What do trees have to do with aperture?

Okay, so we're back to aperture and... trees?  I was asking the teacher to explain the relationship of the aperture to the clarity of the depth of field.  He said it all has to do with geometry! 

On the white board he drew something close to the picture I've drawn below.  Where my arrow is pointing is where the aperture is on the camera (the hole where the light comes through). The f/stop tells us how big the hole is (how much light is coming into the camera). So, again, if the f number is small the hole is big and the other way around.  The light from the top of the tree is scene at the bottom right hand corner of the camera.  And the light coming from the bottom of the tree is projected into the camera at the top right hand corner of the back of the camera.  And of course, whatever light is coming in at dead center goes straight through to the back where the upside tree is drawn (which is where the light sensor is).  In other words, the only light that is going to shine at the bottom right corner of my badly drawn camera is coming from the top of the tree and the other way around for the light coming from the bottom of the tree.

When you have a small hole all the light is focused very narrowly through the hole so the picture is clear.  Like when you squint to see something clearly. 

 Now, if you have a big aperture (small f #), the more light coming into the image which will cause it to become blurry if not properly adjusted by the ISO and Shutter Speed. So to exaggerate the point, if you were to remove the lens entirely from your camera and take a picture, so much light would be flooding the sensor that you'd just have an entirely blurry image.

Here is a good tutorial on the effect of different focal lengths.  50mm and smaller is typically considered a wide angle lens. This is important to know because the longer your focal length (a lens over 50mm) the shorter your depth of field will be.  So, it's hard to get a narrow depth of field with a wide angle lens. 

Imagine you have a person between the camera and the tree in the picture above.  If you are trying to take a picture of the person, with a longer focal length, it will be difficult to focus on the tree and the person at the same time.  Your camera is trying to focus on one thing when you have a longer focal length. So, if you want to take a picture of the person, you can blur the background using a longer lens, but if you want the whole picture in view, it may be best to use a wide angel lens. 

And yes, the tree to the right of the picture is supposed to be upside down.   All photographs that are taken are projected upside down to the light sensor at the back of the camera. The same is the case with the
human eyes.

We also discussed the origin of the camera and what was used before the camera itself, namely the camera obscura Apparently, an enormous camera obscura can been seen at the Whitney Museum of American Art.

One final thought from the class, if you find your camera is having a hard time focusing on something (zooming in and out as if it doesn't know on what to focus), take it off auto focus and put on manual so you can focus on what you want. 

As usual, all thoughts or better ways of explaining this are sincerely welcome!