What Is Depth of Field (DOF)?
Depth of field (DOF) is the distance wherein objects are in focus.
This is the pragmatic explanation.
To be technically correct, DOF is the zone of acceptable sharpness, the area in front of, and behind, a focused subject that appears in focus. Think of it as a range of distances around (in front and behind) a plane of focus that, to our eyes, is acceptably in focus.
A deep DOF means that a lot of objects in your picture from front to back are in focus. You want a deep DOF when you take a landscape picture because you want the maximum “sharpness.”
A shallow DOF means that only a few objects from front to back are in focus. You want a shallow DOF when you want to focus the viewer’s attention only to the main subject (e.g. when you are taking a portrait) and everything else is blurred.
Technically, only the subject in focus — and all other objects at the same distance — are in focus; everything else in front and behind are out of focus. How much out of focus depends on a term called the "Circle of Confusion (COF, or COC)."
If you want to learn more about COF, click on the box below. If you don’t understand it, don’t worry. Many photographers take superb pictures using depth of field to their advantage never having heard of COF. I never did until I started researching this topic. And I find that it just confuses the hell out of most people.
So, here we have these 3 dots, arranged one behind the other (with the closest dot to the right of the middle dot, and the farthest dot to the left of the middle dot, so a camera can take a picture of all 3 dots).
Now, we focus our lens on the middle dot, which comes out in perfect focus. The two other dots also appear in focus, but peering closely at the resultant photograph, we notice, however, that the dot in front of and behind the middle dot appears as circles instead of perfect dots. I.e., technically, they are out of focus, but to our naked eyes (at a "normal" distance), they "appear" in focus.
It is this circle that we call the "Circle of Confusion." So the COF is the diameter of a dot such that when we view it with the naked eye, it appears in focus. If this circle gets past this diameter, our eyes tell us it’s out of focus.
Lens manufacturers have to decide what that diameter is going to be and design their lenses accordingly to be able to resolve a dot within that COF so that it appears sharp to us. For a more technically detailed explanation of how COF is determined by lens manufacturers, read Michael Reichmann’s excellent tutorial: Understanding Depth of Field.
It is about all I am going to say on COF. Do you need to understand COF to use DOF? Personally, I don’t think so. But if you are one of the people who absolutely must understand COF, its history, how lens manufacturers calculate DOF based on the COF, etc. etc., just check out the link above or do a search on Google. There’s enough material there to keep a technical mind happy for many hours.
What Affects DOF
What we really want to know as photographers is what affects DOF so we can control DOF in our pictures. For a long, long time, photographers have gone with the following three criteria:
- lens aperture
- distance from subject
- focal length
Lately, because digital cameras use different sensor size, some have added sensor size as also a factor.
Truth be told, only the distance from subject and the lens aperture directly affect DOF.
The focal length and sensor size can be said to indirectly affect DOF.
Before we continue, we need to make one thing clear. When we say “aperture,” what we really mean is “distance of the entrance pupil.” Technically speaking, it is the “distance of the entrance pupil” that affects DOF. When we dealt only with 35mm film or full-frame sensor, we can interchangeably use “distance of the entrance pupil” and “aperture.” But, with cameras using different sensor sizes, the aperture listed on the lens do not give the same DOF but will depend on the sensor size used.
Now, “distance of entrance pupil” is a mouthful and is not listed anywhere on the lens. But, aperture is. So, photographers have come up with a different term: 35mm equivalent aperture. So, anytime you read “aperture” here, substitute with “35mm equivalent aperture.”
Lens Aperture
The aperture is simply the size of the opening that allows light to go through the lens. It is expressed in f/stops, and a typical aperture range is f/2.8 – f/8, giving the range from maximum (large at f/2.8) to minimum (small at f/8) aperture.
A small f-number (or f/value or aperture value) (e.g. f/2.8) indicates a large aperture.
A large f-number (or f/value or aperture value) (e.g. f/8) indicates a small aperture.
