Depth of Field Simplified

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Depth of field (DOF) is probably one of the least understood principles of photography. Most photographers seem aware of its existence, but few seem able to define exactly how it works and how to put it to use. We believe that part of the reason for this is because it is completely fluid, (meaning constantly changing) element of the photographic process. It has no set parameter, so it is more difficult to define. It's easy to define an f/stop, or a shutter speed, because once you set your camera it stays put. But, as a photographer, if you take so much as one step closer to your subject: the DOF has changed.

Here is a definition of Depth of Field-

In optics, particularly as it relates to film and photographydepth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions. Courtesy of Wikipedia

There are formulas and technical data as relates to DOF, but because this article is titled, Depth of Field Simplified, we are going to forego all that. It's not necessary for the average photographer to know those formulas to put DOF to work in their photography. Over the years camera, and lens, manufacturers have tried to make DOF easier for the photographer. They have done this by incorporating such things as a depth of field preview button on the camera body, and/or, a depth of field scale on the lens barrel. While these are somewhat helpful, they were always cumbersome to use, and they have disappeared on all but a few camera/lens models available today.

The good news is that you don't need those crutches to put DOF to use.

Let's talk about what DOF is, what changes it, and how the average photographer can make use of it.

Try to think of depth of field as a window: a window of “focus” opportunity.

http://farm8.staticflickr.com/7330/9702280468_18e3ef8a33_z.jpg

A window of Focus by Kent DuFault, on Flickr

What is that window of opportunity? It's your opportunity to choose how much of your photograph will be in focus.

http://farm8.staticflickr.com/7398/9699043947_a97548527b_z.jpgDoF_ArticleIII by Kent DuFault, on Flickr

These Elements affect the Fluid Nature of Depth of Field

  1. The focal length of the lens
  2. The f/stop of the lens
  3. The camera to subject distance

Here are the few simple things you need to know to begin putting DOF to work for you.

The Focal Length of the Lens

A shorter focal length lens (wider angle) will have more depth of field at a given f/stop than a normal or telephoto lens. So, remember this- the longer the focal length of your lens, the less depth of field you will have at a given f/stop.

Let's put this into a real world situation. You're going to take a portrait of two people seated in front of a brick wall. You would like to depict the couple from head to toe, while also seeing the brick wall behind them, and you would like the entire scene to be in focus. Lighting conditions have determined your exposure will be 1/125th at F/8.0. You have room to move around so you can frame your subject in exactly the same manner no matter what lens you use.

You start with a moderate telephoto lens of 135mm. Your test shot looks good, with the exception that the people begin to fall out of focus and the brick wall is way out of focus. You change your lens to a 55mm. The people are now completely in focus, but the brick wall is only somewhat in focus. You change your lens again to a 28mm. Now everything is in complete focus. You take your photograph. Congratulations, you've just made a creative decision about depth of field based on the focal length of your lens!

http://farm5.staticflickr.com/4027/4491179701_3e4e5b215b_z.jpg
GQ Engagement by RLHyde, on Flickr

The F/stop of Your Lens

This is a primary decision when trying to take control of your depth of field. This is the simple thing you need to know. The larger (more open) your aperture, the smaller your window of focus will be. Conversely, the smaller (more closed) your aperture, the larger your window of focus will be.

Look at the f/stops on your camera and/or lens. F/2.8 (an example as this various by camera and lens) is a large aperture, (wide open/ small window of focus). F/22 (again an example as this varies by camera and lens) is a small aperture (more closed/ larger window of focus).

Let's put the f/stop decision to a real world test. You would like to take a photograph of a stack of coins.  Your lens is determined because you must use your macro lens. Your lens to subject distance is determined because you want to fill your frame with the coins. There is not a lot of light so you take your first test shot at F/2.8.

http://farm7.staticflickr.com/6086/6054267008_f599f74f7e_z.jpg

Qtr f2.8 120 by ljguitar, on Flickr

Your exposure looks good so you make some camera adjustments and go to F/8.0.

http://farm7.staticflickr.com/6193/6053717973_7ec7357565_z.jpg

Qtr f8 122 by ljguitar, on Flickr

This helped, but you would like a little more “window of focus” on the coins. You make some more exposure adjustments and go to F/32.0

http://farm7.staticflickr.com/6076/6053718283_17802bf861_z.jpg

Qtr f32 125 by ljguitar, on Flickr

Congratulations, you have just made a creative decision based on DOF and your use of an f/stop.

The Camera to Subject Distance

If you've never bothered to closely examine the distance scale on your lens, we would like you to take a moment and do that.

Did you notice how the distance marks become closer together as the focusing ring is rotated toward infinity? There are optical formulas and reasons for this, but again, for this article on, Depth of Field Simplified, we want you to grasp one simple concept-

When you focus on your subject, the further your subject is from the focal plane (sensor) of your camera the more depth of field you will have using the same f/stop and lens.

Let's put the subject to focal plane choice to a real world test. You would like to photograph three toys soldiers in a row, and you would like them all to be in focus. But you have one lens to work with and limited lighting so the smallest f/stop (biggest window) you can use is F/8.0.

So, you take your photograph and no matter which soldier you focus on, the other soldiers are out of focus. What do you do? Take several steps back… This will shift your subject to focal plane distance closer to the infinity end of your focusing ring. And that… will give you more depth of field using the same lens at the same F/stop.

Once you've found the right distance from your subject, (subject to focal plane distance that keeps all three soldiers in focus) take your photograph. Then in post, crop it back to your original framing.

