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When a camera's shutter is opened, if light reaches the sensor instantaneously (speed of light = 300.000 km/s), why does shutter speed modify picture sharpness/detail? Why do pictures get darker with faster shutter speeds, and brighter with slower shutter speeds?

Our eyes are always opened (when we are awake), but images are not "overexposed".

(I think this could be more a physics question than a photo one)

  • How do you define "clarity?" Blur caused by motion (subject or camera motion)? Loss of detail caused by low signal to noise ratio (noise)? Distortion caused by rolling shutter effect? Something else? – Michael C Feb 8 '17 at 20:34
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why does shutter speed modify picture sharpness/detail? Why do pictures get darker with faster shutter speeds, and brighter with slower shutter speeds?

These things happen because the light sensor in the camera doesn't measure the intensity of light instantaneously, but rather measures all the light received during the entire exposure. You could say that the sensor accumulates or sums the light* for the duration of the exposure. Light is made up of discrete photons, and the longer the sensor is exposed, the more time there is for photons to strike the sensor.

If you want a mental model for how a sensor works, imagine putting a bucket outside when it's raining. If the rain's intensity remains constant, leaving the bucket there twice as long will result in twice as much water ending up in the bucket, right? Or, if the intensity of the rain doubles, you'd expect the bucket to fill up twice as fast. That bucket is like one photosite (i.e. one pixel) on a digital sensor, and the raindrops are like photons. The entire sensor is like an array of several million of those buckets, each measuring raindrops/photons in one particular spot.

So, faster shutter speeds mean shorter exposures, which means less time for movement of either the objects in the frame or the camera itself. Motion blur happens when an object in the frame moves relative to the camera, so that light from a given point on the object is recorded in more than one spot on the sensor. The shorter the exposure, the less movement there is, and the sharper the final image.

Similarly, longer exposures allow more time for light to accumulate on the sensor; each photosite will collect more photons and measure a larger value. Those larger values, taken together, create a brighter image. Just as with the rain, the measurement at each photosite is also influenced by the intensity -- brighter light causes the measured value at each point to increase more rapidly. So, if you want a brighter image, you have two options: increase the light's intensity or use a longer exposure. This is why aperture and shutter speed have an inverse relationship: the aperture controls the intensity of light reaching the sensor. If you want to use a shorter shutter speed without affecting the photo's exposure level, you can increase the aperture to let in more light; if you want to use a longer shutter speed, you can reduce the light with a smaller aperture.

*To be really clear about it, what the sensor really does is to accumulate the effect of the light. When a photon hits a photosite on a digital sensor, it creates a small electrical charge; the more photons, the larger the charge. After the shutter closes, the camera measures the stored charge at each photosite. Film works much the same way, except that the light causes a chemical reaction that increases with more light.

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    "If the speed of a raindrop is so fast, why does it matter how long I leave my bucket out for?" +1 – wedstrom Sep 19 '17 at 22:45
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No, it's a photography question alright. But I assume that by "clarity" you mean "sharpness", otherwise the question makes no sense.

If your object is 30 m away, light from it will reach the sensor in 100 ns (billionths of a second). That's several orders of magnitude faster than the shutter's speed, we can actually ignore the 100 ns, and say light arrives instantaneously.

Suppose you have an average shutter speed, say 1/60 of a second. That means that from the moment the shutter opens light from the object reaches the sensor, and it will continue to do so until the shutter closes 17 ms later. Now 17 ms isn't much, but with very fast motion, like a passing high speed train or racing car the scene may change in that time. At 300 km/h a train will move 1.4 m in 1/60 of a second. If the train's front's projection is at the 1000th pixel from the left when the shutter opens, it may have moved to the 1200th pixel from the left when the shutter closes, and you'll get a 200 pixel wide streak for all the train's positions in between.

That's what's called motion blur. Sometimes you want motion blur to give the viewer a sense of the train's speed, and then you'll use slower shutter times. If you move the camera along with the object while you take the picture you also get motion blur, but of a different kind: the train will be sharp, but the background will show motion blur.

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    @downvoters - You'll understand that I can't take downvotes in account if they aren't explained in comments. – stevenvh Feb 9 '17 at 17:39
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    There's been a LOT of unexplained downvotes to this question and several of the answers. – Michael C Feb 9 '17 at 19:55
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You can imagine light as an electromagnetic wave, but for this question I'll use its second "state" as a (humongous) set of particles - photons.

