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photography question - almost, but not quite, entirely unlike tea
if I had to explain, you wouldn't understand
dakegra
dakegra
photography question
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.
14 thoughts or leave a thought
Comments
ayoub From: ayoub Date: November 7th, 2008 03:06 pm (UTC) (linky)
You'd need to set it to six feet...
dakegra From: dakegra Date: November 7th, 2008 03:14 pm (UTC) (linky)
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 From: dave_t_lurker Date: November 7th, 2008 03:28 pm (UTC) (linky)
Optically, the glass is at 3 feet, the image is at 6 feet...
ayoub From: ayoub Date: November 7th, 2008 08:34 pm (UTC) (linky)
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 From: dakegra Date: November 8th, 2008 08:46 pm (UTC) (linky)
yeah, I used to be a physicist too. Long time ago though. :-)
ayoub From: ayoub Date: November 9th, 2008 07:21 pm (UTC) (linky)
:D

Sadly, I remained the right type of geek for it... :P
thebabynancy From: thebabynancy Date: November 7th, 2008 03:08 pm (UTC) (linky)
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 From: dakegra Date: November 7th, 2008 03:15 pm (UTC) (linky)
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 From: thebabynancy Date: November 7th, 2008 03:17 pm (UTC) (linky)
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 From: anonymous_greg Date: November 7th, 2008 03:30 pm (UTC) (linky)
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.
(Deleted comment)
anonymous_greg From: anonymous_greg Date: November 7th, 2008 04:12 pm (UTC) (linky)
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 From: dakegra Date: November 7th, 2008 04:14 pm (UTC) (linky)
me too - that's a great way of describing it.
(Deleted comment)
dakegra From: dakegra Date: November 7th, 2008 04:03 pm (UTC) (linky)
that was this weekend's job. :-)
nextian_cutie From: nextian_cutie Date: November 7th, 2008 04:08 pm (UTC) (linky)
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.
14 thoughts or leave a thought