Hi, I'd like to understand what does the shutter speed of let's say 1/4000s do. I thought it takes a picture in a fraction of 1/4000 second. For example, 1/15 leaves the sensor 'open' for 1/15s. But apparently this is not the case with higher shutter speeds. I can not use speedlight with shutter faster than 1/250s on my camera (DSLR - EOS 90D). When I use external flashlight I can use the 'hight speed sync' (thant's how canon call it at least). I've read how this work and it seems that the curtains are not fast enough to fully open in less than 1/250s. So for higher speeds the rear curtain will start closing before the first curtain is fully opened (doesn't matter whether speeedlight is used or not). If so, then I cannot make sense of it. It would mean that I can not take a picture faster than 1/250 anyway because the narrow line of light created by the space between the two curtains is moving from top to the bottom of the sensor for 1/250s and the actual shutter speed only determines how narrow the line is. But that is clearly not true - I've shot few pictures of flying arrow for example that only show the object at 1/1000 and faster. Also, if the curtain takes 1/250s to open but the space between front and rear is too thin (1/4000 or faster) that would introduce some serios rolling shutter effect - much worse than using electronic shutter at all (on my camera i can ake 1/8000 with mechanical but 1/16000 with electronic). But again, this clearly isn't true. Could you please explain me how it's possible to take pictures with that speed? Thanks
What you have read is correct, it is your thinking that is wrong. Stop thinking about the whole frame and think about each individual pixel of the sensor. Each pixel on the sensor will be exposed to light for the time that the shutter is set at as the slit passes it. This means that the pixels are exposed sequentially the overall time between the first and the last will be greater than the stated shutter speed. The speed you quoted of 1/250 is the fastest that the mechanism can work at due to its physical limitations, hence the need to reduce the slit width. This does give a certain amount of distortion in the final image but unless it is of a subject that is moving very fast then it isn't noticable. The rolling shutter effect only applies to electronic shutters where the information is written/read from the sensor pixels in sequential order and although similar the actual operation varies between different cameras and therefore can require a different explanation.
Ok, so just to be sure I understand it. This means that mechanical shutter is basically the same (in terms of rolling shutter effect) as the electronic one if the electronic can read the whole sensor in about 1/250s (4ms). Is that right? Thanks
It isn't quite that straightforward with an electronic shutter because the information can be read one pixel at a time either vertically or horizontally and even in parallel in groups of pixels, it just depends on how it was designed and what type of sensor the camera has. There is also the added complication where exposure can be started electronically and ended mechanically or vice versa. The term "rolling shutter" is just another of those phrases that a lot of people are using but few people understand, it just means distortion of a moving object in the final image and any type of shutter can cause this.
