This is called out of focus. The circle itself is called the circle of confusion. I really like this name because it adds some drama to the technical description of photography.
The circle of confusion results in a blurry image. Real objects take up nonzero amounts of space, so our previous discussion about points is not really applicable to a real-life situation. However, we can think of a real object as a collection of many different points. P1 has some neighbors: P2 at a distance D2 and P3 at a distance D3. P2 is closer to the lens, while P3 is further away.
As we can see from the picture below, P2 and P3 are are out of focus. This points out a serious problem with our definition of focus: only points that are a specific distance D1 in this case away from the lens can be in focus.
If we stick to this definition, there is no way that an extended object could ever be in focus! It seems that our idealized situation is becoming inadequate. All we need to do, though, is to slightly modify our definition of focus.
Instead of defining focus to be the case where a point object produces a point image on the sensor, we can relax our standards. We can define approximate focus to be the case where a point object produces a circle of confusion that is small enough.
This is not a technical term! I made it up. So what does small enough mean? Well, there is are two versions: the physical small enough , and the physiological small enough. The physical small enough refers to the fact that any real-life camera sensor has finite resolution.
For example, as mentioned before, a digital camera sensor consists of an array of pixels. These pixels are little squares that have finite size. Say one square micrometer. The physiological small enough is a bit more complicated and has to do with the angular resolution of your eye and how your brain processes visual information. If you make it small enough, it will eventually stop looking blurry.
You can also try looking at a picture from very close up and from farther away. It will look better from far away! I have chosen D2 and D3 in the previous picture so that P2 and P3 are at the limits of approximate focus. So the range of approximate focus extends from D2 to D3. I call this range the focus range. This is not standard terminology either. We can now define depth of field : depth of field is just the size of the focus range! Depth of field has huge impacts on photography.
When a photographer chooses to use a small depth of field, he or she can draw attention to certain aspects of a scene. The photographer can choose a small range where things are in focus, while the rest of the image will be blurry. This actually has a name: bokeh. Alternatively, the photographer can choose a large depth of field to emphasize all aspects of a scene. I think this technique is particularly evident in landscape photography.
So that was just the prelude to the main point of this article. Before we continue any further, I should explain what an aperture is. An aperture is basically a hole that you place in front of your lens. You can adjust its size, which essentially shrinks or increases the size of your lens. It turns out that there are actually two reasons for why a small aperture increases depth of field. The first is entirely based on geometry. This allows for a wider focus range and hence a larger depth of field.
The picture below will help you visualize this. The top shows the lens with the widest possible aperture, while the bottom shows the lens with a much smaller aperture. The other reason has to do with the fact that real-world lenses are not perfect. Instead, most cameras use spherical lenses. This effect is shown in the picture below. The top shows a perfect lens, while the bottom shows a more realistic one. The net effect is that spherical aberration increases the size of the circle of confusion.
In fact, it makes it impossible for the sensor to ever measure a single point. By blocking the outside light with an aperture, you can consequently shrink the circle and increase the depth of field.
For example:. Magnification is the third factor that affects depth of field. In this case, magnification can refer to the sensor size, focal length and distance from the subject. For example, a larger sensor size, longer focal length or closer focusing distance will result in shallower depth of field. At the same aperture, full frame cameras will have a shallower depth of field compared to crop sensor cameras.
Again, at the same aperture, medium format cameras will have an even shallower depth of field than full frame cameras. In addition to the aperture, distance from your subject and magnification, the lens you use can also impact depth of field.
Different lenses have different depth of field characteristics, which is caused by the magnification of the lens. The greater the magnification as with telephoto lenses , the shallower the depth of field, and vice versa with wide angle lenses. Interested in creative portrait photography? This natural light portrait image below was shot using an aperture of f1. Interested in natural light portrait photography? Full open apertures, such as was used in the image above, are great for photographing people as it allows us to blur the background and bring attention to the eyes or face of the subject.
Interested in fashion photography? Originally Posted by jcgwakefield. In theory, would it be possible to take an image taken with a wide apperture and process it such that it appears as if it were taken with a small apperture? With sound, I believe its possible to isolate the different wave forms which have been added together to form a rich timbre.
So what I'm getting at is being able to take an image and remove the exposure caused by light waves which result in the image being out of focus or in other words remove from the image the exposure caused by light waves which arrived via a high divergence.
Surely, for every out of focus object, the light waves existed at the time of the photograph for the image to be in perfect focus? The answer is a qualified, "yes" when you shoot digital. See the "Composite Focus" section in this book.
I don't know about earlier editions of Photoshop nor do I know about the "focus stacking" ability of other photo editing programs. Here is a tutorial on "focus stacking" using Photoshop CS3. I have not worked with one of these supplemental programs. Last edited by rpcrowe; 14th September at PM.
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