Camera Motion Blur Shutter Curve
You can use the shutter curve controls (Corona 9 and newer) available in the Corona Camera object or tag, to modify the motion blur (Max | C4D) effect and its appearance. To understand what the shutter curve is, let's first find out how a camera shutter works:
In a real-life camera, the shutter takes some time to open, stays open for some time and closes during another short period of time. All of this affects how the blur from moving objects, including the camera, is captured.
To understand this better, let's create a very simple scene with a sphere and a Corona Camera, animate it from frame 0 to 30 (In and Out tangents for keyframes set to linear, for better visual explanation), enable Motion Blur in the Corona Camera settings for both the Camera and the Geometry, and set the Shutter speed to 1.0. This means the shutter is open for 1/1.0 = 1.0 seconds. This is to make sure that all frames are taken into consideration from the animated 0-30 frames range (this also depends on the current framerate; the default is NTFS, which is 29.97 fps, suitable for this example).
The 3ds Max scene can be downloaded here, and the Cinema 4D scene file can be found here.
If we set the active keyframe at 15, this is what the render will look like with the settings mentioned above:
By default, Corona Camera's shutter is "open" all the time. This results in uniform distribution for the perceived brightness of Motion Blur, as can be seen above.
But, as mentioned before, real-life camera shutters take some time to open, stay open for a bit (shutter speed/duration) and take some time again to close. This can be represented by a simplified shutter "openness" - time graph as below:
Hence, when the shutter is not fully open, the brightness of the image and the motion blur at that moment of time is lower.
The virtual Corona Camera's shutter "openness" on the graph can be represented as below with the green dashed line. Fully "open" all the time (computer graphics let us do so), so there is no time spent on any opening and closing.
The uniform motion blur effect (brightness) distribution in the render above is because of the default, always-open shutter in the Corona Camera. Note again, that the distribution is uniform also because of how the sphere is animated - fixed distance between every next frame - and we did this for better visual understanding and to avoid any possible confusion.
Enabling the shutter curve in the Corona Camera can alter how the motion blur (its perceived brightness at a given frame) is distributed, depending on how much time it takes for the shutter to open and close.
Here is what the motion blur would look like with a linear shutter curve - closed at the beginning (darker) and gradually opening during the whole shutter duration (brighter):
Here is what the motion blur would look like if the shutter was open and gradually closing during the shutter duration:
Having the shutter completely open all the time will result in the same render as using no shutter curve:
Let's simulate the real-world camera shutter curve now, similar to the graphs shown above:
The effect now fades in and out, giving a more natural look of motion blur like the one in real photos.
You can also draw some crazy, non-realistic shutter curves for a more artistic motion blur effect: