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Physics of Depth of Field

Depth of Field Physics

We were recently asked about the inner workings or physics behind depth of field, in other words, why does a smaller aperture create more depth of field whereas a large aperture results in a smaller depth of field.

For those who aren’t familiar with depth of field it’s simply the amount of “stuff” that is “in focus” when you take a picture. A large depth of field means the entire picture appears to be in focus. A small or shallow depth of field means only a small portion of your subject is in focus. To learn more about the topic, you can it all in our handy depth of field guide.


When a camera focuses, the focus is on one point. The light comes from the subject (represented by the X & Y) goes through the lens and the light is then put against film or a digital sensor. When the image is in focus, the light is converging (coming together) just right. When the light doesn’t come back together just right, the result is a blur.

depth of field
Wide Depth of Field

When the opening or aperture is smaller the light does not diverge as much because it is now narrower by nature. So when the rays finally arrive at the “Focal Plane” of the film or sensor, they’re not able to diverge as far to be as noticeably out of focus. So your perception is, with the smaller aperture opening, that most of the image is in focus–even though there is still just one subject or point in focus just like with the more open aperture. The difference is the effects of being out of focus are much more exaggerated when the aperture is wide open.

Free Videos on Depth of Field

Aperture and Depth of Field in Photography

A Simple Guide to Depth of Field

Depth of Field. How the aperture actually affects DOF



  1. Hi,

    I came across your web page as a recommended reference to explain how aperture influences depth of field. The original query was posted at


    on 5 Jan 2015 and a link to your page was posted on page 5 after many totally incorrect responses to the OP. Your graphic explanation of how blur arises and is affected by aperture diameter is excellent and undoubtedly the best answer in principle to the original question, but unfortunately your diagrams are incorrect.

    You show the more distant subject X being focussed further back (behind) the sensor/film when in fact it would be focussed closer to the lens. Surely you know that as you focus on an object further away the lens moves closer to the sensor; close objects are focussed by racking the lens forward because they get focussed farther back from the lens.

    I realize this web page is a few years old, but I think this change to your diagram would be an improvement to your explanation.


  2. When you are experimenting with the exercise“Physics of Depth of Field” with the smallest f stop to the largest f stop. What is the rule to compensate shutter speed?

  3. when doing that exercise, the smallest f stop has given me a much brighter picture than the largest f stop, which makes sense bc of the amount of light being allowed to enter the lens. Do you compensate for that with shutter speed?? my pic on the largest f stop was very dark. Thank you!

  4. These 2 diagrams explain DoF better than any 2500 words I’ve read so far. Thank you very, very much!

  5. I just learned that my rebleXTI dous black and white photos. I can just try everything I have learned so far now with the B&W setting . I will let you know how it works out.

  6. I just understood this after reading it several times over several days. It would be helpful to change the color of the two X points and their reflections through the lens though. It might have clicked a little sooner for me.

    One thing I would like to clarify is, do you need to change your shutter speed when changing aperture settings so you dont over or under expose your subject? I would think so but Im just a beginer.

  7. Sure, You could take a picture with the aperture on the largest setting, the smallest number. This would be taking a picture like the first illustration. You’ll see there is a lot of blur on the things you haven’t focused on.

    Then take a second picture with the highest aperture f-number that will have the smallest opening from light. This will be like the second illustration. You’ll notice that with this photograph the blur on out of focus subjects isn’t as intense as the first.

  8. So I’m still a little confused, are there any exercises I can do to help me understand this better?


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