photography question

[dakegra]

imagine the scene, if you will.

I'm taking a photograph of a framed mirror, about three feet away. I want the frame in focus. I set the lens to focus at 3 feet, and take the photo. I've got a really narrow depth of field, so *only* the frame is in focus and not the background etc.

If I then want to take a photograph of *myself* reflected in the mirror, would I need to adjust the focus? Or would I need to set the focus to six feet?

I'm sure there answer should be obvious, but it's been noodling around in my head for a couple of days now, and I realised that one of you lot would be able to answer it.

Comments

[ayoub.livejournal.com]

You'd need to set it to six feet...

[dakegra.livejournal.com]

that's what I thought, but part of me keeps saying that the image is at three feet, as it's an image in the glass.

[dave-t-lurker.livejournal.com]

Optically, the glass is at 3 feet, the image is at 6 feet...

[ayoub.livejournal.com]

It's reflected light, though, so that doubles the, though... Trust me, I'm a physicist...

Basically, if you focus at three feet, you'll get a perfectly focused picture of dust on the surface of the glass...

[dakegra.livejournal.com]

yeah, I used to be a physicist too. Long time ago though. :-)

[ayoub.livejournal.com]

:D

Sadly, I remained the right type of geek for it... :P

[thebabynancy.livejournal.com]

I forget the actual "rule of thumb"... it may take several tries... doubling the focus sounds like it will work... but you may want to try it at 5' and 7', as well. :)

[dakegra.livejournal.com]

it's more a hypothetical 'what if?' thing. I sort of know that the person is effectively six feet away, but the image of that person is only three feet away, on the surface of the mirror.

[thebabynancy.livejournal.com]

I am fairly certain that one should double the focus... but I am not 100% sure. I am old now. And I forget. *giggle*

[anonymous-greg.livejournal.com]

Six feet. Remember: You are taking a picture of light. The light travels from you to the mirror (3 feet), then back to the camera (another 3 feet), for a total of 6 feet.

[songster.livejournal.com]

What he said. The image isn't "on" the mirror, it's behind the mirror. When you take a photo of something through a window, you focus on the something, not on the window glass. Same applies to mirrors.

[anonymous-greg.livejournal.com]

Ooo, I like that "behind the mirror" description. Likening it to a window is a fabulous analogy. I'll be using that the next time the question comes up.

[dakegra.livejournal.com]

me too - that's a great way of describing it.

[primitivepeople.livejournal.com]

Try some practical experimentation.

[dakegra.livejournal.com]

that was this weekend's job. :-)

[nextian-cutie.livejournal.com]

The frame and the subject in the mirror are at different distances. If you were standing right at 3 feet away, the focus distance to capture you in sharp focus would be 6 feet because the light that is registering as you goes 3 feet to the mirror then bounces back that 3 feet to get to the camera.