DoF and a mirror

[Josh66]

I'll add that each mirror needs to fill the frame to pull it off without cropping. The first mirror can be smaller, since it's closer to the camera.

The camera I used has a minimum focusing distance of .9m. I would estimate that the first mirror was about .5m away, but the total distance of the light path was probably close to 3m.

 

[Low_Sky]

Hamlet, since I suspect you won't trust me, go set this up for yourself.

I put my camera on a tripod, approximately two feet from a mirror. I was looking into the mirror, and seeing the window across the room (about 14 feet from the mirror) and my camera bag sitting on the bed in the middle of the room (about 7 feet from the mirror). I took three photos, all at f4.0, not even at the "razor thin DOF" that you are discussing here. The first photo is focused on the blinds, the second is focused on the camera bag, and the third is focused on the picture frame. By your logic, the third photo should have everything in focus (especially considering the more forgiving f4.0 depth of field), but alas, only the frame is in focus.

Please, take 10 minutes and do this little experiment for yourself instead of arguing with the people who are trying to correctly answer your question.

 

[Josh66]

Measure the camera-to-mirror distance and the mirror-to-subject distance, and add those together (the focus distance might actually be in the exif information). Place an object that distance from the camera and the DoF will be identical.

Same reason a 1000mm reflector telescope is not 1000mm long. The light path is what you are measuring.

 

[480sparky]

Damn, I'm gonna be rich because of this thread.

Anyone ever use one of those cheesy 45° mirror filters Spiratone used to make? It looks like a lens hood, but you put it on a telephoto lens and it allowed you to take photos around corners and hold the camera 90° to your subject?

Image link

I recall Animal using one on the Lou Grant show to nail some mobsters (went to a construction site and pretended to shoot a sexy model, but was recording some illegal action instead).

I'm gonna buy the rights to that from the Spira family and reintroduce it to the market. But it won't be the Spiratone Mirrotach. It'll be the 480 Infinite DOF attachment.


I'm off to file a patent right now! Whoo hoo!

I'm gonna be filthy rich!

New gear for EVERYONE!

 

[hamlet]

The mirror is simply used to get what is being reflected on it.

I know. But how does that get you 'infinite' DOF?

The mirror doesn't have an aperture. The cameras sees what the mirror reflects:

example:

 

[Josh66]

Haha - I've seen those. "Covert" surveillance equipment or whatever.

 

[480sparky]

The mirror doesn't have an aperture. The cameras sees what the mirror reflects:

But the camera does, and the combination of your focus point (which will be the sum of camera-to-mirror distance plus mirror-to-subject distance) and aperture is what will give you your final DOF.

And I guarantee... it ain't infinite.





The proof is in your example. The background is OOF.

Haha - I've seen those. "Covert" surveillance equipment or whatever.

I'd love to sit in on a brain-storming session involving hamlet and good ol' Fred Spira.

 

[Josh66]

The aperture is the aperture. Mirrors (or magnets or any other magic stuff like that) won't change that.

What mirrors will do is increase the distance to the subject, which also increases DoF. Standing farther away does the same thing.

If you have a large enough mirror, you could use it to shoot at f/1.4 and have infinite DoF, but that would be no different than just standing at the hyperfocal distance for f/1.4 and shooting without the mirror. With the mirror, you would only have to be half the hyperfocal distance away. It would have to be a pretty big mirror though.

 

[peter27]

Put a ruler up against a mirror, take a photo of the reflected image focusing on that part of the ruler closest to the mirror's front plane and you will see, depending on what f stop you use, how the ruler goes progressively OOF as it reaches into the mirror.

 

[amolitor]

You're wrong, hamlet.

Try it and see.

 

[pgriz]

Wait - isn't it time for the guy who was arguing that the inverse-square law was not applicable to stars show up to take part in the discussion?

 

[GlennT]

If this concept worked, everything in the viewfinder on any SLR/DSLR would be in focus. A mirror does not put everything on the same focal plane. You don't need a camera to test this. Look at yourself in a mirror from a reasonable distance with one eye closed, then look at the glass. If you're too close, the difference might be minute, but you will be out of focus.

Even if it did work, the mirror would have to be angled, putting one edge considerably closer to the camera than the opposite edge.

 

[hamlet]

So i tested this idea of mine and it didn't pan out. My mirror is clearly in focus, but my backyard dwarves and pretty much everything else are out of focus that is reflected back to me. I don't understand? Its a flat surface with no depth of field.

 

[Josh66]

I don't understand? Its a flat surface with no depth of field.

It's a mirror, not a portal to another dimension.

It is behaving normally.

 

[480sparky]

The image is not ON the surface of the mirror. It's not a print. It's just a highly reflective surface that has the optical property of altering the path of light waves.