Landscape tutorial
Depth Of Field

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Depth Of Field.

Depth of Field or DOF is often one of the most confusing aspects of photography for beginners. This is not because DOF is inherently difficult to understand, but that it is often explained using abstract concepts and confusing terminology. As such, it is easy to get yourself in a muddle.

What is DOF and how do I change the DOF of an image?

Quite simply, depth of field is the portion of the picture that is in focus. This is dependent on four 4 major factors.

These four factors work together to alter the DOF of any given image. I have listed them below in the order that they affect most photographers. Each factor is discussed in more detail later.

1. Lens aperture:
Lenses have an adjustable diaphragm that controls how much light enters the camera. The hole in this diaphragm is referred to as the “aperture” and the diameter of this aperture is expressed in numbers, known as f-stops for example, f2.8, f8 or f11. The higher the number e.g. f11 rather than f4, the smaller the aperture (in diameter) and consequently the greater the Depth of Field.

2. The focal length of the lens:
A basic rule of thumb is: the longer the lens, the shallower the depth of field. Therefore a 28mm lens on a 35mm format camera will have a much greater DOF than a 200mm lens when set to the same aperture.

3. The distance between the camera and the subject:
The greater the distance between the camera and the subject upon which you are focussing, the greater the DOF. This is most apparent in macro photography, where the DOF can be incredibly shallow.

4. The area of the image recording element (be it film
or a digital sensor):

The larger the image recording area the less DOF there is at any given aperture. For example, to get a DOF that extends from 5m to infinity a medium format camera may need to use a higher f-stop (smaller aperture) such as f64. A 35mm film camera, however, may need an f-stop of f22. A standard digital compact which has an even smaller sensor will require an even smaller f-stop such as f8.

I’m kind of ok so far, what’s the confusing stuff?

In my experience the biggest stumbling block that people have is how to express size as it relates to DOF, largely because the terminology used is somewhat confusing. The terms greater, smaller, shallower and larger are used often in relation to the same thing. The result is that they quite often mean the opposite of what you think they might mean, causing confusion. In fact, I’m betting that you are slightly confused now, but hopefully here is where it will start to make sense.

1 – Aperture (f-stop):

F-stops are found on a standard SLR lens or on any digicam that gives you manual controls. They are listed as a series of numbers that don’t necessarily make a great deal of sense when you first encounter them, as there doesn’t seem to be much logic to them or their values. On a typical 35mm SLR lens these f-stops might be: 2.8, 4, 5.6, 8, 11, 16 and 22. They are often referred to as f2.8, f4, and so on, up to f22.

On a manual 35mm SLR lens there will be an unmarked but selectable stop between each of these values, which is known as a “half stop”. On a modern automatic SLR or a digital camera they will most likely be two number values between each full stop, these are 1/3 of a stop for example f13 and f14 located between f11 and f16.

These numbers are perhaps the most important factor that you will use to control DOF. Almost certainly they will be the most commonly used factor.

Small f-stops (those that have a low number, e.g. f2.8) mean a large aperture. For example, f2.8 means that the aperture is larger (or “wider”) than the aperture at f8. This “large” or “wide” aperture creates a “shallow” DOF. F8, or F22, are often referred to as a “greater” or “larger” f-stops, and as such have small apertures, creating a “greater” DOF than f2.8. As you can see, the liberal use of greater, smaller, bigger and shallower makes it easy to get confused.

One way to remember this is:
SMALLER number (f2.8) = BIGGER hole = SHALLOWER DOF
BIGGER number (f22) = SMALLER hole = GREATER DOF

It is even easier of you omit the mention of the hole:
BIGGER number (f22) = GREATER DOF

Most often confusion arises when someone says “use a big aperture”. It can seem more logical that a bigger number would mean a bigger aperture but this isn't the case. However, if they were to refer to a larger f-stop then it would mean a greater DOF.

A note for digital compact users: You may look at your digicam and see that it only goes up to something like f8. Don’t worry; this does not mean that you won’t get lots of DOF, it just means that the sensor on your camera is quite small. Consequently, Factor #4 listed above (“How do I change the DOF on an image?”) will apply. As you have a small image sensor, an f-stop of f8 should be all that is required to obtain a great DOF.

2 – Focal Length:

The concept of the focal length effecting DOF is quite easy to comprehend. As mentioned before the longer the length of the lens (expressed in mm) the less DOF it has. This means that wide-angle lenses, those with a short focal length i.e. 28mm are doubly suited to landscape work, as not only do they allow you to fit more into the frame, but they also give you a greater DOF at any given aperture.

Conversely longer focal length lenses, such as 135mm or higher, are suited to portraiture, as they allow you to focus in areas through having a narrow field of view and less DOF. This allows you to use the DOF creatively drawing attention to parts of the image such as the eyes of the subject.

Once you understand this, you can use the lens and its properties to creative effect, creating the right DOF for the image you are taking.

