
Click to expand...

or look at the sun for 10 minutes..

 
long exposure is for low light conditions where you have to leave the shutter open to get enough light to the sensor. but cheap consumer cams have tiny sensors that can't pickup much light..and so if you leave the shutter open for a long time it'll just get hideously noisy. you need a full size sensor of a digital slr with its higher isos.
 
Think about how film works and why it would be that way with film (makes sense). Digital just attempts to mimic film.

Also, think about what happens in low-light situations. If you have it in an automatic mode the camera will adjust to a longer exposure time to let enough light in
 
Originally posted by: Special K
Originally posted by: aphex
the longer the exposure, the more light you let in, hence the brighter image.
Click to expand...

Well this may sound like a dumb question then, but then why don't we all go blind after a few seconds? Light is constantly being let into our eyes. Shouldn't the image around us keep getting brighter and brighter?
Click to expand...

Your eye is not a camera, it's an eye. They don't work the same.
 
i'm pretty sure u've already noticed that u only really have a usable 100iso iwth that camera anyways right? bright out doors conditions are the only real place where you can be noise free.
 
think of it this way. in order to produce a normal photo, a sensor or film needs X amount of light. Long exposers increase the amount of light, small appertures reduce it. So you adjust those two in order to get X amount of lihgt and whatever effect you want.

the eye works in the same way (ie, it needs Y amount of light to produce a picture), but you can think of it as snapping a photo, sending it to the brain, erasing and repeating - ie it doesn't accumulate light the way a sensor or film would.
 
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