You want to get close; real close to a very nice flower. Your sophisticated and expensive digital camera allows you to get to within a couple of inches so you crawl down on the ground, get real close, focus real carefully and take a real nice picture. Then you get back to your computer and open the picture in Photoshop and only then are you shocked. Only part of the flower is in focus. Right where you focused, the image is, as planned, in perfect focus. But for the parts of the image in the foreground in front of this point and background image beyond our perfect point of focus is also out of focus. What happened? And, more important, would could be done to get the entire image in focus at the same time?
At left is a daisy and it is in focus. But look at the background and notice that the it is intentionally blurred. This effect makes the background seem less important to the overall picture and also makes the primary subject, the single daisy in the upper left, stand out more. It's strictly a creative effect accomplished by controlling the depth of field.
In this article we explore the problem that is creating problems with our poor range of focus, the result of a narrow depth of field. We will learn more about what it is, why we have it and what it means to our photography. Finally, we will explore some ideas of how to use it to our advantage to create interesting photographs.
What is depth of field?
Depth of field is the amount of distance that you subject is still in focus and is expressed as a range from a certain minimum point close to the camera to a certain maximum point as distant from the camera. Everything in this range will be in focus and everything beyond, closer or further, will be out of focus.
Lenses actually focus within a range, not on a point. Wherever the focus is set, objects will appear clear at a certain distance before the setting and beyond. This range is called depth of field and all camera lenses have a depth of field. For example, if your main subject is 5 feet from the lens, the field of focus, just a guess, may be from 2 to 15 feet. This means that if the front and back of your subject falls completely within this range, all of it will be in perfect focus. If you are trying to focus on a flower at about 12 inches away, than your depth of field may only extend from 6 to 18 inches.
The f-stop setting has an effect on the depth of field. With lower f-stop numbers you generally have a shorter depth of field. In the above example with the focus of 5 feet and an f-stop setting of maybe f-2.8, your focus might only have a range from 2 feet to 8 feet (another guess). A higher f-stop setting, such as f-16, will increase the depth of field to (still another guess) 1 foot and maybe even stretch all the way to infinity.
Shown above at the beginning of this section is a pink mountain laurel flower. In order to show the entire picture, it was resized to fit this page. In the resizing process, the original pixel data was recalculated and information was lost making the picture appear somewhat blurred. To compensate, sharpening was applied.
At left is a cropped detail of the original 1024 x 768 digital photograph showing the focused detail in the center of the flower. No resampling was done on this (or detail shown below) picture.
This detail is taken from the back of the picture. The small bud at lower left is at about the extent of the depth of field and is still in focus. The three small dots at the bottom are the upper part of the main flower. But the flower in the background is definitely beyond the depth of field and is therefore out of focus. By the way, it was no more than two inches (about 50 mm) from the front flower! The lens was not much more than an inch from the main focused flower.
Depth of field is simply another tool a photographer has when composing a picture. You should think of it as separate from focus but it also helps you with the basic focus setting. As you study the preview in your viewfinder, study carefully the focus of all the elements that are at different distances. Are they all in focus? Do you want them all in focus? Where, exactly, do you want the range of focus? Do you want a long or short depth of field? Try adjusting the point of focus a bit for you are actually adjusting the depth of field. Study the results of the depth of field and if you don't like where it is, adjust the point of focus which changes the range. These are all controls and adjustments that you should learn and commit to subconscious memory and recall as you work. It is the smart photographer who will use all the tools and tricks available to get that absolutely perfect picture.
To infinity and beyond!
Depth of field is actually a tool for cameras with variable focus. Many low end consumer cameras do not have focusing adjustments and are called fixed focus. This means that you cannot adjust the focus and the camera comes preset to focus almost anything you may wish to photograph. How this works is that the designers of this camera have engineered a lens that has an extremely long field of focus which may be from a couple of feet to infinity. As long as you know the range of focus, all you have to do is make sure that you don't try and photograph something closer or else it will be out of focus. As far as things that are more distant, you don't have to worry because you really can't focus beyond infinity.But designing a lens that has the capability to be in focus from a couple of feet to infinity demands a lot of the lens designer. There are sacrifices in image quality that must be compromised to reach such a flexible depth of field. With low end cameras it is not that serious, but how about more advanced models? What if we do not want to sacrifice image quality?
Autofocus cameras are a solution to the problem. Newer computer electronics available in our newer cameras allow us to happily take pictures without a thought given to focus. The camera will do it for us. Now we can have a higher quality variable focus lens in our camera without the bother of manually focusing. And the quality of our pictures will be higher for the images will be clearer and sharper. By the way, expect to pay more for such a camera but isn't the quality worth it?
With fixed focus and probably autofocus cameras as well, you may not have the ability to work the depth of field to your advantage. You cannot set the point of focus at a distance closer to your subject so that the background will be blurred. Nor can you create a nice picture where your friend and the nice tourist attraction in the background are both in focus at the same time.
Although the autofocus cameras are good for focusing on their own, they might not do what you want them to. The solution would be to find a camera that can allow you to override the auto focus system. In this way you can use the depth of field on those pictures where it is critical or where you are trying to create your most artistic images. Then turn on the autofocus for the average shots.
The best system is, unfortunately, the one that requires the most work on your part. The high end manual focus SLRs offer the highest quality lenses and may not come with autofocus features. But if you learn to work depth of field and consider it useful if not necessary for your work, than this is the system you need. To work a manual focus camera, you must make focusing a natural part of the picture taking process. Realize that by focusing the camera you are actually adjusting the depth of field determining where the actual point of focus must be placed. Professional photographers will constantly move the focus carefully evaluating the effect on the depth of field. The process of focusing is really a complicated process and must be subconscious where your fingers automatically reach for the focusing ring with no thought and concentration on your part. When you want to work the depth of field, then you will concentrate on your preview to insure all the objects, close and far, are in focus at the same time.