So, f/2.8 is a larger aperture than f/8.
Generally, a large aperture gives a shallow DOF, and a small aperture gives great DOF.
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f/2.8
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f/3.5
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f/4
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f/5.6
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f/8
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large / max
aperture |
small / min
aperture |
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less DOF
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more DOF
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great for
portraits |
great for
landscapes |
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Putting Aperture into practice: If you want only the subject the lens focuses on to be sharp, and everything else to be out of focus — such as a portrait with the background nicely blurred — then you would "open up the aperture," i.e. use a large aperture. |
| If you need most of your picture to be in sharp focus — such as a landscape scene — then you would "stop down the aperture," i.e. use a small aperture. |
| Shallow DOF | Great DOF |
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| Note how the use of a large aperture throws the flowers in the background out of focus. Focus has to be precise. |
Using a small aperture extends the DOF from the foreground all the way to the background. |
| 49.8 mm, Av, Spot, 1/30 sec., f/3.5, +0.7EV, Macro, Tripod used |
49.6mm, Av, Spot, 1/5 sec., f/11, +0.7EV, Macro, Tripod used |
| Minolta DiMAGE A2 | |
In most point-and-shoot cameras, you may not be able to directly control the aperture. Many, however, provide a Portrait scene mode and a Landscape scene mode that basically do what we are after, i.e. use a large and small aperture, respectively.
As most of you have found out by now, it is quite difficult to obtain a shallow depth of field with most point-and-shoot digital cameras because the lens attached usually has a very small aperture.
Distance From Subject
When you focus on a subject close to the camera, the DOF is less than when you focus on the subject farther away from the camera.
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Putting Distance From Subject into practice: Step away from your subject to obtain greater DOF, or move in closer to decrease DOF. |
Moving In Close:
Even though we use a small focal length (35mm, 35mm equivalent) and
a small aperture (f/7), by moving in close (macro),
we are able to isolate the subject from the background.
If we had used a larger aperture, we would have obtained even shallower DOF.
Fujifilm FinePix S7000
When you use a camera with a smaller (than full-frame) sensor, you need to be further away from your subject to cover the same field of view that you would get from a full-frame sensor. Being further away increases your DOF. That is why some people say that sensor size “affects” DOF. It does not do so directly, but indirectly by making you increase or decrease the distance from the subject. The only way for you to get the same DOF while getting the same field of view at the same distance is to use a lens with a smaller focal length and a larger aperture.
Focal Length
Set your subject in front of — but not too close to (say, about 3m or more) — a bush (or a tree with lots of leaves, or some other kind of busy background). Use wide-angle and take a picture. Both your subject and the bush would most probably be in sharp focus.
The background may distract from your main subject — unless you’re aiming for a Product Shot where you want to situate your subject in his or her environment, and do want both subject and background to be in sharp focus.
Now zoom in and fill in the screen with your subject’s face and shoulders. Look at the resulting image and you’ll probably notice that, though your subject is still in focus, the bush now appears out of focus, giving a nice blurred background that does not steal attention from your subject.
Photographers use this technique very effectively to "affect" DOF.
So, a wide-angle lens has greater DOF than a telephoto lens. Most consumer digital cameras have very short focal lengths and that is why it is so difficult to obtain shallow DOF, even with the aperture opened up wide.
I promised to come back to the storm of controversy brewing in some circles that disputes that focal length has any effect on DOF.
The contention is that, for a fixed image size in your screen, the DOF is unchanged irrespective of focal length used. And they have pictures to prove it!
However, photography is part technics and part Art. The technics part may well tell us that DOF is the same at all focal lengths for a fixed image size on screen. What this is saying is that we achieve the same DOF whether we fill the screen with the face by walking to the subject, or backing off and zooming in.
