Depth of field explanation

[Mr. Ed]

I like to learn as much as I can about new things I'm trying and I plan on buying a DSLR soon (was going to get a 30D but I'm waiting to see if the 40D shows up at PMA). Anyway, I've been learning about photography for a couple of months and I understand that a wide aperture causes a shallow depth of field but I was just looking for a technical explanation as to why that is so? I can't seem to wrap my brain around letting more light in causing blurriness. I'm sure there's a simple answer and that's exactly what I'm looking for.

Cheers.

[PixelChick78]

Do you wear glasses?

If you do, take a tiny pencil and make a small hole in a peice of paper, then try to read a book through the small opening. It will be clear! Like you put your glasses on!

Now immagine that hole is your apature. Lareger = more blury.
When the hole is smaller, things are more clear.

Try it, it's neat.

There's my bit.

[R Hardman]

Try this link. Your answers are there. ;) http://www.canonfd.com/lenswork.htm

Welcome to the forum.

[Mark_Cohran]

Depth of Field Tutorial (http://www.cambridgeincolour.com/tutorials/depth-of-field.htm)

And a whole set of tutorials on other aspects of photography here: Cambridge Tutorials (http://www.cambridgeincolour.com/tutorials.htm)

[Mark_Cohran]

Oh, and welcome to the forums.

Mark

[StewartR]

Wlelcome to POTN, Mr. Ed. Sadly I think you will find that the links posted by other contributors, whilst interesting and covering depth of field issues reasonably well, do not answer your question. I infer from your post that you understand the "WHAT" perfectly well - that DOF increases with smaller apertures, shorter focal lengths, etc. - but it's the "WHY" that's bothering you. I'm interested myself. I think I once saw a diagram illustrating what happens to rays of light at different apertures, but I can't remember where. Perhaps I should try to reconstruct it myself. (Best way to learn!)

[Mark_Cohran]

Wlelcome to POTN, Mr. Ed. Sadly I think you will find that the links posted by other contributors, whilst interesting and covering depth of field issues reasonably well, do not answer your question. I infer from your post that you understand the "WHAT" perfectly well - that DOF increases with smaller apertures, shorter focal lengths, etc. - but it's the "WHY" that's bothering you. I'm interested myself. I think I once saw a diagram illustrating what happens to rays of light at different apertures, but I can't remember where. Perhaps I should try to reconstruct it myself. (Best way to learn!)

Actually the Cambride Tutorial on Depth of Fields explains the concept quite well, including the circle of confusion (COC), from which depth of field is derived and how aperture affects the COC.

Mark

[Wilt]

Yes, Mark's link is a very good overall discourse on focus, including not only DoField but also DoFocus.

To put it overall in layman's terms, when a point is not precisely at the plane of focus in the scene (but is in front or behind the plane of focus), it is depicted not as a perfect point, but as a circle at the focal plane of the camera. The farther from the plane of focus, the larger the circle (known as the Circle of Confusion -- I'm confused why! :confused: ) And the size of the aperture is inversely related to the absolute size of the CoC for that out-of-plane point...unfortunately I'd have to consult my text on optics for precisely Why, rather that the usual equation which simple captures the variables!

[StewartR]

Actually the Cambride Tutorial on Depth of Fields explains the concept quite well, including the circle of confusion (COC), from which depth of field is derived and how aperture affects the COC.Sorry Mark, I just don't see it. Yes, it is a very good tutorial, and yes it does explain the concept of DOF very well, and yes it does explain the COC. But I couldn't see an explanation for WHY the aperture affects the COC and hence the DOF.The farther from the plane of focus, the larger the circle (known as the Circle of Confusion -- I'm confused why! :confused: ) And the size of the aperture is inversely related to the absolute size of the CoC for that out-of-plane point...unfortunately I'd have to consult my text on optics for precisely Why, rather that the usual equation which simple captures the variables!My point exactly.

[rowdyred94]

Perhaps if there were two or more drawings, each with a different size lens, we could see why aperture affects DOF. I still wrinkle my brow over this almost daily, and I've had several years of physics (though it was long ago).

Edit: Here's a good explanation, but sans diagrams: http://photo.net/bboard/q-and-a-fetch-msg?msg_id=0006DG

Edit again: This might do the trick: http://www.trustedreviews.com/digital-cameras/review/2006/08/16/Digital-Camera-Tutorial-Aperture-Depth-of-Field/p2

[Mark_Cohran]

Sorry Mark, I just don't see it. Yes, it is a very good tutorial, and yes it does explain the concept of DOF very well, and yes it does explain the COC. But I couldn't see an explanation for WHY the aperture affects the COC and hence the DOF.My point exactly.

Hmmm, seems pretty evident to me. Maybe I've got too much of an engineering background. :)

[Wilt]

Hmmm, seems pretty evident to me. Maybe I've got too much of an engineering background. :)

Hey, don't play 'smart'! I don't have an engineering background and that photonet explanation seems pretty evident to me, too. :)

[StewartR]

The second link provided by rowdyred94 - this one (http://www.trustedreviews.com/digital-cameras/review/2006/08/16/Digital-Camera-Tutorial-Aperture-Depth-of-Field/p2) - has a couple of diagrams that explain it brilliantly, even to humble souls like non-engineers.