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The opening of the diaphragm

The opening of the diaphragm

For proper exposure of the photo shoot the parameters that can affect it are the exposure time, the opening of the diaphragm and the ISO. Leaving aside the ISO that to date on DSLRs has become a variable parameter to be set for each individual photo, but that in the past depended on film mounted on the machine body, a fundamental parameter to properly set the amount of light that must compose our shot is precisely to define the amount of light that must pass in the chosen time of shooting. How to adjust the lens opening to properly expose the photo we'll address it in the exposure chapter, now let's see what a given diaphragm choice entails.

The opening of the diaphragm is important because it not only allows the possibility of managing the light that impresses our sensor, but its setting also directly affects the depth of the field.

The Depth of Field,to which we will dedicate a specific lesson, we can define it that area of the photo where all the subjects are perfectly in focus and varies as the opening of the diaphragm changes. A greater aperture corresponds to a lower depth of field and vice versa. This means that on a horizontal plane, subjects that will be in focus towards the horizon with an opening such as f2 will have a shorter distance if we set the diaphragm to f22 instead. This phenomenon is very useful in the customization of our shots. In fact if I have a nearby subject that I want to isolate from the rest of the context I can use a very low setting like f2.8 so that the background is completely out of focus. Conversely if I have subjects near and far I have to increase the depth

and use a higher value, such as an f22, being careful not to run into the problem of diffraction that could make my photo unclear. Even at the lowest values, our photos are likely to be a little less detailed and sharp than in the intermediate values of the lens's diaphragm opening range that we're using. Generally the greater sharpness you should have a few stops above the minimum value and a few stops below the maximum value. These, however, are common sense suggestions that then depend on the quality of the material we are using and the experience in the field.

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However, it is not always easy for the automatic device to be able to distinguish the best point of focus, although on reflexes you can choose the points and areas that should be used to calculate distances, the device could find difficulty in correct setting fire. Or more simply the firing of automation does not coincide with the will of the photographer. To work around these problems, you can switch to manual mode, which in most SLRs is done by moving the switch from AF to M directly on the lens and using the barrel rotation device to approach or move the lens away from the sensor and find the correct fire point.

What does it mean to focus on a composition? It does not necessarily mean that all photography should be in focus perfectly, but it is possible that the need for the composition can make it perfectly sharp only the main subject. Let's see some situations and how to behave in that case.

Main subject is distant objects
It is perhaps the least complex situation. The subject must be pointed at a considerable distance and the diaphragm can be quite open

Subject includes near things and distant things:
If our subject is composed of things near and far you need to make decide how the camera should be set to have the desired focus. Let's look at the cases where:
  • All composition must be in focus (landscape photos)
To achieve the solution to this situation you have to calculate the hyperfocal distance. Hyperfocal distance is the distance beyond which all objects have acceptable sharpness, setting focus to infinity. Once the hyperfocal is calculated, simply focus at that distance to have the entire frame at an acceptable sharpness. But how do you calculate this distance? There is a mathematical formula for calculating this distance
  • The background must be blurred
You point the subject that needs to be focused and open the diaphragm of the lens. The more open the diaphragm, the more blurred the background and the subject is isolated. Keep in mind that the lower the focus, the greater the depth of field and the less blurring the background.

Main subject is made up of things at a little more than a meter
Here the situation is more critical as the closer we get to the lesser subject is the depth of field and the photo may be blurred both behind and in front of the main subject. If the blurring behind the subject can be useful in isolating the subject and is accepted by the human eye, a blur in front of the subject is not tolerated. To work around this problem it may be enough to focus at a smaller distance than the main subject by paying close attention to the depth of field that can be very limited

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The focal length

The focal length

Digital Reflex and Crop Factor

Depending on the reflex model used, the field angle may vary depending on the sensor model that our reflex mounts. The smaller our sensor, the smaller the field angle, with the same focus, it will be small. As mentioned above, focal measurements refer to film or a 35mm (full frame) sensor. Many reflex models use sensors smaller than the full frame, the most common and amateur-level use the APS-C model which has a multiplication ratio of 1.6x compared to 35mm. A sensor covers a closer view than the one captured by the lens, as shown in the figure, but the lenses are designed to make the most of a full frame sensor. Using an APS-C sensor we will cover a smaller surface area and proportionally reduce the field angle equal to the multiplication or crop ratio. This means that we need to recalculate the focal effect according to the crop factor of the sensor that mounts our reflex. In the case of APS-C sensors, a 50mm lens will be equivalent to an 80mm (50mm x 1.6) lens. The advantage of the crop factor of the APS-C sensors is definitely to decrease the field angle to the same goal on FF with the same effective Focal length (LFE). This can be an advantage in sports and naturalist photography in view of the fact that decreasing the angle of the field we have a magnifiing effect of the image. On the contrary, it is negative in photographs where you need to use the big angle. This inconvenience led to the construction of specific large-agonal optics for APS-C sensors with a focus equal to 10mm

How to choose the focal length to use.

There is no rule to say what is the right goal to be mounted to take a photograph, but we can give some important indications to make the correct choice for the shot of the photograph that we have in mind. By summarizing what has been said so far, we can see what are the main characteristics of the three macro categories of the focal.

  • GRANDAGOLI Wide Field Angle allows you to store a large amount of elements in the frame and has a wide depth of field.
  • NORMAL or STANDARD function. They provide a field angle similar to that of the human eye
  • THE TELESORS. Narrow field angle, reduced field profodity. In addition, the telephoto lenses crush the image by reducing the distances between the various planes of the photograph.
Knowing the unique characteristics of the various types of lenses helps in deciding which one to use to make our shots.
In this paragraph we mentioned the depth of field that we will see specifically in the dedicated chapter.

Two exercises to see in the field the differences that you have using different focal points on the same subject:

1. Shoot a subject from the same distance using a wide-angle, standard focal, and a lens.

2. Fill the frame with the same subject as you approach or move away from the subject and using a grand-agolare, standard and telephoto lens optics.

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