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All points of each plane in a scene converge at points that are contained in the corresponding image plane. In the accompanying figure, although not explicitly depicted, the plane of the scene that passes through the point P has a corresponding image plane through the point P '. When using the focus ring, what we are doing is to vary the distance between the plane in which the sensor is located (so-called focal plane) and the plane of the lens. (NOTE: The focal plane is located on focal length F ', only when focused at infinity).
This leads to select one of the planes in the scene so that its image plane is formed in the focal plane. In this plane will be called the focal plane. In our image the focal plane is passing through P and the image plane passes through P '.
Depth of field is the set of planes before and after the plane of focus forming circles in the focal plane images below in size to the circle of confusion. Due to limited human vision, we observe on the image to be perfectly clear, even when in fact, perfectly clear, there is only one: the focus plane. Therefore, and importantly, the depth of field refers to the planes of the scene, in particular those who appreciate sharply. The attached figure
this definition of Depth of Field is manifested in a slightly different way. Here we take the planes A 'and B' as those in the circle formed by the cone of light rays converge in P 'and then diverge again has the size for the circle of confusion. From these image planes can provide that depth of field is limited by the corresponding planes A and B, of which A 'and B' are flat image. It is important to note that the definition of depth of field is underway in Actually, depending on the depth of focus.
The summary is that although the first definition is considered to be more common in the literature on photography, the second is more simply helps us understand what is happening in diagrammatic form. Obviously the two do not cease to be two sides of the same coin.

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Photo Features

The most accepted definition photography approach is the assertion that the size of the image of the subject should be, at most, 10 times smaller than the subject itself. That is, a photograph taken with a wide angle that allows a minimum focusing distance of 40 cm. not be classified as close-up photography and, instead, taken with a 180 mm macro lens at a focal distance of 1 m. yes it would. Therefore, the focus distance is not a defining feature of photography. But ...., in fact, the vast majority of the photographs classified as approach is taken with a very small focal length.
This actually triggered some features that are present in a large number of cases and what happened to list:

a) Approach
When the focus distance is small, the depth of focus is great. This means it is difficult to focus. Therefore, usually we can not use our camera autofocus and manual focus we do. In fact, in the limits of photomacrographs-size image of the subject similar to that of the subject itself, not even this will in many cases, be necessary to move the camera as a whole. A professional level camera is mounted on a rail.

b) Depth of Field
When the focus distance is small, the depth of field is also very small. In the limit of photomacrographs mm. So we shut everything we can to get the diaphragm to increase the depth of field. We must also align the plane of the lens to the maximum with the main plane the subject.

c) Shutter speed
If we close a lot of the diaphragm, enters low light. So shutter speeds are great. This has two important consequences:
c.1) We use a tripod and remote trigger.
c.2) If we do outdoor photography, we have a problem. Any little breeze to move the subject out of focus the picture. So it is hard ta take pictures of flowers, plants or insects on the outside. These wonderful macro pictures we see in magazines, of course, are under study. At least the vast majority.

d) Diffraction diaphgram
When we close we must be careful not cross a red line. From a certain value of f is the phenomenon of diffraction. A practical result-and inconvenient-is that the sharpness of the picture suffers. (More on this in another article.)

When the focus distance is small, the image is formed farther from the goal than you would for a greater focus distance. But the sensor is located far inside the camera body and we can not get more back, is it? . Of In fact, most articles on the topic describe an extensive set of gadgets, used for this very purpose: the separation between the sensor's goal as possible.
These are in a very brief summary of the main features of the photographic techniques employed in the field of photography and photomacrographs approach.

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photomacrographs Approach and normal lens APS-C Format and Full Format

Fact 1:
According to the figure below, the same that appeared in the article on the viewing angle, we can write the following formulas:
as α = m/2f
where:
α is half the solid angle.
m is the diameter of the circle circumscribing the sensor, the same for both the diagonal of the sensor.
f is the lens focal length.
If we know the value of F from the other two parameters, we find that the correct formula is:
; f = m / (2tan α)
Fact 2:
has been determined by empirical methods the angle of vision of the human being is, on average, 45 degrees. Or what is the same PI / 4 radians, and the tangent of the angle is well known by everyone. The unit. So 1.
Fact 3:
By trigonometry, the following formula gives the so-called double-angle tangent:
, so (2α) = (2tan α) / (1 - so ² α)
Since we know that tan (2α) = 1, if we want to know what the value of tan α, the sensible thing is to make the substitution: x = tan α
thus the above formula is reduced to us:
, 1 = (2x) / (1 - x ²)
which can be written as the following quadratic equation.
, x ² + 2x -1 = 0.
The solutions are, with two significant figures:
x1 = x2 = 0.41 and -2.41
Taking the positive solution, as in the physical world we are only in this case interesandos values absolute, we find that as α = 0.41
From this we can establish with relative ease which is the focal length "normal" an objective, defining this as providing a viewing angle similar to that of being human. Given that the formula appears as a parameter m, which is the diagonal of the sensor, it seems clear that the fact of having our camera with a sensor of APS-C or of a full format will provide different results. 's Check.
Complete Format sensor
In this case, the diameter of the sensor is 43 mm. Therefore:
f = 43 / (2 0.41) = 52.4
sensor APS-C
In this case, the diameter of the sensor is 30 mm. Therefore:
f = 30 / (2 0.41) = 36.5
For this reason it is not surprising that a target of 50 mm is considered normal when using a full format and 35mm when used APS-C format. It is clear that manufacturers of goals would not use decimal numbers in them. The only thing missing.

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APS-C Format and Full Format

For many, many years, the most popular movie format was known as the 35 mm format. This measure indicates the width of the film, including perforations drag. The frame size was 24 x 36mm, which gives a diagonal of 43 mm. .
In 1966 he introduced a revolutionary film format known as Advanced Photography System (APS) that incorporated many new features. Among the most important, the possibility of developing the film without removing the casing and the ability recording, on magnetic media adhered to the edges of the film, information about the camera and the (a precursor of the EXIF \u200b\u200bdata).
The APS film width, including perforations, was 24 mm and the frame size of 16.7mm x 30.2mm was something less than the 35 mm format. Here the bias was about 39 mm.
From the point of view of the subsequent history of photography, the most momentous was the possibility that the photographer could choose, for each frame, the proportions of film prints for the final print. There were three options


APS-C:
proportions 3:2 (Classic)
25.1mm. x 16.7 mm.
Diagonal: 30.1

APS-P:
2.5:1 Ratio (Widescreen)
30.2mm. x 9.5 mm

APS-H:
ratios 16:9 (High Definition (HD))
30.2mm. x 16.7 mm.

photographer, when he took the picture, impressed the entire surface of APS, but could decide to "despise" a part of the socket.
Manufacturers of digital SLR cameras, at the beginning of the first decade of the century, decided to use measures such as APS-C size standard (with minor variations) for their sensors. In recent years, Some manufacturers began to use sensors with a size similar to the old format and 35mm format sensors called Full. The rest is history all more or less already know.