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I'm trying to simulate some motion blur caused by camera shake and object movements. I checked motion blur, and tried to customize the shutter speed of the camera to see the differences. In the real world, when you increase the camera shutter speed the output image would be darker with less blur, but in my experiments,increasing shutter speed has no effects on the brightness of the output image, but it causes less blur effect. It just makes no sense. Do you know why it behaves like that?

Shutter speed at 2.0

Shutter speed at 0.1

So it seems there is no advantage in slow shutter configuration which is not true in the real world.

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    $\begingroup$ I assume, you are using the "Shutter" Slider under the Motion Blur settings. As far as I understand it, this is only meant to affect the Motion Blur, but not the Exposure of your image. You can change that independently in the Scene Settings under "Color Management" - In 3D Graphics, we can "break" some of the rules we and our Tech have to obey in the real world. $\endgroup$ – michaelh Dec 17 '17 at 17:49
  • $\begingroup$ @michaelh Thanks for your response. I see your point but I actually want to see the differences in brighness and blur effect together caused by changing shutter speed. So when they are independant, I can't get result for my experiment. And my question is that is there a way to apply the dependancy. For example calculating the exposure based on shutter speed (while ISO and aperture are fixed) ? $\endgroup$ – Erfan Samieyan Dec 17 '17 at 21:17
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Unlike a real camera, in blender the brightness on the scene will not be affected by the shutter speed (or angle), or the f stop used by the camera.

If you want to change the overall brightness of a scene change the

Film->Exposure control.

enter image description here

There is no ISO, or photosensitive anything in blender as on a real camera. You can of course change the exposure of your scene to simulate the effect it would have with changes in exposure time.

If you think of a shutter shutter angle of 0.5 as the normal exposure, changing the value to 0.25 would result in an image that would be darker since it only captures half of the light than the normal exposure. To simulate the exposure change you would need to make your image darker by changing the Exposure in the film section to 0.5

Conversely, if you changed the shutter speed to a setting of 1.0, it would result in twice the brightness, so to simulate the change you would need to have an exposure value of 2.0.

There might be a way to create a script or a driver to do the computations for f stop, and shutter... but there is no built-in functionality for it by default.

Just to be clear, the term "exposure' in blender is quite different than it is in film or digital sensors. In cycles, the value of a given pixel is calculated by the amount of light being reflected by objects based both on the intensity of lights, the albedo (reflectance) and other characteristics of the material and other atmospheric factors (if you use volume scattering). The resulting values are then processed through the color transforms on the color management section, which will determine what the pixel's brightness and color will be. There is no notion of "exposure time" or "ISO" (in terms of sensitivity that would affect brightness).

The shutter speed option in blender is there to simulate motion blur (as pointed by @michaelh).

The F stop is there also to simulate the effects of depth of field.

Changing the values of either one of them will not affect the brightness of the rendered image.

If you want blender to behave like real camera in terms of exposure there is no automatic option for it, you will have to do it by hand (or through a script)

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  • $\begingroup$ Related links: Which Exposure control to use? and Does Cycles f-stop effect the brightness of the render $\endgroup$ – user1853 Dec 17 '17 at 18:05
  • $\begingroup$ You could also add to your answer that as michaelh mentioned Shutter speed is used exclusively to control the motion blur effect $\endgroup$ – Duarte Farrajota Ramos Dec 17 '17 at 18:16
  • $\begingroup$ @cegaton Thanks for the answer. Actually I saw this setting but it is effecting on the image independantly while I'm looking for the exposure caused by shutter speed. Do you think I can set the shutter speed and calculate the exposure (based on fixed ISO and aperture) in order to get the realistic result? $\endgroup$ – Erfan Samieyan Dec 17 '17 at 21:20
  • $\begingroup$ @ErfanSamieyan see edited answer $\endgroup$ – user1853 Dec 17 '17 at 21:33
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    $\begingroup$ @3pointedit in terms of noise, probably, but certainly not in terms of brightness $\endgroup$ – user1853 Dec 17 '17 at 22:51

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