Measuring depth of field when shooting a mirror...

ImmixFilms · March 11, 2005

I'm a film student and at my school we've been getting conflicting reports on how to measure for depth of field when shooting a subject's reflection in a mirror.

Do you measure from your camera to the mirror's surface, or

Do you measure to the mirror and then from the mirror to the subject to find the total distance? And why?

I'm not the only one at my school that is questioning this, so your answer would help out a lot of future filmmakers.

Thanx.

Dominik Muench · March 11, 2005

you measure from the cam to the subject in the mirror, make sure its in focus, and then take your t stop accordingly from there, since you focus on the subject in the mirror thats where your t stop is lying....i hope im correct.

Dominic Case · March 11, 2005

The image in a mirror is as far behind the surface as the object itself is in front of the mirror.

So you need to meaasure from the camera to the part of the mirror where you see the subject reflected. then from that part of the mirror to the subject itself, and add the distances together.

Laurent Andrieux · March 12, 2005

As Immixfilms was also asking why, I'll try to put it like this. Imagine a ray of light coming from the subject to the camera, thru the mirror. Its path describes the distance you are searching, ie the distance you have to focus on. As to go to the camera, this ray goes straigth to the mirror, is then reflected and goes from the mirror to the camera. and the path describes an angle.

Michael Nash · March 12, 2005

I've never understood why there's so much confusion about mirrors and reflections. It's pretty straightforward, and if the laws of physics don't give you the answer, then at least common sense should...

Of course you measure from the lens to the mirror, and then from the mirror to the subject. It's just light rays. That said, it's best to double check focus by eye, as the glass can add some distortion and throw off critical focus in some cases ( not to mention refraction at extreme angles).

I don't know, it's just a pet peeve of mine I guess, when I see an entire crew start to look in all kinds of crazy directions and come up with weird solutions to resolve reflection issues, as if the laws of physics break down as soon as a mirror is involved, instead of proceding simply and methodically to find the angle. Angle of incidence = angle of refelction; compound as necessary for the reflections involved. Learned that in 10th grade physics.

As a DP I once worked for said: "are you pool playing man?..."