curious - how does vray and mental ray calculate its dof?

[joconnell]

Heya Folks,

Bit of a pub argument type question here - what method does vray and mental ray use to makes it depth of field? Is it like the scanline trick of rotating the camera around the focal point and using motion blur to knock bits out of focus or does it take the point that the camera ray hits an object then jitter it based on how far away it is from the focal point?

Cheers!

[vlado]

It is similar to the scanline trick, but without any motion blur involved. Also, the camera position is jittered over a disc area, rather than just a circle (or an N-gon area depending on the options). This jittering is done per pixel separately, so in each pixel that jittered positions are different (which is what causes the noise).

Best regards,
Vlado

[joconnell]

Super - thanks for that.

[joconnell]

Just another quick query - I wanted to do a shallow focus shot with an animated rack focus recently and it would have been convenient to do this using a focus control so I went for the vray physical camera. Since it wasn't a realistic scene I didn't need exposure so that was turned off. I also used quite small numbers with the dof to try and get the right level of blurriness. What I found though is that below an fstop value of 1.0, the vray cam starts darkening everything hugely despite exposure not being a factor - I needed to go down to values like 0.02 on the fstop to get the right level of focus but this returned a near black rgb channel. I've attached two pictures from a simple test showing the difference between f 1.0 (very little blurring from the dof) and 0.25 (a little more blurring but very dark by comparison) and a regular max cam with the render dialog settings tweaked to get a similar level of blurring.

No biggy since I'm not using it for a job at the moment but just out of curiosity?

Cheers,

John

[vlado]

It might be because of the vignetting; have you tried disabling that?

Best regards,
Vlado

[joconnell]

Bingo. Once again I don't bother looking at the simpler solutions