The Art side, however, tells us that a long lens (i.e. long focal length) reduces the distance from the subject and thus provides a shallow DOF. Yes, we can achieve the same narrow DOF by walking close to the subject. However, the results look very different! Why? Because a long lens also provides a "flattened" perspective which makes the narrow depth of field much more prominentthan walking closer to the subject using a lens with normal focal length does. The link above with all the pictures prove it.
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Putting Focal Length into practice: Use long focal lengths to achieve the effect of a shallow DOF. |
Long Focal Length:
Bu using a long focal length (380mm, 35mm equivalent), we are able to
"throw the background nicely out of focus"
Minolta DiMAGE Z1
Putting It All Together
Putting it all into practice:
A large aperture (f/3.2), moving in close to your subject (macro) and using a long focal length (112mm, 35mm equivalent) has thrown the grass into a nice green blur
Olympus Camedia C-8080 Wide Zoom
Macro Photography
In close up shots, should we use a small or large aperture? It’s really up to you and the effect you are trying to achieve.
If you want to isolate your subject from a distracting background, then use a large aperture for a shallow depth of field.
But if your subject is facing you or at an angle, only a small part of its body may be in focus. To get as much of your subject in focus as possible, you’d use a small aperture for maximum depth of field.
A Word On Minimum Aperture
Following our rule of thumb on using a small aperture to obtain a great DOF, does this mean that we should always use the minimum aperture if we want the greatest DOF?
It sounds logical, but because of diffraction that comes into play when the aperture is too small, this may affect the quality of the image, giving the appearance that some objects (especially light sources) are not in focus (but hey, use this to your advantage to obtain starry effects when photographing light sources!). So you don’t have to stop down to the smallest aperture for maximum DOF. For example, on 35mm cameras (where the aperture stops all the way down to f/11 or even f/16), f/5.6-f/8 are usually the best apertures to use for maximum DOF. On digital cameras, try f/5.6.
Pseudo DOF
A word of caution. Some use post-processing and apply a Gaussian (or other type of) Blur to achieve a blurred background effect. Depending on your skills, this may be quite effective but, to a trained eye, this is almost always apparent and looks "false." So never try to pawn off a processed blur as naturally obtained shallow DOF. Also, be careful not to apply this blur effect to all your pictures, but only to a few whose subject matter will benefit from it and where it won’t matter that the blur is processed, not natural.
Summary
Even though the subject you focus on is the only thing technically in focus, some objects in front and behind your focused subject also appear to our eyes to be acceptably in focus (thanks to the COF). This zone of acceptable sharpness is the DOF.
We can increase the DOF (or the appearance of it) by using a small aperture, moving away from our subject, or using a wide-angle lens.
We can decrease the DOF (or the appearance of it) by using a large aperture, moving closer to our subject, or using a long focal length.
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Our Readers Write Back
From: Richard Crossley
Feb 8, 2006
Differences in perspective are not the result of focal length, but of physical distance, as [we] move the camera from shot to shot.
The truth is that focal length does not affect either the DOF or the perspective, at least technically. A 50mm shot and a 300mm shot from the same spot would of course appear different, but that is the result of magnification. If the 50mm shot is blown up until the image of the 300mm shot could be cropped from it, they would be identical. The longer focal length does not change the DOF: it is simply magnifying the out of focus area.
In the movie making business, it is critical to understand that, so we don’t automatically just run back 100 feet and create audio problems, and also so that we focus on the framing and desired image size, knowing that even though our small LCD display may not be giving us that ‘soft background’ we want, that when the entire picture is blown up, the true effect will indeed be seen.
Also, as a filmmaker, I find you miss a large and important factor in your DOF discussion. Whereas you list focal length as a true determining factor when in fact it is not, you fail to mention target size at all, which is a huge factor, and represents the single biggest reason why it is harder to get a shallow DOF from a digital camera. The relationship of the physical target size (1/4 inch CCD as opposed to 35mm film) makes a HUGE difference.
[Editor: emphasis ours, and a lengthier version of the discussion can be found here. ]
