Physics: Why exactly does a large aperture make the background blurry?

[jsl]

Hey guys,

I love portrait shots and often have my Canon EF 50mm f/1.8 set to a low f value (around 2.2). I know this gives me wonderful portrait shots with the subject in focus and the background blurry and all bokehry (I know, I'm making up words!). But I've often wondered.... WHY?!? Why is it that widening the hole in a lens and allowing more light through makes the background nice and blurry? Is it to do with more light coming through or is it to do with the reduced exposure time? I suspect it is the former. When more light comes through, is it scattered in a different way?

Can anyone explain this to me? I'd just be super interested to know... there must be someone who knows or has done physics before... or is much better at using Google than me ;-)

Hope you can help!
JSL

[teaking]

Its to do with circles of confusion, light bounces off on object and when those beams cross before they hit the sensor this causes a circle of confusion and the object to be out of focus more light coming in causes more so does a bigger distance from the focus point and magnification causes these circles to be bigger.

For a good explanation on DOF I would recomend watching this episode of meet the GIMP at the end he discusses DOF so if you dont use GIMP just fast forward to the end

Meet the GIMP » Blog Archive » Episode 130: Getting the Bugs out

[sybren]

It doesn't have anything to do with the amount of light that enters the lens, but the angle at which this happens. By opening up your aperture, you use more of the outside of the lens.

Just think of it - if you could close your aperture so that only a single ray of light would be able to enter your camera, you wouldn't need a lens. This is because that single ray would pass through the middle of the lens, and in the middle the lens does nothing - it's just a flat piece of glass there. You wouldn't need to focus, as everything would be perfectly sharp (I'm ignoring diffraction on purpose).

[kencaleno]

All to do with depth of field:

Depth of field appears to be the most confusing of focus elements. Depth of field is a range of acceptable sharpness in an image, from near to far. Three factors control Depth of field: Aperture: The wider the aperture, the shallower the depth of field. Focal length of lens, The longer the focal length of the lens, the shallower the depth of field will be. and Distance from subject, Landscape images have great depth of field, whereas Macro (extreme close-up) images have very shallow depth of field. Regards, Ken

[rahuldg11]

One thing that helped me to understand the elation between aperture and depth of field is the pin hole camera.
The smaller you make the hole the sharper and better the image you get. This is because of an infinite depth of field. But when you make the hole bigger, light rays from one part of the image reaches the screen at more than one single spot and the image becomes blurry. This is a short depth of field.
I may not have been able to fully explain it, but i think i am clear at what i wanted to explain.

This is the site from where i learnt this. http://digital-photography-school.co...pinhole-camera

[lputman]

This might help you out, Understanding Depth of Field in Photography

[mng100]

i found this article helpful... Depth of field - Wikipedia, the free encyclopedia

[inkista]

Found it. It's in the first paragraph of the Wikipedia entry on Aperture:

...the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If the admitted rays also pass through a lens, highly collimated rays (narrow aperture) will result in sharpness at the image plane, while uncollimated rays (wide aperture) will result in sharpness for rays with the right focal length only.

Basically, the wider open the aperture is, the fewer rays of light are going to be hitting the sensor perpendicularly. So, more rays are going to be "defocused" (from travelling a different distance?) and ending up getting blurred on the sensor. Or so I'm reading it. I could be wrong, and hell, it's still just Wikipedia.