This, by the way, is the biggest advantage of higher megapixel cameras.

One Final Tip

Many photographers (even experienced ones) are often not aware of this aspect of depth of field. The window of focus does not occur evenly in front of, and behind, your focal point. Approximately 1/3 of the DOF window will occur in front of your focal point and 2/3 will occur behind your focal point.

http://farm8.staticflickr.com/7429/9699043835_c87afb3dc5_z.jpgDoF_ArticleIV by Kent DuFault, on Flickr

This is why the earliest portrait photographers learned to focus on their subjects eyes. The 1/3 DOF window in front of the eyes kept the nose in focus, and the 2/3 DOF window behind the eyes kept the head in focus. This is why if you inadvertently focus on someone's ear while taking their portrait, their face will be mostly out of focus; it is outside the DOF window.

http://farm8.staticflickr.com/7075/7232627280_c13d1e2645_z.jpg
Nice portrait of the young male jaguar by Tambako the Jaguar, on Flickr

This head shot of a young jaguar is an excellent example of the 1/3 in front, 2/3 in back rule. If you examine this photograph closely, you will see that the point of focus is on the bridge of the nose between the eyes. This POF, slightly forward of the eyes, kept the tip of the cat's nose in focus. The 2/3 behind was enough to keep the left ear in focus while the right ear is already going soft. The background is completely out of focus. This is an excellent use of the DOF window- as all the important elements are in focus.

We hope that these simple tips have helped you understand depth of field and how you might now be able to put to work for you in your own photography!

About Author

Kent is an occasional writer at our place, and also handles the weekly “Picture of the Week” contest. He has been involved with photography since 1974 and you can get to know him better here

Is DOF really that hard to understand that most photographers don’t get it?? And in reference to the line above that states something about DOF being “fluid, constantly changing, if ou step closer o the subject your DOF changes…”… No it doesn’t!! DOF is relative to the photographer, so f they take a step closer the window of focus moves 1 step further also, the DOF relative to photographer is the same!?

Craig- You’re right that if the camera moves, the window of DOF moves as well. But you’re wrong- in that it does not stay the same. The reason being that the subject to focal plane distance changes. If you have access to a lens that has a depth of field scale on it. Read the scale when the lens is focused near infinity mark and read the scale when the lens is focused near its closest setting. At the infinity end of the focus scale, the DOF window will cover many feet. At the closest focus setting the DOF window will cover a matter of inches. The window of DOF expands or contracts depending whether the camera is moving closer or further away from the subject.

Lance- You got me on this one, and I had to do a little studying. This is what I have learned. The sensor size does not change the physics of DOF. The reason that it appears that a FF camera has less depth of field is because it needs a longer focal length lens, or, needs to be closer to the subject to achieve the same composition of a smaller sensor camera. Example- I want to take a picture of a car and fill the frame. With an APS-C sensor I can complete that task with a 50mm lens at F/8.0 from 20 feet away. With a FF camera I can complete that task with an 85mm lens at F/8.0 from 20 feet away, or, if I use a 50mm lens I must move closer than 20 feet to the subject to fill the frame. The DOF appears to be less using the FF camera because of an increase in focal length, or a decrease in subject to focal plane distance to achieve the same photograph. (by the way those are just sample numbers I chose out of the air to illustrate the point.)

I think, there’s one more element that influences to the DOF.
The sensor size, the more of the size, the less of DOF, and the less of the size, the more of the DOF.
So when we use a compact camera (very small sensor), the DOF is so large that, we cannot make the background blur in portrait.
But in macro work, compact camera is excelent in DOF.

“You start with a moderate telephoto lens of 135mm. Your test shot looks good, with the exception that the people begin to fall out of focus and the brick wall is way out of focus. You change your lens to a 55mm. The people are now completely in focus, but the brick wall is only somewhat in focus. You change your lens again to a 28mm. Now everything is in complete focus. You take your photograph. Congratulations, you’ve just made a creative decision about depth of field based on the focal length of your lens!”

This section is flawed because when you change Focal Length, in order to maintain equal Magnification of the subject you must also move. So you move an amount basically equal to the change in Focal length. So with Equal magnification comes Equal DOF (plug it into your DOF calculator) in this situation. The exception would be, if one of the combinations would be close to the Hyper-Focal distance in which case the argument goes out the window because that combination WILL have a greater DOF. So because we both change position and focal length there will NOT be difference in DOF in the 3 examples. Remember it’s not that Focal length OR Distance change DOF, They do, But in combination resulting inEQUAL MAGNIFICATION, they are basically the same DOF

The 1/3 and 2/3 of DOF is actually a myth. It only applies to ‘average’ focal lengths at an ‘average’ subject distance. Take one of thos DOF apps and try dufferent scenarios. You will find that the DOF will vary between 50% in front and 50% behind, and 10% in front and 90% behind.

Hello Kent DuFault,

A very well written article. It was easy to understand through graphics. Really, 1/3 and 2/3 rule of DOF was new to me as well. Thanks a lot for sharing this knowledge. I enjoyed reading it.

What I could understand that, exactly, how wider the aperture opens, the more background gets blurry based on the distance. Smallest aperture value like f/1.8 or f/2.8 will blur the background more.

I also shared the information based on the same on my blog. Please go through it and leave your feedback. I would appreciate it. http://siddharthmalkania.com/value-of-aperture-in-photography/

Regards,

Siddharth Malkania

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