3 – Distance from the subject:

The third largest influence on DOF is the distance between the camera and the object that you are focussing on. The greater the distance between the camera and the subject the greater the DOF; the closer the object is the shallower the DOF. This is why in a lot of macro photography you will see a DOF that is almost unbelievably small. The lens is so close to the subject that maybe only an edge of a leaf or the eye of an insect is in focus.

The opposite is also true if you focus on an object that is located near the horizon; your image will have an extremely large DOF. This also leads to what is known as the hyperfocal point, something that is covered later in this article.

4 – Film / Sensor size:

Again this is essentially a simple concept, the larger the film / sensor, the shallower the DOF you will get at a given aperture and focal length lens. Unless you switch formats, say from a digital SLR with an APS sized sensor, to one of the full frame 35mm SLRs (digital or film) or even to a medium format camera.

This is really a constant unless you do switch between film / sensor sizes. However, it is useful to bear in mind if you are not getting the results you want because a switch of systems may help you out. Smaller film sizes or sensors, such as those on most digital cameras, will give you greater DOF. This makes them good for landscapes and gives you a bit more leeway with sports or wildlife shots, unless you want a really shallow DOF that is. The downside is getting lenses that give you a very shallow DOF for portraiture becomes harder and much more expensive.

A lot of digital compacts have a limited aperture range some going up to only f8, this is because of their sensor size, the manufacturers can keep costs down by reducing the f-stop range and still retain a good DOF.

Taking this all in

This is quite a lot to absorb in one sitting, but here I’m going to use something that helped me first get my head around the first three factors and how they relate to each other and DOF. As mentioned above the fourth s pretty much a constant and I have included it more for completeness and as a reference.

My first camera was a manual SLR that had no aperture priority, shutter priority, or auto focus. The closest it got to any automation was through the lens metering, activated by a lever on the side. While this slowed my photography down, it sped up my learning as I was forced to understand what I was doing rather than let the camera do the work for me.

The lens that came with this camera was a 35mm to 70mm zoom. On this zoom were a series of marks on either side of the focusing mark on the lens barrel. These really helped the ideas of f-stop, focal length of the lens and distance-to-subject sink in.

Adjusting the aperture
In this first image the lens is at the 35mm setting and is set to focus on an object located 2m away. Using the lines on either side of the thicker focus line, we can see how much of the scene would be in focus. The outer line represents f8; at this focal length of 35mm everything from under 1m to about 15m is in focus (giving a total DOF of just over 14m). In the second image, the aperture is set at f5.6, which is represented by all of the innermost lines save one, the DOF would now be between 1.5m and just under 3m, giving a total DOF of just under 1.5m.

Adjusting the point of focus
In Image three I have adjusted the focus to 5m but kept the aperture at 5.6. The DOF has now changed, encompassing everything from 2.5m up to near the infinity symbol 8. As you can see from this example, when it is compared to image 2, if the point that I am focussing on is farther away, then the DOF will be greater at any given aperture, in this case f5.6.

Adjusting the focal length of the lens
In Image four, I have adjusted the focal length to 70mm (as opposed to 35mm in the previous examples). As you can see from this image the lines on the lens barrel are almost totally hidden. Set again to focus at 5m with an aperture of f5.6 (the same as used for image 3) the DOF is from 4m to 8m, this is a big change compared to 35mm. Here you can see the effect that focal length can play in effecting the DOF on any given image.

The amount that a lens alters the DOF depends on the individual lens and its characteristics as well as the camera format that the lens is connected to. Markings like this are useful on a lens, especially for the beginner or when focussing using the normal method becomes difficult. I do lament the fact that my wide-angle lens that I currently use on my Canon lacks these markings as well as manual focus distance markings because they are useful for night photography!

Wasting DOF and the Hyperfocal distance

Hyperfocal distance is a somewhat difficult concept to implement on a daily basis, because it does involve maths or a good memory or experimentation. When you focus on an object, the area that is in focus extends in front of this object and behind it, this is you DOF. If you focus your camera to infinity the leading edge of the area of focus (the edge that is closest to you) is the Hyperfocal distance.

Obviously this distance will vary depending on what aperture and lens you use. There is a formula for working this out, or a quick search will reveal plenty of websites that have tables based for focal length and aperture. Alternatively you can experiment with your own camera to give you a rough guide of how much DOF you can achieve. If you focus at infinity and set your camera to the smallest aperture, you should be able to work out from the resulting image roughly how far towards the camera your DOF extends. Refocusing on that point would mean that the DOF would extend up to infinity as well as even closer towards the camera. Obviously this method is less accurate than using maths or a table of distances, however, it does mean that if you are working on landscapes or have a long time to set a shot up you can use it in the field. It is certainly worth remembering for those times when you need to squeeze out as much DOF as you possibly can.

If you want some more information a quick google search pulls up lots of pages.

I will be adding to this article in the future. If you wish to comment on this article, please email me with this link. I will be adding in a comments page later. If you wish to ask me a question email me from this question link, I will also extend this article by answering any questions submitted.






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