Why do pictures get darker with faster shutter speeds, and brighter with lower shutter speeds?

In a given period of time some amount of photons pass through the lens and excite parts of the semiconductor chip (pixels).
The level of excitement is proportional to the incident photon count and is represented by the brightness of the displayed pixel. If you double the shutter speed, the time of exposition halves and the brightness is also halved. If you halve the shutter speed you double the exposition time and double the resulting brightness.

Why does shutter speed modify picture sharpness/detail?

During the time each pixel gathers the photons hitting it. The camera and the scene are not in perfect still position. Photographer's hands shake slightly and the object in scene may move. This causes the light gathered in the chip to be (motion) blurred. The significance of the motion blur is proportional to the exposure time and inverse proportional to the shutter speed.
For faster shutter speeds you get darker images; to compensate this effect you have to open the aperture and/or increase (ISO) sensitivity.

  • Aperture: Opened aperture results in stronger aberrations and more shallow depth of focus.
  • ISO: Higher sensitivity results in brighter images. But the shorter shutter times enabled by higher sensitivity also result in lower signal-to-noise ratio which usually leads to higher noise.

Our eyes are always opened (when we are awake), but images are not "overexposed."

Our eyes have automatic aperture setting (iris) and our brains provide automatic ISO correction. That's why our eyes can be fooled :)
Look at your friend's eye when there is sunny day, you will see iris and small black dot. When you look at it in the dark night you will see small ring of iris and large black circle. The iris is automatically tuning the amount of light reaching your retina.
The iris also has its limits. If someone bursts flash in your eyes in the night you are blind for a while - your wide-opened iris couldn't close fast enough to accommodate to the fast change of light and your retina was overexposed. It then took some time for your iris to open wide again.

The signals the brain receives from the retina are also accommodated in its sensitivity to the incident light and to the scene. Try skiing with amber goggles for whole day. After you remove the goggles a blue thing will look green to you.
It also accommodates locally. Here you can see green dot between pink ones. Or you cannot? Another trick: Stare at inversed image for a long time and then look at white wall. You will see the original image.
Your eye and brain reduce automatically their sensitivity according to exposure and there is some delay between the change of light and change of sensitivity.

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The camera lens is designed to project an image of the outside world onto surface of an imaging chip inside and at the rear of the camera. However, a mechanical door called a shutter prevents the imaging light rays from playing on the imaging chip. To take a picture, the shutter is briefly opened and then closed. This act permits the image forming light rays contact the imaging chip.

On the surface of the imaging chip are millions of photo sites. Each receives light energy during the exposure and this energy is proportional in intensity and color to the actual vista. As the light rays play on these sites an electrical charge is induced. The amount of charge corresponds to the light intensities of the vista.

Nevertheless the charges are very feeble and require software in the camera amplify them to a usable level. The software also coverts each charge to a numerical (digital) value. The result is an image made up of a “paint by number” system.

Because the scene brightness is a variable, the duration of the exposure is adjustable. If the vista is dimly lighted, the exposure time will be increased to compensate. Conversely, if the scene is brightly lit, the exposure time will be shortened. The main reason that the shutter speed is adjusted in its duration is to allow time for the charge at each photo site to accumulate and become manageable.

The speed of light is so fantastically fast and the distance, vista-to-camera and the distance lens-to-image sensor is moot.

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It's about the duration of the source of light, not the speed of the light. If I speak a sentence it may take 15 seconds to say it. The word travel to your ears at the speed of sound. if I say the sentence faster each word gets to your ears at the same speed but the "sharpness" or clarity of the words changes as I speed up or slow down.

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The actual speed at which light travels is immaterial. The fact that it is not instantaneous is very important. Even though light travels very fast, the light from the subject or scene doesn't strike the sensor or film all at the same instant. Light reaches the camera from the subject in a stream of energy that is spread over a duration of time. For the time the shutter is open this stream of light is recorded in a photograph. If the scene changes over the course of the exposure, the shape of the stream of light reaching the camera during the exposure changes as well.

