Focus project

as seen on TV!

Intro

Having indulged in way too much camera equipment for my own good - I decided I needed to justify some of the expenditure by doing some science {gasp} with the equipment.

For my University 2 Physics class, we study mirrors and lenses. Where exactly the camera has to focus when looking in a mirror can be illustrated with photographs. I started with the bathroom mirror, a camera and some signs (it's not what you think!!). Flushed with success (a favorite quote of my Dad!), I decided to branch out to the side mirrors on my truck (a slightly different focusing situation) and even to the mini convex mirrors you attach to the drivers' side mirror [if you don't have one on your car already .. oh baby, you have to get one, they are great!].

First Project - Flat mirror

There is a camera halfway between a wall and a mirror. We can focus on the mirror surface, and the camera is out of focus, and the wall is really out of focus. (Double click on the image to view it in more detail.)
Now if we extend the distance at which the camera focuses, we can see the camera itself. (Notice the word camera is reversed by the mirror!). The image of the camera is "behind" the mirror, according to the camera .. it needs to use a distance more than the distance to the mirror itself. Notice the mirror sign is now out of focus, but the back wall seems slightly more in focus. (The camera is stopped all the way down {meaning a very large aperature} - this gives a very flat focal plane - very precise in terms of distance.)
Now we can focus on the back wall. The camera is slightly out of focus, the front mirror sign is completely blurry. [The back wall sign is supposed to say "Farther Away" - reversed!]
If I put my hand between the camera and the mirror .. notice that the mirror sign is not too far out of focus and the camera is not too far out (we've gone "past" the mirror, but not quite to the camera image). Notice the very blurry image of the tips of the fingers to the far right - that is way too close to the camera lens to be focused properly, since the camera is "reaching behind" the mirror to get the image of the fingers.

Second Project - Convex Passenger side mirror

The passenger side mirror is convex - this creates a virtual image of the objects in the mirror - the image appears to be "behind" the mirror's surface (so the camera would have to focus there). Notice the image of me and the camera are really out of focus, (but the "Objects may be closer than they appear" message is in focus). Remember the scene in Jurrasic Park where the T-Rex is chasing the car? Look at the image of the T-Rex in that mirror again - what is the error made?
Now if we extend the distance at which the camera focuses, we can see the camera itself. The image of the camera is "behind" the mirror, according to the camera .. it needs to use a distance more than the distance to the mirror itself. Notice the mirror surface is out of focus .. but the tape roll behind the mirror is much more in focus .. this shows how the distance has to increase slightly to get the image of me in focus.
Here we can see the layout of the "shoot". Notice the tape is slightly behind the mirror (much closer to the mirror than the camera is). The camera had to focus only slightly more than the distance to the mirror (remember the flat mirror .. we would have to focus all the way back).
If we try to focus on something farther back (the sign on that distant tripod is supposed to say "Farther Away"), we can see the camera and photographer go out of focus, also the roll of tape - we have gone farther behind the mirror to catch this distant image "in" the mirror. Compare the trees in this image with the second image .. they are slightly more in focus here - we have gone slightly farther behind the mirror to catch objects much farther back.

Third Project - Mini-Convex driver's side mirror

Here is the setup - notice the small convex mirror added to the side of the drivers' side mirror.
Focus on the mirror surface, and notice that the image in the flat mirror is very out of focus, but the image in the convex mirror is not so far out of focus. Notice the fingers just behind the mirror .. out of focus, just slightly.
If we focus on the camera and photographer, the sign on the surface and the small convex mirror are so out of focus, we almost can't see them. (We've "doubled" the distance of focus).
Adjust the camera so that we can see its image in the small convex mirror .. and the other mirror surface goes out of focus (we are slightly "behind" the mirror in terms of the focusing distance). Notice the fingers behind the mirror are now in better focus. Notice the image of the camera is much smaller than the image from the flat mirror. "Smaller" appears to be "farther away" to our eyes - hence the warning on the Passenger side Mirror!

Hey, if you have any camera/lens/mirror ideas you can suggest to me .. suggest away!!!

An extra bonus ... look deep into the mirror!

Follow this link for an animated GIF (about 500K) of a camera looking into the multiple reflections of two facing mirrors : Multiple Mirror focusing