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You have control over how many intermediate frames After Effects creates for each frame of your animation, basically how smooth your blur is. To get access to that, you need to open Composition > Composition Settings, then go underneath the Advanced tab. That's where the Motion Blur settings reside. There are two sets of parameters: number of samples and shutter angle and phase. Motion Blur is not defined by the shutter speed in After Effects. Instead it is defined by the Shutter Angle.
This is borrowed from film-camera terminology. One entire film frame is considered to be 360 degrees. The rotating film shutter in a mechanical film camera typically exposes that film for only half the duration of a frame or 180 degrees of the 360 degree rotation that the film shutter goes through. That's why After Effects uses degrees to define how long the shutter is open for each frame and therefore how long the blur trail is going to be.
180 as I mentioned is a default for filmic look. You can see how that looks here. With After Effects, you can go ahead and reduce it to a smaller number. As I go small, you see the blur is much less. That's because the imaginary shutter is open for a smaller percentage of time during the frame. Or you can increase it. In After Effects you can actually beyond 360 degrees, beyond one frame of duration to create unnaturally long blurs. By the way, this preview we are seeing is courtesy this brand-new Preview switch, which was added in After Effects CS5.
Before After Effects CS5, you actually had to close this dialog to see the results of changing these numbers. In CS5, you now get a more or less live preview. You have to release the mouse and then the screen updates. In After Effects, you can go ahead and increase the Shutter Angle to two whole frames. 720 degrees . Now close this for a second. Step through and you can see what this looks like. The frames now actually overlap the blur so long for a very cool, surreal unnatural look out. I will open up Composition Settings again and go back underneath the Advanced tab.
I am going to set this temporarily back to 180 degrees. Don't press Enter. That would close this dialog. Just click off to accept that new value. Shutter Phase has to do with the point in time where After Effects starts calculating these additional frames. If I set Shutter Phase to 0 degrees, After Effects will start at the current time and then start creating these intermediate frames to bind together from the current time forward.
However, that's unnatural. It creates a blur that basically leaves your animation. In reality, blur seems to be centered around the current time. Therefore, the new default in After Effects these days is to set this to -90 degrees. In other words, half of the default Shutter Angle. When you do that, you have backed up the calculation of Motion Blur to start half of the entire Motion Blur duration before the current time and that extends to half of the Motion Blur duration after the current time.
Whenever you edit Shutter Angle, you want to then go set Shutter Phase to -180 of whatever you chose for Shutter Angle. Now, of course, you can do funny things with these numbers. You can go ahead and make these artificially strange numbers to go ahead and create special effects, a blur that seems to lag or lead the object. This becomes really fun if you happened to have more than one copy of the same layer. Blur them and put different shutter angles on different copies of the layers. Some will lead, some will lag.
You can create some kind of fun special effects that way. The other half of the equation is how many samples are used to calculate this blurred image? How many intermediate frames are calculated? Internally After Effects breaks layers down into two different camps, basically ones that are easy to calculate and ones that are harder to calculate. Most frames such as most 2D animations of layers, objects, et cetera, are easy to calculate and that's where the second number Adaptive Sample Limit comes in.
This tells After Effects this is the maximum number of frames I want you to calculate to create blur for these easy layers. You can go ahead and use fewer if it's moving more slowly, but don't ever use more than this. Let's put in there in case you have a slow computer. You can reduce this number all the way down to 64. In reality, I found the Adaptive Sample Limit renders pretty quickly. After Effects is pretty smart about calculating how many frames it actually needs to use. I tend to leave this at its maximum value, and then leave it to After Effects to use as many as it needs to create smooth animations.
Then there is the harder to calculate layers. 3D layers take more calculation time. Also shape layers in After Effects. And other certain effects take more calculation time and After Effects is concerned about using this many samples to calculate the blur. That's why those examples, those types of layers, get separate samples per frame number, such as 3D layers. This is not adaptive at all in this case. You set precisely how many samples should be calculated. 16 used to be kind like to default in older versions of After Effects. 8 and under to my eye is strobey in most situations.
I would use a minimum of 12. But frankly again, After Effects computers are so fast these days. Most of the time, I leave these at its maximum value for the default and only if I am having rendering speed issues, do I then play with reducing this. Or if I am trying to create special effects like strobing, I might reduce this. Speaking of Special Effects like Strobing, let me show me what strobing might look like. I am going to temporarily set this Adaptive Sample Limit down to as minimum of 64 and I am going to speed up our animation to be unnaturally fast.
I will put the time right here in the middle. It's kind of hard to see what's going on here over the background. So I am going to temporarily turn off our background layer and set the background color to be white to get a better idea of what's going on here. I deselect the layer. Now you can see the strobing. Now you can see the individual copies of the layer that are being calculated at different points in time and merged together to create this Motion Blur effect. In this case, with this extremely fast motion, 64 samples is not enough.
So that's a case where I might go back in my Composition Settings, Command+K or Ctrl+K as the shortcut. Go to Advanced tab, increase the sample limit and see how much smoother that is, as After Effects uses as many samples as it needs, up to 256 to create a smooth looking blur. I will close out of this. Now Motion Blur is used to be a real render hit back in earlier versions of After Effects. It is much more efficient now. So I don't worry about it as much I used to. But you do need to remember the defaults to off for all layers.
So you need to turn it on for anything that's moving fast that you want to smooth out. If you find that it is bogging down your comp, particularly to say 3D layers or some effects, you would always turn off the preview and look at things unblurred and let me turn it on just to check your work. What's important is having it turned on for the layers that you wished to be blurred when it comes render time.
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