Ever wonder what it looks like inside your DSLR when you trigger the shutter? So did the team over at the Slo Mo Guys and, luckily, they have a Phantom to record the action at 10,000 frames per second. In the video below, you can witness what your shutter looks like as it opens and closes at various frame rates. When watching the shutter fire in real time, it’s sometimes difficult to even notice a difference; however in slow motion you can really get good a good look at the mechanism.
Take a look:
As Gavin points out in the video, the DSLR he’s using as an example (Canon 7D) uses a rolling shutter for taking stills. But, in the second half of the video, he shows some pretty cool slow motion examples of how your shutter works when taking live video.
In the image above, the cork, which is destined to hit Dan in the face, hasn’t yet blasted him in the face yet. But, according to the corks shadow, the cork has already made contact. This artifact is the result of how your shutter works. It starts recording image data at the top of the photo then rolls down the frame, recording as it goes. This results in a very slight delay. In other words, the top section of your image is actually older than the bottom portion of the photo.
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This is certainly an awesome video~~
But it has got me thinking though….
1. At high shutter speed (in this case, say its 1/8000) the curtain creates a small opening for the light to go through. It starts at the top (or at the bottom) and stops on the other side. So is that the reason why we don’t use fast shutter speed for high speed photography-basically rolling shutter for still photo? I know they use long exposure and use the flash to freeze motion instead.
Also, when you are taking a photo with a CMOS sensor, does the exposure starts at the moment the curtain opens (global) or it acts as the rolling shutter that the active part of the sensor follows the opening of the curtains?
2. Let’s say we now use a CCD sensor instead of the CMOS, which uses globe shutter instead of rolling shutter. How does the fast shutter speed effect the exposure-Does the exposure starts at the moment the curtain starts to go down?
If you know the answers, please let me know~!
To answer:
1. Well practically yes, this is the reason why you can only use your flash with a certain, lower shutter speed than your max shutter speed. Flashes have super short burn times (much faster than 1/8000) so you would only be lighting a small part of the sensor as the small slit moves across the sensor. Your flash sync speed is basically the shortest shutter speed which still has the shutter completely open as in: no slit moving across but 1st curtain sailing alone to the bottom and just when it hits the bottom (or stops moving and thus showing the whole sensor) the 2nd curtain will start moving. So your whole sensor can take light from the flash.
Your exposure starts right when the first curtain begins to move (or a tiny little bit before that moment) and it will read the whole sensor for the duration of exposure. The curtains make sure that it will basically only read the part that is exposed to light as photons now can freely hit the sensor. For example you could theoretically even start counting the photons (“exposure” reading) a minute before the curtains open and end up with the same exposure as the path to the sensor is blocked as long as the shutter curtains are closed. The sensor readout is another problem and this is where things get confused easily.
Let’s get to it! With still images there is the following procedure: shutter is reset (read: all charge that is on the shutter will be zeroed); shutter opens; sensor is exposed; shutter cloeses; sensor is now charged with photons (actually electrical charge from converted photon energy, but let’s keep it simple); sensor is the read out line by line from top to bottom or vice versa. The charge on the sensor and each individual pixel will not change because there is no more light hitting it. Therefor the picture is basically set in stone. The camera could now take as much time as it wanted because there would be no chance to get skewed lines.
With video it is a different thing: shutter is reset; shutter opens; stays open!; sensor gets charged all the time; sensor is read line by line, as one line is read it will be reset, leaving it to accumulate new charge; while that happens all the other lines are also gathering new charge, thus the “scene” on the sensor changes. You get rolling shutter effect…
2. Same as with the CMOS sensor. Only difference is when using the “electronic shutter” needed for video. You wouldn’t get the rolling shutter effect because you read and reset the whole sensore at a time.
This is it, basically. I am no scientist and my technical knowledge is also not fundet but I’m pretty sure this is how it works. Feel free to contribute and correct.
awesome!