DoF and a mirror

[480sparky]

We've now turned an aperture 1.4 into infinite.

Huh?

If you're saying you now have infinite DOF, then why is the mirror frame (and the wall it's hanging on) horribly OOF?

 

[Josh66]

No. The image is not ON the mirror. It would simply be further away.

But you put the mirror at the distance you would photograph. So that would cancel that out.

The distance you would be measuring is the distance the light is traveling, which will be camera-to-mirror plus mirror-to-subject.

My current avatar was taken using two mirrors. One problem you will encounter is that fine details will often have some "ghosting". It's not very noticeable, but it's there. Quality of the mirror is a factor in this, I'm sure.

edit
Nevermind. It's not my current avatar, lol. It is everywhere but here.

This is the picture I was referring to:


I can see in that picture that the camera was set to f/8. Shutter speed was probably 1/60 or 1/125 - I can't remember for sure. I have no problems hand holding this camera down to 1/8 second... Subject movement is usually the deciding factor when choosing a shutter speed.

 

[480sparky]

No. The image is not ON the mirror. It would simply be further away.

But you put the mirror at the distance you would photograph. So that would cancel that out.

If you can put the mirror at a distance you can photograph an entire group, why not just put the whole group there instead?

 

[Josh66]

No. The image is not ON the mirror. It would simply be further away.

But you put the mirror at the distance you would photograph. So that would cancel that out.

If you can put the mirror at a distance you can photograph an entire group, why not just put the whole group there instead?

Also, the farther away the mirror is, the larger it has to be.

In a way, I can see a benefit... Not the one hamlet is talking about though. In the shot I posted above, the first mirror was closer than the minimum focusing distance for that lens. It only works because of the mirrors.

I think that's actually the opposite of what he's talking about. He's wanting to get farther away, not closer.

For what it sounds like he's trying to do, the mirror would have to be unrealistically large. The easiest solution would be to just stand farther away. Or use a shorter lens.

The mirror will have to fill the frame - that would very quickly become a very large mirror.

 

[hamlet]

No. The image is not ON the mirror. It would simply be further away.

But you put the mirror at the distance you would photograph. So that would cancel that out.

If you can put the mirror at a distance you can photograph an entire group, why not just put the whole group there instead?

Because i want to shoot at 1.4.

 

[Josh66]

But you put the mirror at the distance you would photograph. So that would cancel that out.

If you can put the mirror at a distance you can photograph an entire group, why not just put the whole group there instead?

Because i want to shoot at 1.4.

And you need mirrors to do that ... because?

 

[480sparky]

Because i want to shoot at 1.4.

I understand that. Explain how a mirror turns the razor-thin DOF of f/1.4 into a magical infinite DOF.

By your own examples, it doesn't.

 

[hamlet]

But you put the mirror at the distance you would photograph. So that would cancel that out.

If you can put the mirror at a distance you can photograph an entire group, why not just put the whole group there instead?

In a way, I can see a benefit... Not the one hamlet is talking about though. In the shot I posted above, the first mirror was closer than the minimum focusing distance for that lens. It only works because of the mirrors.

That is actually an awesome idea.

 

[hamlet]

Because i want to shoot at 1.4.

I understand that. Explain how a mirror turns the razor-thin DOF of f/1.4 into a magical infinite DOF.

By your own examples, it doesn't.

The dof is a thin slice, the mirror is a thin slice. put them together and everything is within dof.

 

[480sparky]

The dof is a thin slice, the mirror is a thin slice. put them together and everything is within dof.

Then why is the mirror frame and the wall it's attached to out of focus?


(Me suspects your thin slice of DOF is placed at a distance that is the sum of camera-to-mirror plus mirror-to-subject)

 

[Josh66]

Because i want to shoot at 1.4.

I understand that. Explain how a mirror turns the razor-thin DOF of f/1.4 into a magical infinite DOF.

By your own examples, it doesn't.

The dof is a thin slice, the mirror is a thin slice. put them together and everything is within dof.

By using the mirror(s), all you are doing is increasing the distance to the subject. Yes, that will also increase DoF - but unless you're trying to do something like that self portrait I posted, the same thing can be achieved by simply standing farther away.

 

[hamlet]

The dof is a thin slice, the mirror is a thin slice. put them together and everything is within dof.

Then why is the mirror frame and the wall it's attached to out of focus?


(Me suspects your thin slice of DOF is placed at a distance that is the sum of camera-to-mirror plus mirror-to-subject)

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


Your shot would look like this:

This is the picture I was referring to:


.

(Very good picture btw)

 

[Josh66]

This is a very crappy drawing of how I did that.

Focus distance that the lens must be set to will be equal to the sum of the red lines.

 

[480sparky]

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

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

 

[480sparky]

This is a very crappy drawing of how I did that.

Focus distance that the lens must be set to will be equal to the sum of the red lines.

Is that one of them fancy tilt/shift lenses? :mrgreen: