Depth of field and Iray render times

mtl1

A bit of an odd question/observation regarding render times using the Depth of Field option within cameras: Why do render times shoot up astronomically as an object is placed closer towards the camera and away from the plane of focus?

I never noticed this until I was setting up a render with rain using SY's Rigged Water. My initial renders with 'rain' in the background were of decent rendering time, given my GTX 960. However, the render times shot up dramatically when I placed more rain before my subject -- that is, in between the focal plane and the camera itself. The only way I could resolve this issue was to: a) turn off depth of field, b) use architectural/caustic samplers, or c) move the rain closer to the plane of focus.

Volumetric effects due to water definitely increase rendering times, but is there an interaction between volumetric effects and depth of field that I should be aware of?

jeff_someone

Depth of Field causes longer render time because the renderer must sample additional points to compute the proper blurring/focus.  Also the unfocused portions of the render take longer to converge. That said, I've not seen a substantial increase in render times due to DoF.  

Mattymanx

Funny, for me I see faster render times with DOF enabled cause the render engine is not busy giving you all the detail.  Same with 3DL renders.

Arnold C

Mattymanx said:

Funny, for me I see faster render times with DOF enabled cause the render engine is not busy giving you all the detail.  Same with 3DL renders.

Working with Iray I experience that, too. I ceased working with 3DL some time ago, but the last I remember rendering with DOF enabled, you had to raise the Pixel Samples to at least twice default. Which resulted in increased rendertimes.

mtl1

Yes, I agree. DOF usually improves or decreases rendering times for me in Iray.

I'm starting to think it's purely a volumetric issue, causing lensing effects when it's between the Plane of Focus and the camera, thus throwing off path tracing and convergence.

Arnold C

mtl1 said:

I'm starting to think it's purely a volumetric issue, causing lensing effects when it's between the Plane of Focus and the camera, thus throwing off path tracing and convergence.

I guess you're on the right track, the "Water" objects in this produkt are using refraction and volume material settings, things which usually slow Iray down a bit.

TheKD

You know, I never even bothered trying DOF in iray, in every render engine I ever tried before, DOF trippled or quadrupaled render times. It was always way easier and faster to do in post(I mostly do still images). I usually render really large resolution, so that would mean adding many hours to days lol. 

mtl1

TheKD said:

You know, I never even bothered trying DOF in iray, in every render engine I ever tried before, DOF trippled or quadrupaled render times. It was always way easier and faster to do in post(I mostly do still images). I usually render really large resolution, so that would mean adding many hours to days lol. 

I've actually considered releasing a product to do fast DOF/bokeh in DAZ, but there are already dozens of post-process products out there that can do the same thing already...