Camera and Reflections - Mirror DOF bug

[salvadoresz]

Hello,

I found a bug. I?ll try to describe it, I also attached a very simple scene file and screenshots.
Vray 3.6 for 3ds max 2017
Windows 10 home
Nvidia driver 385.28

I created a mirror, placed a camera that is focusing on the mirror, with a small Aperture (f-stop) so that the background is very blurry (out of focus). I places a vray light and a red ball far in the back, so that they reflect in the mirror. When I turn depth of field ON, the ball and light source?s reflection in the mirror becomes blurry, but they shouldn?t, because the camera is focusing on the mirror?s surface.
Practical example: if there?s a tv in the middle of the room, and the screen is reflecting a far away window from the room, and the camera is focusing on the tv screen, the reflected image of the window should be crisp, not blurry.
https://www.dropbox.com/s/bu3s54rak3...F_Bug.max?dl=0

[Rens]

Actually it's accurate, because you have to measure the distance the light is travelling, regardless of whether it's mirrored or not. It is the same as if you would link the ball and light to the mirror, then rotate the mirror 180 degrees (swinging the ball and light in front of the camera), and then delete the mirror. You're actually just focusing on an area in front of the ball and light when you're looking at the light rays. (The mirror image is called a virtual image)
You'll have to set the focus distance to the distance from the camera to the mirror plus the distance from the mirror to the ball. You cannot have both the mirror itself and the ball in perfect focus.

If you have a photo camera and auto-focus on the TV screen in your example, chances are that it will focus on the reflection, not the sceen itself.

[Raven]

This is behaving exactly correctly for real world.
If your focal point is the mirror, the ball and light should be out of focus.



ha ha Rens you beat me to it.

[Rens]

Haha very close.

[salvadoresz]

Hi Rens,
Thanks for your reply. I think what you are saying is not quite true, because the surface of a mirror is 2d, the image on it is a 2d reflection of the 3d space that it reflects. The image on the mirror doesnt have 3d depth, just like a camera lens projects a 3d environment image (light) to a 2d surface, the film or digital sensor. Since that projected image doesnt have a 3d depth, you can't focus on independent objects. You can surely try to stand in front of a mirror with a camera, and try to focus on a furniture behind you, and see if your image gets blurry (assuming the furniture is far behind you so the depth of field is very clear)

[salvadoresz]

WOW, holy s***! I just took out my dslr and tried the mirror thing. Seems you guys are right. I does focus way behind me, and blurs everything else. I stand corrected. I really thought this works like how a projector beams a 2d image on a wall. Learning something everyday. Thanks alot guys, i owe you a few beers.

[salvadoresz]

i'm sorry i wasted everyone's time with this, and also made myself look really bad.

[salvadoresz]

It's still a bit weird, because i worked on alot of studio scenes, lighting and rendering sometimes pretty small objects. Sometimes i used hdri lighting, and as an environment light, i presumed the light source in a hdri is pretty "far" away from the objects, yet when i rendered with a very small f-stop, i never got alot blurry reflections. Same with vray lights. It was the first time i got blurry reflections and i didnt know why. I'll have to look into it and figure out something, i can't have the tv or the reflections get blurry, because it's a product shot, has to have nice controlled and crisp reflections. Any suggestions are welcome.

[Rens]

No worries! Maybe a larger aperture or larger distance from cam to screen could work?

[salvadoresz]

Thank you! Will try that out today.