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Technically, why is the out of focus area blurred more when using a bigger aperture?

I understand that a small aperture (large aperture number eg. F/22) means large depth of field. It is simple to undersand that small aperture size means, less light is allowed into the camera sensor. If less light is allowed how is the large depth of field is acheived?. I always believed that "Large" means you need more light, but in this case, if aperture is small, large depth of field is achieved. Could someone please help me understand this from form technical perspective.

I have seen many threads related to somewhat same question, but I don't think I have got much help, so I am posting this question

(I own a D90)

  • The links in the answer here photo.stackexchange.com/questions/16513/… will explain it, and the wiki has a fairly simple diagram – Dreamager Dec 4 '11 at 13:22
  • I think the answers in the question I linked explain this pretty well. If there's a follow-up needed, either comment there or else ask a new, focused (no pun intended) question about that specific aspect that's unclear. – mattdm Dec 4 '11 at 14:32
  • Thanks mattdm. The link provided by you gives me useful information to answer my question. Basically this is what I saw in the link."Basically, the smaller the aperture, the more restricted-to-exactly-in-focus the light is. A bigger aperture lets in more light, but the "price" is that it's less controlled" – pradeeptp Dec 4 '11 at 15:20

Watch these series of videos (the link leads to the first part), that made me understand it right away. The idea is in the way the light travels, and a smaller opening makes light travel through it differently than when the aperture is wide. Anyway, one picture is worth a thousand words:



This question has more to do with optics (physics) than photography. This link shows you how the DOF is derived. You can easily check you hypothesis by substitution. This link provides a bit more verbose explanation.

Summary: The following equations describe the boundaries of the field of view. For a given set of focal lenth, sensor size and subject distance, substitute different values of N. (the online calculators do this)

DOF equation Source: http://berniesumption.com

  • I think understanding the physics behind photography has lots to do with photography! – mattdm Dec 8 '11 at 23:34
  • The basic generalization that the DOF increases with aperture is good enough for photography. Isn't going into the gory mathematical detail just an overkill? – Sridhar Iyer Dec 8 '11 at 23:40
  • There's an in-between that is more useful than either dismissing or going to equations without explanation. In fact, I don't think the equations you've given explain the "why" at all! They just describe the what. – mattdm Dec 8 '11 at 23:48
  • That said, the second link you provide does have a nice explanation. – mattdm Dec 9 '11 at 3:11

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