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Now another handy feature of Lens Blur is the ability to use a Depth Map to determine what sections of the image to blur. Let's open up another project of a scene that was rendered from a 3D program and I have also rendered a separate depth map that shows the distance of various objects from the camera. White is closer, black is further away. I will select the image itself, not the depth map, and apply Effect > Lens Blur. I'll crank up the blur a little bit, so you can see what's going on, took it up 20 pixels.
And for my Depth Map I will select my special render from my 3D program. This will now cause some objects to appear sharply in focus, in this case the background, another objects to appear out of focus. In other words that grayscale depth map determines what is in or out of focus. I can invert the Depth Map, so instead close objects are in focus and far away objects are out of focus. I can also play around with the Blur Focal Distance. What this parameter does is it determines what values of gray are in focus and then values on either side will go out of focus. For example, if I set this to 120, you'll see that these buildings in the back are in focus, while things in the foreground, or things in the very back are out of focus. If I go ahead and move the focus a bit closer, 180, you'll see that the focus moves so that these objects are now in focus, the closest once are slightly out of focus, and objects behind are also out of focus.
So again, this allows you to add real world camera effects to 3D renders where normally everything would be in perfect focus. Let's go ahead and look at another project. This is another case of a bit more fanciful 3D render. Again I have objects that are rendered in 3D and I have rendered a separate depth map that shows what's close, such as the leading edges of these screens and what's far away. The darker the pixel, the further away it is. I have already got Lens Blur applied, I will enable it, and I'll go ahead and calculate a RAM preview to show you what's like to animate the Blur Focal Distance over the length of this composition. In essence, I'll be pulling focus from near objects to far objects.
Now Lens Blur takes a while to render, so I am going to go ahead and jump ahead to after this render is done. And here is a RAM preview of animating the Blur Focal Distance so objects close are initially in focus and then moves to objects far away being in focus. Another useful application of the Lens Blur effect. Now you might not always have a separate depth map for your scene. It is possible to create a depth map using a Lens Blur. I am going to go back to our first project, where we have this night sky scene, and you notice that the whole scene is blurry. The blue section as well as the stars. This is with Lens Blur turned off.
We can go ahead and pick a layer to be its own depth map. And what you want to do is pick whatever is going to be the best channel to work for depth. So I have my effect turned off, I'll go to my Comp panel and I want to find what color channel provides the greatest contrast between objects that are supposed to be out focus, the stars, and objects are supposed to be in focus such as this blue haze. I'll go ahead and look at the Red channel and it shows me a lot of contrast. The sky drops out to black, all the stars are white. Look at other channels for contrast.
Pick the Green. Hmmm, fair. Blue, almost no contrast at all. That won't be useful at all. The Red channel is going to be my best map. I'll go back to RGB, turn on Lens Blur, choose from my Depth Map Channel the Red channel and crank up my Iris Radius a little bit on my stars. We'll now see that the blue haze is remained in focus, but the stars have gone out of focus. I can go ahead and play around with things such as Specular Threshold to pump up the stars in the final render. Lens Blur off, Lens Blur on, and it does take a moment to render.
Now these show some more realistic applications of the Lens Blur effect. You can also use it for interesting graphical looks. Go watch the movie on Compound Blur. You can do all those same things with Lens Blur, except for Lens Blur is going to look better. And that's a quick overview of Lens Blur. It's a very versatile effect and is particularly useful for visual effects work if you are trying to make scenes look a bit more realistic, particularly ones rendered in 3D. It does take some time to render, and it does take some time to tweak and master, but if you are trying to get perfection, I think it's a good tool to use. And if nothing else, it's a much better version of a Compound Blur effect. So rather than using Compound Blur, you might look at Lens Blur every now and then, just to get a little smoother image.
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