Depth of Field, Part I: The Basics


Depth of field (DOF) is defined as the area in a projected image, forward and aft of the focal plane, which also appears to be in focus in the image. When you pass light through a lens and focus that light to form an image on a piece of film, digital sensor, projection screen, etc., the area of the image that is in true focus is razor thin—the focal plane. Everything else is out of focus, to some degree. However, because of the subtlety of the out-of focus regions, we do not notice the softness of the image until, as objects are located farther away from that plane of focus, the blur reaches a certain level; our eyes, and cameras, see a region of depth in an image where everything appears to be in focus.

Let’s set up a visual aid to help you “see” the phenomenon. Instead of visualizing arbitrary light rays being reflected from a subject, for the purposes of this article, we can imagine we are taking photographs of a single tiny point of light. If that glowing point of light is in focus, it looks like the single point of light that it is. If it is out of focus, it looks like a larger, blurrier point of light. Right?

The camera and lens. The lens focuses the light onto the image plane.

If you review my article about focus, you will see simplified diagrams showing rays of light reflecting from a subject, passing through a lens with a moving focus element, and then being focused, ideally, onto the sensor/film or image plane. The rays of light that converge before or after that image plane, when the focus lens is moved toward or away from the subject, are rendered out of focus at the plane. Now, let us imagine fixing the focus lens element at a constant distance from the subject. Assuming the subject is three-dimensional, only those light rays that are at the focus distance are converging correctly onto the image plane. Any light rays reflecting from the subject at different distances from the lens are going to either converge before or after the image plane. This creates a blurry spot at the image plane. However, many of those convergences are happening just prior to or just after the plane; close enough that the blurry spots are tiny and we do not notice that those reflected areas of the subject or scene are not in true focus.

The bottom two illustrations illustrate what happens when light emitted aft or
before the focus plane intersects the image plane. A blurry spot is created. If the intersection of light is close
to the image plane, the blurry spot is small and the light appears in focus.


Now, here is the cool part: there are physical variables in photography that allow us to adjust the depth of the region of what looks to be sharp before and after the plane of focus—the DOF.

Factors Affecting DOF

The depth of the DOF region is described in several ways. A DOF that does not encompass a great distance is called shallow, small, narrow, or short. A DOF that covers a great distance is known as deep, large, wide, or long. These terms are synonymous and can be used interchangeably. No term is more correct than another, but I have found that, for simplicity, describing DOF as “long” or "shallow" works the best and helps to reduce misunderstanding.

There are four factors that affect, or control, the DOF in an image:

  1. Circle of confusion (COC)
  2. Aperture of the lens
  3. Lens focal length
  4. Focus distance (distance between lens and subject)

DOF is not specific to a certain camera or lens. It is a combination of these four things that creates the DOF we get to observe in a projected image. The first factor is somewhat confusing and intricate. The final three factors are easily controlled by the photographer.

If you feel a wave of confusion bearing down on you and you do not want to reference diagrams, talk about blurry spots, or look at formulas, the following is for you:

If you want to take an image with a softer focused background, a shallower DOF, you may use a larger aperture, a longer focal length lens, or get closer to your subject with a given lens. To get an image where the background appears more in focus, a longer DOF, you should narrow your lens’s aperture, use a shorter focal length lens, or get further away from your subject.


If your curiosity is heightened and you want to know more about how and why this happens, please keep reading! Then, if you want to know the math behind it, we have you covered in Part II.

Circle of Confusion, Simplified

Confusingly known as the disk of confusion, blur circle, blur spot, and circle of indistinctness, the circle of confusion (COC) is the most complex and difficult DOF concept to grasp.

Here is an illustration of how objects at different distances, although identical in size,
will create spots of different sizes (and blur) at the image plane.

The COC is defined as the size of the largest blur spot that still appears as a single point (in focus) in an image. What COC brings to the DOF equation is a standard criterion for sharpness. You may have seen other definitions of DOF that refer to the term: “acceptable sharpness” or “acceptable focus.” By itself, “acceptable” is a subjective term. What might be acceptably sharp or in focus to one viewer might be horribly blurry to another. So, for optics and camera engineers, we need to make this a definitive measurement and not subjective. The term “acceptable” will reappear in this article, but know that, when calculating DOF, “acceptable” is a mathematically defined quantity when discussing sharpness or focus.

COC is a function of:

  1. Viewing distance
  2. Enlargement
  3. Visual acuity

Viewing distance: The blurry spot above looks like a blurry spot when viewed from a normal distance from your computer
or tablet screen. However, if you view this same spot from across a room, it will look like a crisp, clean, green dot.


