Rolling Shutter VS FPS confusion

Started 2 months ago | Discussions thread
Erik Kaffehr
Erik Kaffehr Veteran Member • Posts: 4,645
Re: Rolling Shutter VS FPS confusion
2

Currantos wrote:

So I am a little confused.

Silent shooting/electronic shutter is supposed to have worse/worst rolling shutter effect than mechanical. So the recommendation is to avoid it for fast moving objects.

For fast moving objects the recommendation is to use highest possible FPS to catch that best moment in an action sequence. Electronic shutter allows for higher FPS so one would switch to that.

? Conflict?

So which is it? Avoid it due to rolling shutter or use it to get the FPS?

Natural light situation?

Continuous stage/regular lights?

Advice on shooting athletes or a dance performance on stage? I want high FPS but I don't want rolling shutter. Looking for advice from mirrorless users that can choose from full mechanical, full electronic or EFCS which to use when.

Hi,

Rolling shutter is coming from the focal plane exposed at different time at top and bottom of the image.

At short exposures, faster than the X-sync time, the image is not exposed at the same time, but through a slit that moves across the frame, that movement is the cause of the rolling shutter.

With modern cameras we often have three options:

  • The old mechanical shutter. Shutter speed is closely related to X-sync time. So, with 1/300s X-sync speed the shutter moves across the frame in 3 ms, or so. 
  • Resetting a CMOS sensor is very fast. So reset of the sensor can be used instead of a fast moving mechanical shutter blade. This is used with Electronic First Shutter Curtain. But exposure is terminated by reading out the sensor, which is slow. So, EFCS uses the rear curtain to terminate exposure. It has two advantages on mirrorless. The first one is that exposure starts with shutter open. That gives short response times and zero vibration.
  • Electronic shutter uses linewise sensor reset to start exposure, but exposure is terminated by linewise readout. Reading the sensor output is a slow process, so scan time for ES is normally very long.

A camera like the Sony A9 has a sensor with very fast readout, using parallel processing. So the shutter may scan the sensor area in say 10 ms. That means it can be used in many situations, with small rolling shutter effects.
The ideal solution is a global shutter, that exposes the whole sensor at the same time. Leaf shutters work that way, but they have some limitations, too.
An electrical global shutter is quite possible, but it needs to be able to shift the charge from the photodiodes to a storage area in a very short time frame. In practice it means halving the dynamic range of the sensor and increasing noise with 41%. So it can be done, but it comes at a price.

In the end, for silent or fast shooting you need very fast readout, or a global shutter.

Leaf shutters have a great advantage with electronic flash, especially if the electronic flash is short duration. With short duration flash all the output of the flash can pass through the shutter while it is fully open. Most flashes are not fast enough to make full use of a modern leaf shutter.
HSS prolongs flash exposure time to perhaps 10ms by pulsating the flash. So if you shoot at 1/1000s you will waste 90% of the flash output.
Global shutters would be the ideal solution, but it seems that vendors are not willing to make the image quality sacrifice needed for an improvement that doesn't help the many.
Best regards

Erik

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Erik Kaffehr
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