In physics there is often a phrase used to describe the way that light simultaneously demonstrates properties of both wave energy and particle energy: the duality of light. For the purpose of photography we normally treat light as a stream of photons flowing from the scene onto the sensor (or film). As they strike the sensor they are transformed into electrons in each pixel well that a photon strikes. As they strike film their energy results in chemical reactions to the grains of chemicals in the film's emulsion.

Why does shutter speed modify picture sharpness/detail?

The shutter time determines how long the stream of photons from the scene are allowed to strike the sensor. If things change position in the scene during the exposure duration then the light from the part of the scene that has moved will move across the surface of the sensor and fall on different pixels. If the camera itself is the source of the motion then the entire scene will shift and each point in the scene will fall on different pixels on the sensor. Whatever the source of the motion, the result is blur as the light from a single point in the scene is spread across multiple pixels. The longer the shutter is held open the greater the blur for the same rate of motion.

On the flip side of the same coin, the longer the shutter is held open the more light is captured in the photo. The more light that is captured by the sensor, the higher the proportion of electrons collected by the sensor from the light from the scene (we call this signal) will be to the electrons produced by the electronics of the camera that are also recorded along with the current from the sensor pixels. These stray electrons are what we call noise. Read noise is produced by the camera's electronics. Photo (shot) noise is produced by the random nature of light due to the duality of light. Those photon particles are travelling along a wave shaped path defined by the wavelength of each bit of light. The more signal (light) we have in proportion to the noise, the more detail we will be able to produce in our photograph. This is called the signal-to-noise ratio.

So a shorter shutter time minimizes the effect of motion but can lead to a loss of detail due to a poor signal-to-noise ratio. A longer shutter time increases the signal-to-noise ratio but can lead to a loss of detail due to motion blur.

Why do pictures get darker with faster shutter speeds, and brighter with slower shutter speeds?

Because the longer the shutter is held open, the more light is captured in the photograph. It is the same thing as turning a faucet on and off while holding a cup under the spigot. The longer the faucet is held open, the more water will be collected in the cup. The longer the shutter is held open, the more light particles (photons) will be collected by the sensor (or film).

Our eyes are always opened (when we are awake), but images are not "overexposed".

Again, the light is striking our eyes in a continuous stream, not in a singular instant. All of the light collected by our retinas over the course of a day, or a year, or our entire lifetime does not get transmitted to our brain in a single instant! The electrochemical signal from our eyes to our brains are continuously changing as the scene in front of our eyes change.


(Note: the below was written before the question above was significantly re-edited into its current form)

Light is electromagnetic energy. As such there are two components that must be measured with regard to a photograph: field strength and time duration. Field strength measure how strong the light is over a specific area. Time duration measures how long that field strength is maintained.

It is just the same as any other form of energy. If one were to apply a constant force against a body the body would accelerate. The longer that force is applied the longer the body will accelerate and the faster the body would be moving relative to its beginning state.

A piece of photographic film collects information about the energy falling upon it in the form of light. The longer the shutter is left open the more information is collected. If a shutter is left open twice as long it will collect twice as much information from that light assuming the strength of the light is constant.

The problem in photography is that the light is often not constant. As things in the world in front of the camera move the field strength of light over any particular point of the film or sensor changes. As long as the shutter is open it continues to collect information about the light falling upon each point of the film or sensor. If something in the view of the camera is moving the information about all of the positions it passed through during the time the shutter is open will be recorded. Instead of being recorded on the same spot on the film or sensor the image of the moving subject will be spread across the area over which it moves. This will result in blur. Even if nothing in front of the camera moves, if the camera itself moves the same thing will happen. Light from a particular spot in the scene will be spread over the area of the film or sensor upon which it falls as the camera is moved.

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It's not about the light's speed really, it is the amount of light that matters

It is also the same reason why it gets darker as the sun sets; light's speed is not really relevant in this case

Well, it's no different in photography!!

In a very dark scene, the number of photons could be so less that the pixels would barely collect any photons even at longer exposures

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There are too much to say if you wish details, you need to learn about lenses, see this:

https://www.youtube.com/watch?v=1YIvvXxsR5Y

To answer your question, the faster shutter speed prevents smearing of pixels on your sensor, resulting in better details and clarity. It's not the speed of light, but the freezing of motions that causes it.

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