Viewing distance: Let’s get back to that point of light in an image. If you put your nose to the computer screen, or up to the print or poster, that point of light might look like a blurry spot at very close range. However, if you stand across the room and look at that same spot, it will now look like a single point.

If your eye is closer to the reproduced image, the DOF is shallower. Points of light must be points to not look blurry. If you are farther from the image, the DOF is longer. Blurry points might look not-so-blurry from a distance.

This animation simulates viewing an object at different distances.

Enlargement: The larger the image is reproduced, the shallower the DOF. The smaller the reproduction, the longer the DOF. This factor is similar to the viewing distance described above. Reproduce a blurry spot on a giant highway billboard and it’s a huge, blurry spot. Reproduce it on a tiny wallet-sized print and it might look like a single point of light.

Enlargement: An identical image is reproduced at different sizes.
Note how,
in the smaller image, the golf ball’s texture is more consistent where, in the larger image,
the image shows short depth of field in which only a portion of the dimples are in focus.


Visual acuity: We apologize to those with less-than-perfect eyesight. If that point of light looks blurry to you because your eyes are not focusing correctly, your DOF is longer than those who can see the point of light more clearly. The better your vision, the shallower the DOF.

Now that we have COC out of the way, the DOF discussion gets much easier to understand moving forward. We are also about to discuss the elements of DOF that we can control directly with our camera and lens.


Aperture, the relative size of the opening of the lens, not only controls how much light enters a lens, it has an effect on DOF.

Aperture DOF comparison

The larger the aperture, the shallower the DOF. The smaller the aperture, the longer the DOF.

Light entering a lens with a diaphragm that has been opened to its maximum aperture will have to be bent more to meet at a single point at the sensor. Many light rays are passing through the aperture, far from the optical axis of the lens. Because they are bent more, they intersect the image plane at a greater angle than light passing nearer the optical axis. When the light rays converge before or after the image plane—out of focus—the greater angle causes a larger blurry spot to be produced at the image plane and, therefore, produces shallower DOF.

Illustration of light passing through a large aperture versus a smaller aperture

Light passing through a narrow opening on the lens, a smaller aperture, is passing through the lens much closer to the optical axis and is not refracted as much as light entering at the extreme edges of the lens. This reduction in refraction means that the defocused light rays are closer together when they intersect before or after the image plane—forming a smaller blurry spot at the image plane. Even though the light rays are intersecting before or after the image plane, the smaller angle creates a smaller blurry spot and this gives you a longer DOF.

So, to give your photographs longer depth of field, dial in a smaller aperture (larger F-stop number). If you want shallow depth of field, open the aperture (smaller F-stop number).

Focal Length

The focal length of the lens, the distance from the rear nodal point of the lens to the image plane when the lens is focused at infinity, has an effect on both your field of view and your DOF.

Focal length DOF comparison

The shorter the lens focal length, the longer the DOF. The longer the lens focal length, the shallower the DOF.

With aperture, we noted that the more refracting of light rays meant a larger blurry spot at the image plane and, therefore a shallower DOF. Here, we have the greater refraction due to the focal length, but, because the lens is a different physical size, we are also changing the distance at which those points are reproduced before and after the focal plane. This change of distance is having an effect on DOF.

Why is that? A shorter focal length lens has to bend the light entering the lens at a sharper angle to meet the image plane, because the lens and image plane are closer together. Because of the greater angle of refraction, the out of focus intersections happen closer to the image plane. This shorter distance causes a smaller blurry spot to be created, resulting in longer DOF.

Illustration of light from finite subjects passing through lenses of different focal lengths shows the relative distance of the image points.

The farther you move the lens from the image plane—a longer focal length—the less the light needs to bend to intersect at the image plane. This means the out-of-focus intersections will happen at a greater distance from the image plane, meaning a larger blurry spot, thereby causing a shallow DOF.

If you want comparatively shallower DOF, use a lens with a longer focal length. A wider-angle lens will give you longer DOF.

Focus Distance or Subject-to-Lens Distance

The true variable for DOF equations is: focus distance, assuming you have focused on a finite subject. This is the distance from the lens where the focal plane exists. Older lenses had markings on the barrel that told you exactly how far the lens was focused, but many modern lenses have eliminated those markings.

Focal plane or subject-to-camera distance DOF comparison

Having the focal plane at a short distance from the image plane renders shallow DOF. Conversely, the farther the image plane is from the focal plane, the longer the DOF.

This is most easily visualized when you think about some photographs you have likely seen. A close-up portrait features a background that is not in focus, while a landscape photograph taken across an expansive view will show the foreground river and trees to be in focus, as well as the mountains dozens of miles beyond it.

At a greater distance from the subject, light rays will have a narrowed path through the lens and, when the out-of-focus rays intersect before or after the image plane, they will produce a smaller blurry spot and create longer DOF.

