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I was checking out the working principle behind a camera recently. I was amazed after learning how you can use slow shutter speed settings to take shots like this:

enter image description here

So, if I set the shutter speed to say, an hour, will the camera keep collecting light as long as the shutter stays opened? How does it create the final image?

For example: I don't see any traffic at all in the final image but the lights only. So, what factors decide that image?

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So, If I set the shutter speed to say, an hour. Will the camera keep collecting light as long as the shutter stays opened?

Yes. But an hour is quite a bit longer than you'd typically want unless you take steps to drastically limit the amount of light entering the camera, like using a very dark neutral density filter.

How does it create the final image?

Same as for any other exposure: the sensor is a large array of photosites, where each photosite corresponds to a pixel in the final image, and each photosite accumulates a voltage as photons strike it. After the exposure, the voltage at each site is read, and that set of values is the image.

I don't see any traffic at all in the final image but the lights only. So, what factors decide that image?

You don't see cars because cars don't emit any light, aside from their headlights and tail lights (which you do see). Cars often do reflect light from other sources, but there aren't any light sources in the image that are close enough to be reflected by the cars.

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    Is there a limit to how much light can be accumulated by photosites in sensor e.g. like a memory limit? Also, Is there a chance that the gathered light information from photosites starts to leak into other sites at some point and the final image will come out blurry? – cpx Aug 22 '16 at 1:38
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    There's absolutely a limit. You see it every time you overexposed an image and the detail in the overexposed part of the scene is lost. I can't say about leakage, but to the extent that it might happen it's a weakness if the sensor. – Caleb Aug 22 '16 at 2:03
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The exposure is an accumulation of light energy. A correct exposure occurs when the accumulated light energy is sufficient to make a good image. Too little and the resulting image will weak with lack of detail (under-exposure) Too much and the rescuing image will be over-exposed thus the tones will be distorted. A good image results when the exposure is just right.

Now think about it, the exposing light is accumulating all the time the shutter is open. This is true regardless of the exposure time. If we use a super-fast shutter we must counter the short time the exposing light is allowed to play on the image sensor. We do this by using a fast lens. This is a large aperture that is able to act like a funnel in that it gathers vast amounts of exposing light energy. If we use a super long exposure, likely we will over-expose. We resort to using a tiny lens aperture or limit the light that transverses the lens by means of a dark filter (neutral density) that acts like sunglass for the camera.

We can use the exposure time to do amazing imagery. If we set the camera to a super-fast shutter, the resulting picture will be super under-exposed. Suppose we make a series exposures using super-fast shutters speeds. What happens is, a shot of a busy city street will look like a ghost town. Only motionless objects like buildings and parked cars will image. Moving objects will not record. This is the way they show an empty city in a science fiction movie. The invers is to use a super long exposure. Objects that are moving will not record because the lens aperture is set supper small. However the headlights and taillights of the cars are bright and they do record.

It’s neat to work with extrema shutter setting. We can stop a bullet in flight with a fast exposure or we make time laps movies and watch flowers growing and blossoming. It’s lots of fun.

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Shutter speed is simply the time between when the shutter opens until it closes. The camera does not do anything different if the shutter speed is set to 1/30s or 30s, except in some cases apply different noise-reduction since longer shutter-speeds can show more noise.

Modern cameras have a fixed shutter-speed range, typically 1/4000s-30s, but some go both faster and slower, up to 1/32000s until 250s, depending on the model. To get longer shutter-speeds, DSLRs and a number of mirrorless models offer a Bulb mode which lets the user control when the shutter is open and closed, thereby creating longer shutter-speeds. There is still usually a limit though, sometimes in minutes and sometimes in hours, mostly for DSLRs.

Light gets collected during the entire exposure which is why long exposures are taken at night usually. One can do so during the day by using an ND filter which reduces the amount of light that reaches the sensor.

The key as to what appears in the image is to understand exposure. Car lights are much brighter, so they register on the sensor quickly. Car bodies are comparatively darker which is why they look as if they were not there. None of them sent enough light to register on the sensor. Should you have had a slow moving vehicle, it might have registered partially and would appear ghostly.

  • Since longer shutter speeds what? :) – mattdm Aug 22 '16 at 2:14
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When using a long shutter speed you have to adjust your way of thinking. When the shutter is opened everything gets recorded, but when something moves, whats behind it will leave its mark over the previous imprint.

This is why the traffic gets washed out, it moves too fast sso the imprint is too small and gets overwritten by the road. The headlights are so bright they make a huge imprint and because of this they kind of make it impossible to overwrite.

Hope this makes sense : )

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