Why are exposure times so long if light travels so fast?

[snapsnap1973]

How come the camera needs say even 1 second to capture light if light travels at 186,000 miles a second? Also, I'm having trouble understanding WHY if the shutter is open longer (say 4 seconds) why there would be motion blur.

For example, say for the first second the scene is recorded by the sensor and then the 2nd second a bird flys by. How do those 2 events get recorded into one image and WHAT determines WHAT gets recorded at what time?

For example, say there was a red house behind the bird, but before the bird flew by in the 2nd second (of 4 seconds) the entire red house was recorded somewhere (where?) and then the bird flys in front of the red house. What time frame in those 4 seconds determines if the bird blocks out part of the red house as a blur maybe?

I mean say for the first second there's NO BIRD and then it flys by in the 2nd second and then he's gone in the 3rd second and 4th second of exposure. Does the bird appear in the final picture as a blur?

This is kind of confusing.

I'm especially though having trouble with the fact that the shutter being open a little longer would allow more light in. If light travels so quickly why wouldn't it all end up at the sensor in a fraction of a second and just be recorded?

How would allowing more light in over a period of time alter the picture?

Say, I draw my blinds in the morning. The light just shoots in and lights up the room instantaneously and everything is visible just as it is. It doesn't become MORE visible in a matter of seconds unless of course the sun gets higher or a cloud moves out of the way, etc.

This is got me a little irritated thinking about it.

 

[KmH]

Because of the inverse square law.
The longer a shutter is open, the more chance moving objects will be blurred in a photo because the camera is not also moving.

Motion blur can be mitigated by using a technique known as panning. Panning allows the main subject to be in focus but the background is blurred.

 

[Vtec44]

Read up on exposure triangle (ISO, shutter speed, aperture) and try to understand the function of each in relation to exposure, flash, and motion.

The speed of light has nothing to do with how much of that light can be captured by the camera sensor. If I'm in a dark room completely closed with no windows, speed of light has no effect on the brightness of that room until I have a window, and open it, to let more light into the room.

 

[480sparky]

It don't matter HOW fast light travels. It's a matter of how MUCH light the camera receives.

Motion blur occurs because the subject MOVES across the field of view in relation to the camera.

 

[bratkinson]

It don't matter HOW fast light travels. It's a matter of how MUCH light the camera receives.

^^^^^^^^
That!

I think of exposure like filling a swimming pool...The diameter of a soda straw, garden hose, fire hose are like the aperture of a lens. How long the water runs is shutter speed. The water pressure used...dribble to city water main pressure(GPM) is how slow or fast the sensor reacts to the light (sort of). Needless to say, using a soda straw at 500 PSI will shoot a stream a good distance (if it doesn't burst), but it will still take nearly forever to fill the pool. Whereas a fire hose at 500PSI will fill it in an hour or so.

 

[JustJazzie]

Think of your sensor as a "cup" the longer the faucet stays open, the more "water" (light) reaches the vessel. close the faucet and no more water (light) enters.

You need a certian amount of light (correct exposure) to "fill" your sensor . Too much light and it "over flows" (turns white) Too little light and it's not "full" (underexposed or black frame results)

Your aperture is how "open" your "faucet" is. Your shutter speed is the amount of time you allow light to "fill" the "cup" (sensor). Your Iso is how large of a "cup" you need to fill.

Probably a terrible analogy, but it works.

 

[Designer]

The camera sensor starts adding up all the light "chunks" (photons) that fall upon it for as long as the shutter is open. If the shutter is open for a longer time, more of those photons are accumulated, giving you a brighter exposure.

Not all sensors can grab photons at the same rate. Some sensors are more sensitive than others, and you can shift the sensitivity of your sensor by increasing the ISO number.

 

[qleak]

Woo hoo that's quite a few questions:

How come the camera needs say even 1 second to capture light if light travels at 186,000 miles a second?

Photographs are on a recording medium. It's usually either a sensor or film. The photons (light) carry energy. It takes a certain amount of energy to produce a picture.

Also, I'm having trouble understanding WHY if the shutter is open longer (say 4 seconds) why there would be motion blur?