Illustration of light from finite subjects at different distances passing through lenses shows the relative distance of the image points and the difference in the size of the infinite blur spots.

Conversely, place the camera close to your subject and the light rays must be bent more to intersect at or near the image plane. Again, just as with focal length, the change of distance produces a larger blurry circle and, therefore shallower DOF.

If you want to have a shallow DOF effect on your image, get closer to your subject. Conversely, move farther away from your subject if you want longer DOF where more of the background will appear in focus.

Seeing DOF While You Photograph

Let’s finish with: How do I “see” DOF?

Way, way back, when you connected a lens to a camera, the image you saw on the ground glass or viewfinder showed you the light that was making its way through the camera through whatever aperture opening you had dialed in, or at which the lens was fixed. Auto-diaphragm changed this on SLR and other types of cameras. Auto-diaphragm means that when you attached a lens, the camera automatically opened the aperture on the lens to its maximum setting. Why do we want this? If the lens aperture is open fully, you will get the brightest possible image in the viewfinder from which to compose your image.

Without auto-diaphragm, what you saw on the ground glass or through the viewfinder showed your depth of field (and sometimes a very dark image). The invention of the “depth of field preview” button allowed photographers with SLR cameras to depress a button and close down the diaphragm to whatever F-stop they had selected. The image would get darker (if you chose an aperture smaller than maximum) and then you could pre-visualize the depth of field for that scene. Release the button and the auto-diaphragm feature would open the lens to its maximum aperture again.

Today, with mirrorless interchangeable-lens cameras and point-and-shoot digital cameras, as well as DSLR cameras with live view, the image you see on your LCD monitor or in the electronic viewfinder will show you the live DOF without having to press a dedicated DOF preview button.

Part I Conclusion

The bottom line is that, if you know that you can change your DOF by changing aperture, changing your distance from the subject (or focus distance), or by changing the focal length of your lens, and you know how those three things affect your DOF, then you are all set and you can go out and start manipulating DOF while you take photos!

Adjusting your DOF changes the way your image looks. Shallow DOF is sometimes used to enhance portraits or close-up macro images, aesthetically. Long DOF is used often by landscape photographers to make sure that both the foreground and the background appear to be in focus.  

One of the most common questions asked at the photo counter at B&H is, “How can I take portraits with a blurry background?” Utilizing your knowledge of DOF to open up your aperture and close the distance between you and your subject will help you achieve this.

The possibilities of creative DOF are endless, and it all comes down to how you want your image to look, and what DOF your lens is capable of producing on your camera.

If you care to dive into the proverbial weeds of DOF, click on Part II to see how mathematics is at the heart of the way DOF is calculated. If you don't give a hoot about the math, and are more into the mythology, proceed directly to Part III.


There are a great many articles on DOF online. I sometimes find mistakes in the articles, or conflicting information. What you have read above has been carefully researched and is the best information I feel that I can present. However, if you have a question, comment, or see something that you feel is inaccurate, please bring it to my attention in the Comments section, below. Thanks for reading!



This is a very detailed article on depth of focus not depth of field. 

Hi John,

From our friends at Nikon: "The term depth of focus, which refers to image space, is often used interchangeably with depth of field, which refers to object space."

In another explaination, I found this: depth of field is the area in front of the lens that is in focus—what this and the two follow-on articles discuss—and depth of focus is the area aft of the lens where the film plane is placed to allow a focused image.

Thanks for reading!


really i loved the article soo much having good information

I especially loved this series of articles.  The illustrations are fantastic.  Salute, to the illustrator.

Thanks Waddizzle! I will pass your props to our graphics team!

Many thanks fro the clear and informative article. Requires a couple of reading, but that is the way it should be. Thanks again. John

Hey John,

Thank you for reading! I hope the reason it took more than one reading was not illegible writing!

Depth of field can be kept really simple, or, it can get deep! I tried to give the readers options on how deep they wanted to get.

It is certainly "legible" - I suspect you mean to say you hope it is "intelligible."     ;-)

One simple way to see the influence of the apperture of the lens in the DOF is through the concept of The Camara Obscura, used for some painters  more than a centuy ago. Then they didn't used lenses, but only a pinhole in a window or wall in a dark room or space. They saw inverted images as with any simple lens, and didin't need a way of focusing because de size of the COC was the same of the pinhole at every distance of it, being always "in focus". The more you close de diafragm, the smaller the COC that defines the image on the negative, sensor or wall, and the better the image quality. The ninth illustration shows it clearly.

All you explanation about the DOF theme, also serves to explain why "slow" lenses are cheaper than "fast" ones. The more DOF you get at wide open, the less de defects in the lens are shown, for the same Focal length, and the leeser influence of field curvature in the periferical quality of the images in some lenses al higher f number. At least some kind of them.

