Focusing on reflected images

[blu82]

Can someone explain the following?
I manually set a camera to focus on an object 3m in front of me at f1.2.
I then photograph a mirror also at 3m so that the mirror is in perfect focus.
Why is the image in the mirror also in perfect focus even though it is 1000m away?
Shouldn't the mirror be in focus and the reflected image be blurred?

Dave

[E-K]

Is the object that is 1000m look the same if you turn around and shoot it directly? Is the mirror flat?

Since you say f/1.2 I'm assuming maybe a 50mm or 85mm focal length so in theory yes at 3m the DOF would be fairly shallow.

Can you post an example?

e-k

[hollis_f]

Shouldn't the mirror be in focus and the reflected image be blurred?[

Yes.

[Mike]

Can you post an example?

That would help. Here's a similar shot that I took last year:

359281

As you can see, the frame is oof and the couple are in focus. I don't have the exif to hand but I think it was at f/2.8

[IslandCrow]

You have to understand the way light works. By photographing the image in the mirror, you're not photographing the image itself, but the light reflected by the mirror. It's really no different than photographing a framed picture. The light from the reflected image is only travelling the distance from the mirror to your camera. For that matter, the issue of DoF is moot. The mirror "flattens" the original image, so it's 2 dimensional and there's no DoF. Your entire reflected image will be in focus (assuming you focused correctly) whether you use f/2 or f22. In the above example, the only reason the mirror frame is out of focus is because it's not in the same plane as the mirror itself. Which I think is what the poster was trying to illustrate, but I don't believe that's what you were asking.

[Mike]

You have to understand the way light works. By photographing the image in the mirror, you're not photographing the image itself, but the light reflected by the mirror. It's really no different than photographing a framed picture. The light from the reflected image is only travelling the distance from the mirror to your camera. For that matter, the issue of DoF is moot. The mirror "flattens" the original image, so it's 2 dimensional and there's no DoF. Your entire reflected image will be in focus (assuming you focused correctly) whether you use f/2 or f22. In the above example, the only reason the mirror frame is out of focus is because it's not in the same plane as the mirror itself. Which I think is what the poster was trying to illustrate, but I don't believe that's what you were asking.

I can't agree with this. The light reflected from a mirror has to travel the distance from the mirror to the camera PLUS the distance from the subject to the mirror.

DOF still applies I'm sure.

[Mike]

Ok, I just went and shot these in the bathroom mirror, 30mm f/1.4, focused on the word Canon on the top of the camera:

http://img90.imageshack.us/img90/138/mirrordof1.jpg

http://img90.imageshack.us/img90/5554/mirrordof2.jpg

As you can see, depth of field is very evident. I didn't shoot one at f/22 as there's not enough light in there but I'm sure that there would be an increase in the dof if I had.

[IslandCrow]

Interesting. I believe I stand corrected.

[ImRaptor]

You have to understand the way light works. By photographing the image in the mirror, you're not photographing the image itself, but the light reflected by the mirror. It's really no different than photographing a framed picture. The light from the reflected image is only travelling the distance from the mirror to your camera. For that matter, the issue of DoF is moot. The mirror "flattens" the original image, so it's 2 dimensional and there's no DoF. Your entire reflected image will be in focus (assuming you focused correctly) whether you use f/2 or f22. In the above example, the only reason the mirror frame is out of focus is because it's not in the same plane as the mirror itself. Which I think is what the poster was trying to illustrate, but I don't believe that's what you were asking.

That is way off.
Principle distance in regard to light is the distance of the light path, adding a mirror is nothing more than extending the distance to the object.
A mirror itself has about the same effect on focus distance as a window, ie nothing.

[hollis_f]

If one thinks about it there's no way a mirror could 'flatten' the image. If it did then macro photographers wouldn't have to use such narrow apertures to try and get a reasonable depth of field - they'd just stick a mirror in there somewhere.