After a recent conversation, I really got into thinking about Depth of Field, and thought I would post something about it. My girlfriend was taking some pictures and we got to talking about depth of field. Well, it seems that she had been taking pictures at f/1.8, from about 3′, which equals about 1.25″ depth of field with her lens. This made me look around for information about depth of field. I took 4 pictures with my 50mm Canon macro lens. I took the pictures at 2.5, 5.6, 9, and 13 aperture. This gives a visual representation for my example. This is paired with an application at dofmaster.com. These pictures are taken on a Canon XSi, about 7 inches away.
This picture is at f/2.5 – The depth of field is about .03″. The applications shows .02″ in front and behind my focal plane. There is not a lot of depth of field here.
This picture is at f/5.6. The Depth of field is a total of .08″. The applications shows .04″ in front and behind the focal plane.
This picture is at f/9. The depth of field on this one is about .12″. The applications shows about .06″ in front of and behind the focal plane.
This picture is at f/13. The depth of field on this one is about .18″. The applications shows about .09″ in front of and behind the focal plane.
What I find very interesting, is what happens as you move to longer distances, the balance of the depth of field goes from nearly even, to more and more depth of field behind the subject. For example, with this combination, at 10′, the depth of field is balanced at 35% in front of the plane of sharp focus and 65% behind the plane of sharp focus. At double that distance, the depth of field extends 19% in front of the subject, to 81% behind the subject.
Before looking at the calculator, I had been wondering what the actual depth of field “spread” was. Finding that it is 100% dependent on subject distance should have been obvious to me, and I understood how it worked, without thinking about it. By learning the technical specifications of how it works, actually adds it to my photography toolbox, to use later. For the most part, I do not think about depth of field, beyond the fact that a smaller aperture, gives me a greater depth of field. If you need to get technical, many modern smart phones (like the G1, iPhone, and Windows Mobile based devices) have DoF calculators, and/or you may be able to use the mobile web to access the Online Depth of Field calculator I have been using.
Adding tools to your personal toolbox helps you to become a better photographer. I hope this quick primer on DoF helps you, on your way in this wonderful hobby, called Photography.
I want to share one more picture. This is one of my all-time favorite pictures I have taken. It also happens to be one of the earliest pictures I took with my SLR. The Depth of Field really stands out, with the gosling’s head in focus, and you can see the grass below it, how shallow the actual Depth of Field was. But, since I was taking the picture at it’s profile, I didn’t need a lot of depth of field. I was shooting as open as possible, on that lens, since the goslings were moving so fast.
Thanks to Baz from dpreview for help clarifying the difference between plane of sharp focus and focal plane. (The focal plane actually being the point where the surface of the sensor is)
Thanks to WilbaW from dpreview for a link to a technical article about Depth of Field or Field of Focus. The link can be found here: http://www.hobbymaker.narod.ru/English/Articles/sharpness_eng.htm. It is pretty techincal, and I plan on figuring it out, in the near future.
Thanks for helping to describe how DoF changes with subject distance. I didn’t have that clear of an understanding. Saw the link on DP review.
Interesting article, I already have a pretty good understanding of DOF but for someone unsure on this subject, this will be a good guide. I’ll bookmark it and give a link to anyone asking for a DOF explanation.
Thanks for the post. You explained it well. I love to see good use of DOF. Not so good at it myself sometimes:)
Thanks Ryfter, I’ll add my penny’s worth below!
To understand depth of field you need to understand what the circle of confusion is.
Basically the circle of confusion is what you get when a point isn’t focused properly. When this happens you get a circle rather than a point. As long as this circle is smaller than the resolving ability of the recording media (in our case a digital sensor) then the point will look sharp.
The normal measurement for the circle of confusion limit in photography, or the circle of confusion diameter limit, is the largest blur circle that will still be perceived by the human eye as a point when viewed at a distance of 25 cm.
The focal plane is the distance from the lens at which a point will resolve on the sensor as a point. It’s in focus. This only happens at one distance, there’s no range of distances where everything is perfectly focused.
If you imagine a point on a piece of paper in front of the camera, at exactly the focal plane of the lens. This point is perfectly in focus. If we move it even a tiny amount towards or away from the lens it will be out of focus, albeit very slightly. If we move it steadily toward the camera we’ll see the point will form a circle rather than a point, and this circle will get bigger and bigger the further it moves from the focal plane. Once the circle reaches the size of the circle of confusion diameter limit we have reached the near end of the depth of field for the lens at this focal distance and aperture. If we return the paper to the focal plane and then move it away from the lens until the point becomes a circle the size of the circle of confusion diameter limit we have found the far end of the depth of field.
The maths behind all this is fairly simple trigonometry and can be looked up on wikipedia. Some interesting points to make though:
1. The depth of field calculators are working to a definition for the circle of confusion which is subjective. As I’ve said above, the normal measurement is the largest blur circle that will still be perceived by the human eye as a point when viewed at a distance of 25 cm. If you’re viewing from nearer or further then the whole calculation is shot, as you’ve changed one of the most important variables.
2. The resolving power of the sensor is very important. The more pixels, the higher the resolving power and the lower the threshold for the circle of confusion limit. A point will spread over more than one pixel sooner making the image look blurred. Another good reason not to get sucked into the megapixel myth.
3. The size of the sensor changes things too. The larger the sensor, the smaller the depth of field. For this reason it’s virtually impossible to get a narrow depth of field with a point and shoot camera. Also, it’s hard to get everything in focus with a medium/large format camera.
4. Depth of field can be changed by altering the aperture or changing the focal length and moving your point of view. Zoom in and walk back to isolate your subject against a blurry background.
5. As the focal plane moves away from the camera (as you focus on things further and further away) a point is reached where everything behind the focal plane falls within the depth of field. This is known as the hyperfocal length and is very useful for landscape photography.
Wikipedia is a great place to look if you’re not sure about any of the terms used here or fancy looking up more detail.
Good work! Thank you very much!
I always wanted to write in my blog something like that. Can I take part of your post to my blog?
Of course, I will add backlink?
Sincerely, Timur I.