Why larger aperture = shallower DoF?

[mitawa]

I understand that larger the aperture, shallower is the DoF and vice-versa. But why is it so? I mean, larger the aperture, more light comes in. With more light allowed to come in, shouldn't the entire object be in focus then? Similarly how come the narrow aperture, (i.e. with limited light intake), allow greater DoF? Why is it exactly opposite?
Well, I tried to research; but the terms like Circles of Confusion, lens front cardinal points etc. only added to my confusion. Any simpler way to explain this phenomenon?

[Photologyst]

Here's an analogy. The camera lens functions to some extent like the human eye. When your eyes are dilated, you don’t see as well.

[RecurrentNerve]

Someone mentioned an analogy a photography tutor used a while back that I thought was good.

Imagine you have a large hole (aperture) in a piece of wood, held horizontally above a large piece of paper. If you pour paint through the hole from a height it goes through the hole and splashes all over the place.

Now imagine the same thing with a small hole. The paint will drizzle through in a thin stream.

[mitawa]

Now imagine the same thing with a small hole. The paint will drizzle through in a thin stream.

That's my point! That means with smaller hole, focus should be narrow and entire image should NOT come out crisp. Only part of it should, as aperture is small. And as I increase the width of aperture (larger hole), light should (paint in your analogy) spread across the whole image giving greater DoF.
But what happens is actually opposite. Isn't it? DoF is inversely proportional to width of aperture; not directly proportional.
Or am I missing something?

[RichardTaylor]

Thin stream means smaller out of focus areas, circles, hence more depth of field.

[RecurrentNerve]

That's my point! That means with smaller hole, focus should be narrow and entire image should NOT come out crisp.

No because the 'paint' isn't 'spattering' through a small hole. Just because you have a narrow aperture doesn't mean the focus is narrow.

[prabbit237]

Originally Posted by mitawa

That's my point! That means with smaller hole, focus should be narrow and entire image should NOT come out crisp. Only part of it should, as aperture is small. And as I increase the width of aperture (larger hole), light should (paint in your analogy) spread across the whole image giving greater DoF.
But what happens is actually opposite. Isn't it? DoF is inversely proportional to width of aperture; not directly proportional.
Or am I missing something?

Refer to the following page for some info about how it all works without too much technical jargon but also I'll refer to some things in that page's diagrams.

Digital Camera Tutorial: Aperture & Depth of Field - TrustedReviews - TrustedReviews

Basically, the size of the aperture doesn't matter as to focus for any given point that is in the focal plain. The larger aperture would allow in more light from that point and the light would spread out more but then the lens would focus it right back to a point again. Look at either of the diagrams and at the green point. In both, no matter if it's a large or small aperture, the green dot's light is refocused to a point with no problem. The only difference there is the TOTAL amount of light and the smaller opening requires a longer shutter speed (or higher ISO.)

However, for the red or blue sources, the lens can't focus those back to perfect focus. So with the larger aperture, the light for either of those sources DOES spread out more and when the lens tries to pull it back in, it has less success with the larger opening, thus the points are more out of focus at a given distance (and the greater that distance the point is from the "plain of perfect focus" (the green dot) the more and more it gets out of focus. With the smaller aperture, the light doesn't spread as much so the object has to be further away from the green point for the focus to be off.

Basically, ANYTHING that's not at the exact same distance as the green point would be out of focus but our eyes can't detect it when it's within a given range (the "depth of field") so we don't notice it.

Hope this helps.

[dcclark]

You may prefer a visualization. Check this out:

DOF Visualized.

Specifically, look at the diagram in the "Depth of focus & Aperture visualization" part. Notice how, when the aperture is large, the light "spreads out" quickly on either side of the sensor/film. That corresponds to narrow DOF, because the "spread out" light means lack of focus. With a narrow aperture, the angle that the light "spreads out" at is much narrower, keeping more things in focus.

[Sterling]

With more light allowed to come in, shouldn't the entire object be in focus then?

The amount of light coming through the aperture is not what creates focus.

[mitawa]

Thank you all... Now I understand.

Thanks prabbit237, your link really helped.

So with the larger aperture, the light for either of those sources DOES spread out more and when the lens tries to pull it back in, it has less success with the larger opening, thus the points are more out of focus at a given distance (and the greater that distance the point is from the "plain of perfect focus" (the green dot) the more and more it gets out of focus. With the smaller aperture, the light doesn't spread as much so the object has to be further away from the green point for the focus to be off.

Thanks dcclark for your link.

That corresponds to narrow DOF, because the "spread out" light means lack of focus. With a narrow aperture, the angle that the light "spreads out" at is much narrower, keeping more things in focus.

The amount of light coming through the aperture is not what creates focus.

No because the 'paint' isn't 'spattering' through a small hole. Just because you have a narrow aperture doesn't mean the focus is narrow.

Now I really understood the concept. Thanks a lot!