The key is that focusing must be natural and subconscious. As a beginner this may be a major annoyance but with time it will become easier. Don't give in and declare that the camera is too hard to use. Work at it and eventually it will become easier. The result will be that you will be able to use the flexibility and capabilities of the best lens systems available. Otherwise, if you trade it in for a fixed focus camera, you will forever be limited by that design compromised lens.
Let's see what your camera can do!
Test the depth of field by placing a ruler next to the lens and focus on the unit measurements. By twisting the focus ring and observing the lines on the ruler that are in and out of focus you will develop an understanding of the depth of field your camera lens is capable of. Understanding that each and every camera lens is different and will produce different results. Further, you may be able to purchase close-up lens attachments which will bring the minimum focus closer. As you focus closer to your subject, the depth of field will be shorter. If you could attach your camera to a microscope, the depth of field may be so short that you might not even clearly focus a single microorganism!In the following table we use our Sony FD91 digital camera for this test. The camera is on a table and the ruler is held vertical up in a saw cut through a piece of scrap wood. The end of the ruler is as close to the lens as possible, almost touching the glass. Auto focus has been turned off.
At left we focus the lens as close as possible. The end of the ruler is not much more than 1/8" from touching the glass, resting against the black focus ring.
We can see, in clear focus, the lines on the ruler almost to zero and almost to 3/4 or an inch. The one inch marker is just beyond the range of focus.
The depth of field ranges from about 1/8" to about 5/8" or about a half inch.
In this illustration the focus is adjusted to between one and two inches. Notice that the lines before one inch are out of focus as are those beyond two inches.
1-1/4" to about 2" for a depth of field range of 3/4". This is an increase above having the focus on the end of the ruler but it really isn't that much.
On many cameras the range increases until you reach infinity. When shooting outdoors subjects you may not notice much of a depth of field problem unless you have subjects that are a couple of feet and maybe miles away that you want in focus at the same time. Your lens may not be able to produce such a wide range and therefore it may not be possible to take that picture. More often you may find that ranges of a few to a lot of feet at the same time may present problems that can be solved by carefully adjusting the depth of field.
Adjusting the f-stop has an effect on the depth of field. In the examples below we again photograph our ruler. The focus was aimed at the 1 inch mark and maintained in the same place for both pictures. The only difference is the f-stop settings. But as you increase the f-stop number, you are actually allowing less light to strike the CCD and the pictures will be darker. In the example below, the bottom picture was artificially lightened in Photoshop using Levels and Curves. Without these adjustments we would have a hard time seeing the image because the picture was so dark. To avoid dark pictures, decrease the shutter speed. Both pictures, below, were exposed at 1/60th second, the slowest speed for the Sony FD91.
At f-2 our depth of field of focus is very narrow as we focus around the one inch marker. This could produce difficulties with close-up photography.
No sharpen filter was applied to these illustrations in Photoshop for that would affect the test results! No, sharpen does not improve the focus, it merely increases the contrast between adjacent pixels creating the illusion of sharper focus. Think "sharpen contrast", not "sharpen focus"!
At left we increase the depth of field to f-8 and you can see a definite increase in the depth of field. The picture, however, was very dark. To compensate for a higher f-stop we could decrease the shutter speed. Since, in this test shot, we are at our minimum shutter speed our alternative would be to increase the amount of available light by turning on the flash or finding a brighter location for the test.
Your final tests will obviously be in the field where you will have to take a lot of pictures and study the results carefully. After all, once you gain an understanding of the depth of field concept on your camera, you're ready to put it to good use. So let's take our camera outdoors and take some pictures!
Close-up photography using the depth of field
Technical knowledge aside, it's time to put the theory into practice and try and produce some nice pictures. We will concentrate our efforts on close-up photography because that is where the depth of field problem could easily ruin your pictures if you're not careful. Further, with close-up photography you could use depth of field to your advantage. The results should always be as artistic as possible. After all, photography is an art form and that makes you an artist!
Above, a wild strawberry is photographed with a narrow depth of field. On the original, the entire strawberry was not in focus. But the distracting background is gently out of focus; as was intended. In the version above, the strawberry and background is in focus and we can now see it relative to it's natural surroundings. The f-stop was increased and autoexposure compensated for the differences in the light.
Above, our single flower stands out from the blurred background and we naturally focus on it as our main subject. The plant in the background is not the same as the flower so it is just simple background. Above, the background is brought in focus and the flower seems to be competing with the background for attention. But look carefully, for now the flower and background share the picture making an entirely different photographic statement.
Conclusion
Depth of field is one of those things that makes using sophisticated cameras difficult, confusing and unpredictable. Yet it is one of those tools that will help you make some of the most spectacular photos because it allows you to exert some control over what is in focus (your main subject) and what is out of focus (distracting foreground or background image).
Photography is an art and you are the artist. You will have study the depth of field concept for two reasons. First, you should understand depth of field as a problem you should avoid. Learn when to increase the f-stop or carefully adjust focus to avoid having parts of your subject (or subjects) out of focus. The second reason to explore depth of field is to be able to use it as a tool in creating just the effect you want. You may or may not like the idea of parts of your picture intentionally blurred, but that is a purely subjective conclusion on your part and subjectivity is what art is all about. It is your artistic sense that will lead you to use depth of field or intentionally eliminate it from your pictures. After all is said and done, you are the artist and they are your pictures!
