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When it comes down to it, depth of field is synonymous with range of focus. It only seems natural that a photographer would want to produce a picture with a wide range of focus, but this is not always the case. If everything in the image is tack sharp, it tells the viewer to look at every detail and that each is important. Conversely, if only certain subjects in the image are sharp, the viewer is drawn to those areas and the rest of the elements become secondary. In the case of landscapes, images work better when everything is sharp from the foreground to the background. But for portraits, and many other subjects, the image is more successful if just the main subject is sharp and the remaining elements fade into softness. So how does a photographer create specific ranges of focus?
Depth of field is controlled by a combination of the following: the f stop used to make the picture, the focal length of the lens, subject distance from the camera, and subject distance from the background. With regards to f stop, the higher the number of the f stop, the greater the depth of field. In other words, with all other factors being equal, f22 will create much more depth of field than f4. This translates to a more sharply rendered foreground and background. With this in mind, as a guideline, for landscapes use f stops closer to f22 and for portraits, use those close to f4.
With regards to focal length, the wider the angle of the lens, the more inherent depth of field it will produce. If the goal is to create images with lots of depth of field, stick with wider lenses. Conversely, as one progresses from medium to long telephotos, the depth of field becomes more and more narrow. This is why many landscapes are made with wide angle lenses and portraits are made with medium telephotos. Once you get to 300mm and greater, depth of field becomes narrow and careful placement of the focusing sensor over the part of the subject that is most important is critical.
The last two factors that impact the range of sharpness both deal with distance. The closer you get to your subject, the more the background falls out of focus. This happens because the lens has to focus closer to its closest focus point which translates to distant elements falling out of the range of focus. The same principal holds true given the relationship of the distance of the subject from the background. If the subject and background are close to each other, then the proximity of all elements near and the lens will see everything in fairly sharp detail. Conversely, if the subject is moved far away from the background, the lens isn’t able to sharply render both the subject and the background which is far away. Both of these effects are enhanced using a telephoto lens with a wide open aperture (f4) and both are reduced using a wide angle lens with a closed down aperture (f22). Experiment using all the above variables so you can take charge of your depth of field and learn how to MAKE a picture rather than TAKE a picture.
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It is my understanding of optics that lens opening and magnification of the subject are the main contributors to DOF. The reason that distance or focal length affect the DOF is that they change magnification. If you make an image with a 50mm lens at f/8 from 25 feet away from your main subject and then change to a 100mm lens at f/8 and make an image of the same subject from 50 feet away, the DOF will be the same. (The main subject will be the same size in both. Objects in front of or behind the main subject might look a little sharper in the wider angle image because they are smaller. But if you pick an object in the background and enlarge the wider angle image so the size of the background image is the same in each, the background objects will be equally unsharp in both images.)
Bob,
You’re correct in your good example. Another way to put it, which I prefer, is mathematically.
DOF is directly proportional to both lens aperture (or the ‘#’ in the f/#) and square of the object’s distance, d, and inversely proportional to the square of the lens’ focal length, F:
DOF ~ (f/#) x [d/F]^2,
where the units of d and F should naturally be the same.
In your example, your aperture is 8 in both situations, so it cancels out in the comparison of the two DOF’s in each situation, and d/F is also the same before and after. As you say, DOF is then the same before and after.
So, what’s the advantage of the formula above? It provides a feel for how the DOF changes with a change in those parameters.
Russ: thanks for the article.
Antonio, thanks for the formula. I am a retired mathematics professor, so I appreciate seeing the formula. Russ, thanks for the article and for sharing it. The f/5.6 flower image and the landscaped image are stunning.