VRayZDepth Question (focus in middle of long passage)

[Morne]

Hi all

So I have a 50m passage. I want to focus on 20m to 30m, rest should be out of focus. For min range I set 20m and for max 30m. But when I look at the element, Everything from 0m to 20m is white, then it greys out to 30m and from 30m onwards it is black. Is this correct?

Isn't it suppose to fade from black at 0m to white at 20m, then from 30m again white to black onwards? I mean how would I go about blurring things close to the camera and far from the camera, but in the middle it is in focus? Is it even possible?

[joconnell]

If you were obeying how camera focus works, there'll only ever be one distance that's totally sharp so in your case if you've got the camera focal distance set to 20m, you'll have a very thin section 20m in front of the camera that's white - it might only be a single line of pixels and then from this point towards and away from camera it'll start to go towards black. Real life cameras only ever have one tiny part in focus and then everything in front and behind this gets gradually blurrier. Since you're doing your dof in post though, there's nothing to say you can't colour correct your black and white depth pass so that the grey values that lie between 20m and 30m get pushed to white, and everything else outside of that gradually falls off and then you can feed this into your dof filter.

In your case I'd set your focal distance to 25m so that your focus is bang in the middle and that it renders with white values at this depth, and then you can use levels or curves in whatever program you're comping in to push the light grey values that lie around 20m up so that they're pure white. Your dof filter will then treat this entire section as being totally in focus and come out sharp. There's no way using Vrays dof to set a large span to be in focus - it's one thing at a time!

[Morne]

I don't think I understand. So the whiter it is, the more in focus it is and the more black it is the more blurry it is?

Then how can I get it white in the middle, and black on both ends? I'm happy to get it pure white in the middle, and fade to black on both ends. But currently I'm only getting black on one end. The other end is pure white untill the middle, then it fades to black.

[rivoli]

you can even encompass the whole scene: min distance set from the camera to the nearest objects, and max distance to the farthest, and then control the focal point in post.
this is a quick example made with AE, and the blur it's quite exaggerated. it sucks, big time, but so does any dof made in post (if you don't own any good plugin that is).

focal point in the foreground



mid



background



edit:
I didn't even bother to render the Z pass at a bigger resolution and with no filtering, but I guess you may want to do that to get somehow better results..

[Morne]

How do I go about adjusting the focal point in post? I mean can you put the focal point wherever you want to with the same Zdepth pass? or for example where the middle part is the focal point, I need a different Zdepth pass for that? ie adjust min and max ranges?

[joconnell]

Back to you shortly on this - just tied up at the minute.

[rivoli]

in AE, the lens blur filter has a slider with which you can change the blur focal distance. it works, to some extent, but I don't find any of these native filters doing a very good job. it may works for very subtle effects, but it falls apart pretty easily.
I believe PS has a similar control for the focal distance.

[simmsimaging]

I think the thing you need to keep in mind is that Zdepth is not a DOF pass - it's just a measure of the distance between objects in the scene, represented as a greyscale. White is closer, black is farther. In most cases I just set the min to be at or near the most foreground object in the scene, and set the max just beyond the furthest. This gives a nice subtle gradient across the whole set.

This map you load in Photoshop (or wherever) and the filter there (Pshop has Lens Blur, or plugins like Lenscare or DOFPro) read that in as an alpha map. Inside the filters you define where the "focal plane" will be along the gradient of white to black. In your case you would select somewhere in the middle range of grey, which would correspond to the middle distance in your scene. The size of your "in focus" area is defined by the filter strength in part, and in part by the contrast of your ZD map you are using as the alpha for the filter.

Most filters offer built-in contrast controls so you can tweak the ZD map and see the DOF change, but sometimes it's easier or more controllable to simply adjust it by hand in Photoshop prior to loading it in the filter.

Hope that helps.
b

[Morne]

thanks, makes sense now