Regards. Roberto.

Hey Roberto,

Thanks for your comments and sharing the insights on the camera obscura. That method of photography is still very viable and fun!

Another reason that "slow" lenses are less expensive is that they contain smaller amounts of glass. Glass is the expensive part of the lens, not the metal and plastic!

Thanks for reading!

Todd, enjoyed the article.

It was interesting and timely for me, because I just recently scanned a large group of 35mm images I made in Okinawa and southest asia in 1958-1959.  The slides and negatives have been preseved fairly well, but the constant I kept seeing was that scanned at high resolutions, when you got down to the nitty gritty, thay weren't all that sharp.  Most of the imges were made with a Contax lllA, with a 1.5 Sonar or one of a couple of Nikon lenses I used. Most always my 35mm Nikor lens gave sharper results (expected from a wide angle lens).  I had concluded that the reason for the unsharp images was due to my almost complete reliance on using the depth of field scales.  And your article leads me to believe that is a correct conclusion. This problem wasn't so much noticed before, since the viewing methods for such images then were generally eyballing the slide, or projecting it on a screen.  But the new high resolution scans revealed what I had never realized.  There were some that were pretty sharp, and I concluded these were ones that were focused on the primary subject. 


G. Gloyd

Hi G.,

Thanks so much for your comments and perspective. I guarantee that if we all looked at our old film prints or slides with the same kind of eye for sharpness that we have today (not to even mention the ability to zoom in on a digital image on a computer screen), we would be horrified at the "sharpness" of some of our "sharp" images.

Sometimes, it might be best to just have a 5x7" print in your hand, hold it in front of you, and enjoy the photo!

Thanks for reading and sharing!


Great article!  Besides being a working professional, I teach a photography class at a local unicersity.  May I reference your illustrations to help my students?


Hey john,

Reference away! Feel free to credit the source, however. Our graphics folks work hard on these products and I want them to get the credit when they can.

Thanks for the well written insights, Todd.  Any chance your tutorials are available in .pdf format for saving?

Hi Todd,

Great description and teaching tool of what DOP is.

I have the same question as Larry. I often get asked to explain DOP to new photographers, and struggle. People just starting out, have trouble getting their head around apertures, shutter speeds, exposures, focal lenghts etc. I know I could refer them to this page, but I would love a pdf print out that I could put in front of them and explain it that way. Hope this request doesn't come across as rude, because of all your hard work. Thanks

Hey Gerard,

Thanks for the compliments! Not rude at all!

As I told Larry, see if your web browser has a "Save as .PDF" or "Export as .PDF" option. It sometimes isn't perfect, but its a quick and easy way to save the article and print it out. Or, you can manually cut-and-paste the images and text.

Hey Larry,

Unfortunately, we don't have .pdf files available for download, but your web browser might have the ability to save the page(s) as a .pdf or another type of file. You can also cut-and-paste the images and text into a word processor if you have the time.

Thanks for your compliment and thanks for reading!

Excellent article! Two minor comments for whatever they might be worth:


First, an admittedly picky quibble: In the usual terminology, the "focal plane" is behind the lens (on the film or image sensor). What's labeled the "focal plane" in the diagrams here is more usually called the "field plane." On the other hand, one could quite reasonably argue that the clearest, most self-defining term for the field plane might actually be the "in-focus plane." Still, it might help to mention in passing the usual terms to help avoid confusion for people reading other articles or textbooks on optics.


Also, with respect to the circle of confusion, it might help to point out the connection with bokeh, since most people are at least somewhat familiar with that concept.

Quick addendum: One advantage to the term "field plane" is that it's consistent with the term "depth of field." (There's also "depth of focus" in on both sides of the focal plane behind the lens, but that's mainly of intererst with respect to pro cameras with interchangeable lenses and adjustable backfocus (and of course to designers of cameras, lenses, and imaging systems).

Hey D Gary,

Trust me, on this topic, you can easily dive into the weeds inside the weeds! Luckily for me, I am not a lens designer!

Thanks again! 

Hey D Gary,

Thanks for the comments! You are certainly correct. Having said that, I have found, especially when writing these pieces, is that the world of photographic terminology is very fluid and not always consistent from book to book or website to website. It is immensely frustrating! I purposely chose terms that, I felt, were the least confusing.

In regards to bokeh, while related, has the potential to derail the DOF discussion as its mention might create questions that could only be answered by a discussion of a lens' optical and aperture design. Gotta keep on topic! We should probably have a separate bokeh article. I'll look into it!

Thanks for the compliments and comments, and thanks for reading!