When a subject is moving, the light can be transmitted to different parts of the sensor or film. If enough energy from the same place hits on different parts of the sensor the result will be blur or multiple images depending on if the light is constant or not.

For example, say for the first second the scene is recorded by the sensor and then the 2nd second a bird flys by. How do those 2 events get recorded into one image and WHAT determines WHAT gets recorded at what time?

For example, say there was a red house behind the bird, but before the bird flew by in the 2nd second (of 4 seconds) the entire red house was recorded somewhere (where?) and then the bird flys in front of the red house. What time frame in those 4 seconds determines if the bird blocks out part of the red house as a blur maybe?

If the bird is flying and the amount of energy transmitted by the light off the bird is low enough it will not appear at all. If a strobe light fires as the bird flys across you may get multiple sharp birds. If you shine a flashlight on the bird as it flies across it will be blur.

I mean say for the first second there's NO BIRD and then it flys by in the 2nd second and then he's gone in the 3rd second and 4th second of exposure. Does the bird appear in the final picture as a blur?

It depends.

I'm especially though having trouble with the fact that the shutter being open a little longer would allow more light in. If light travels so quickly why wouldn't it all end up at the sensor in a fraction of a second and just be recorded?

Speed doesn't mean there is more of it. Think about a motorway, 10 cars can travel down the road at 50mph or 1 car at 50mph. The 10 cars will have more energy than the 1 car. So the important thing for exposure is the amount of light reaching the camera, not the speed of it.

How would allowing more light in over a period of time alter the picture?

It's like taking several smaller pictures and adding them together. Here's how this has been done:
Faking an ND Filter for Long Exposure Photography - DIY Photography

 

[photoguy99]

Light flows in on to whatever. It is then for the most part absorbed. Some reflects , usually to be absorbed by something else.

The more intense the light, the more is absorbed, so light never 'piles up'.

Your sensor absorbs light just like anything else. If there's not much light falling on it, it's not absorbing much. If there's more it is.

Imagine you're drawing a picture by pouring ink, very carefully, on a sheet of blotter paper. If you have a microscopic pitcher of ink, it'll take a while. If you have a great big bucket, your image will appear pretty fast.

The important difference between light and ink here is that the light will eventually... evaporate? Vanish? It goes away after a while, unlike ink.

 

[Trever1t]

Yes, if you have a 4 second exposure (and it's proper for the conditions) and bird was in the frame for one second...it WOULD BE a blurr...if we even saw it.

 

[sashbar]

Light is a flow of tiny "packets" called photons and exhibits properties of both waves and particles hence it's wave-particle duality. A sensor needs to collect a certain amount of photons for the analogue signal to reach a certain strength. In low light where the number of emitted and absorbed photons is small, it takes time. It does not matter how fast are photons, what matters is the cumulative energy collected by a sensor i.e. the number of photons.

 

[Designer]

In case anyone is interested in how light theory is changing, I found this:

new theory of light

 

[snapsnap1973]

Ok thanks all that helped a little bit, BUT I'm still having some trouble understaning something.

Say, we've got a scene (hipothetically) with NOTHING moving in it and no more or less light entering it, it's stagnant. How would it make a difference in the final picture if I left the shutter open for 1/1000 of a second or 30 seconds??

I mean why the heck would a sensor need a whole 30 seconds to capture an image?? I really don't get it. I mean if I wanna take a picture of a blue mailbox, why would I want to leave the shutter open for 30 seconds instead of just snapping the same exact image in 1/1000 of a second?

Also, during a whole 30 seconds, what's going on with the camera's sensor? What info is being saved and where? Is that entire 30 seconds getting written to a file on the SD Card and isn't the stuff that happened in the first 10 seconds being overwritten anyway?

For example, what if I press the shutter and it's set to 30 seonds and a lightning bolt strikes in the first second. What happens to that lightning bolt image? If it's not recorded in the final image then what's the point of having the shutter open for a whole 30 seconds?

This is confusing. I can't find any info on the web to read up on it either. All the article just repeat the whole birds eye view of the process, so that it all remains a mystery, but I really want to understand it.

I understand ISO and APERTURE that I can grasp, but I just don't get the shutter speed thing and how with todays fast sensors 30 seconds would do something that couldn't be done in a fraction of a second.

UPDATE** Darned after posting this I found this article. Looks like someone was just as confused as I was:
Digital camera sensors and long exposures - Electrical Engineering Stack Exchange

sorry

 

[TCampbell]

Image sensors have something called "wells" which collect light. You can think of each of these as if it's a tiny little bucket. The bucket collects "photons" of light... but you can think of these like drops of water.

If I have a bucket which can hold precisely 100 drops of water (at which point it would be "full"), then a bucket which hasn't collected any drops while the shutter is open will render as "black" and a bucket which received the full 100 drops would render as "white". A bucket which receives somewhere between 1 to 99 drops would be rendered as some shade of gray.

But note my bucket has a limit... if I try to collect 101 drops in a bucket, I can't do it... at 100 drops my bucket is "full" and the 101st drop will spill over. This means it is not possible for my "bucket" to sense whether there were 101 drops... vs. 200 drops...vs. 2000 drops because 100 is the limit. That's all it can register. This limit is referred to as the "well depth".

Suppose I have a camera that has a well-depth that allows it to collect 1000 drops of water... instead of 100. That sensor has greater "dynamic range". It can detect more gradients between black and white. It is more difficult to over-flow the wells on that sensor.

The drops of water (photons) are all received at the speed of light. But some areas are more saturated with photons than other areas. Think of dark areas as having very few photons (like it's barely even sprinkling). Think of bright areas as having many photons (it's a torrential downpour of "rain drops".)

When you meter an exposure, you are trying to determine what settings to use so that the sensor will become "about half full" of light in those "buckets". This way... you have lots of wiggle room for dark areas (where you get hardly any drops of water) and bright areas (where you get lots of drops of water) and you can do this without clipping or over-flowing the limits of the "bucket".

"Wells" and "well depth" are usually not listed as specs on a typical camera. Cameras used for astro-imaging usually will list this information. But all digital cameras have the concept whether the manufacturer lists it or not.

 

[sashbar]

Ok thanks all that helped a little bit, BUT I'm still having some trouble understaning something.

Say, we've got a scene (hipothetically) with NOTHING moving in it and no more or less light entering it, it's stagnant. How would it make a difference in the final picture if I left the shutter open for 1/1000 of a second or 30 seconds??

I mean why the heck would a sensor need a whole 30 seconds to capture an image?? I really don't get it. I mean if I wanna take a picture of a blue mailbox, why would I want to leave the shutter open for 30 seconds instead of just snapping the same exact image in 1/1000 of a second?

Also, during a whole 30 seconds, what's going on with the camera's sensor? What info is being saved and where? Is that entire 30 seconds getting written to a file on the SD Card and isn't the stuff that happened in the first 10 seconds being overwritten anyway?

For example, what if I press the shutter and it's set to 30 seonds and a lightning bolt strikes in the first second. What happens to that lightning bolt image? If it's not recorded in the final image then what's the point of having the shutter open for a whole 30 seconds?

This is confusing. I can't find any info on the web to read up on it either. All the article just repeat the whole birds eye view of the process, so that it all remains a mystery, but I really want to understand it.

I understand ISO and APERTURE that I can grasp, but I just don't get the shutter speed thing and how with todays fast sensors 30 seconds would do something that couldn't be done in a fraction of a second.

UPDATE** Darned after posting this I found this article. Looks like someone was just as confused as I was:
Digital camera sensors and long exposures - Electrical Engineering Stack Exchange

sorry

The sensor is collecting photons emitting from the blue box for the whole 30 second. You are completely wrong thinking that it takes the initial moment to capture an image. Imagine a water running from a tap. You need to hold a cup for a certain time to fill it with water. You do not fill the cup in the first 1/1000 of a second. The stronger the flow, the less time you need. It is exactly the same with light - we just have photons instead of water molecules. The brighter is the scene the stronger is the flow of photons and the less time you need to "fill the sensor". If you shoot in the dark, just imagine the tab that is just dripping. That is how long you will hold the cup, and that is how slow will be your shutter speed.
Now, imagine a tap that is moving around while you hold the cup. The water is being splashed all over the sink, your shirt, your trousers etc. Do you feel your trousers are wet? That is where the blur comes from.