اختيار الفتحة المناسبة للعدسة .. كتاب فن التصوير الفوتوغرافي

Choose the appropriate aperture for the lens

To take a successful photo, a number of preparations must be made. One of the most important of these preparations is to choose a suitable aperture for the lens, because choosing a good aperture enables you to avoid many minor defects that harm the image, and you can also choose this from:
1- Regulating the degree of exposure (Exposure)
2- Increasing the depth of field
3- Increase the overall clarity of the image.
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1- Choose the lens aperture to regulate the degree of exposure:

The presence of a diaphragm of variable width (diaphragme) in the lens is mainly to regulate the process of exposing the film to light, because the film is sensitive to light within narrow limits. If the amount of light falling on it is too little or too much, then the image is not good. The regulation of this quantity is done by means of the variable-width veil and the moving shutter together.
The important thing to remember here is that the wider the aperture, the smaller the f/number.

A way to remember this is that f is the available light. If the light is small, a small f-number should be used, and if the light is strong, a large f-number should be used.

2- Choose the aperture to increase the depth of field:

Suppose you want to take a picture of a horse on a farm, and the horse is close to you. You can easily make his image clear and pure. But you want to show the horse with its background, landscape, or farm buildings clearly as well.

If you use a camera whose lens has opened a large aperture with a high shutter speed of its moving shutter (Shutter), you can focus the horse clearly, but the background and distant objects will not be clear.
You can increase the depth of field by narrowing the aperture and decreasing the shutter speed. In this way, the film receives the same amount of light as it received in the previous mode (ie a large aperture and a large shutter speed), and the depth of field increases to the extent that the image of distant objects appears clear.
In lenses with a short focal length, the depth of field is large, so large apertures can be used in this type while maintaining a large depth of field.

3 - Choose the lens aperture to increase the overall clarity of the image:

When we studied the aberrations of the lenses, we saw how they could be avoided and corrected, with a suitable design for these lenses. However, in the best designed lenses, errors cannot be completely corrected. As some of them remain effective, especially when using a very large aperture (using a large flat lens).
With high-quality lenses, excellent aperture clarity can be obtained by using an aperture that is one degree less than the maximum lens. As for normal quality lenses, or high speed lenses, it is recommended to stop the aperture at two degrees less than its maximum aperture.
Using a smaller aperture increases the depth of field and reduces focusing errors, as well as errors caused by slight twisting of the film in the camera.
As for lenses with a small focal length, such as the 35 mm lens, or cinematographic cameras, it is recommended not to reduce the aperture much less than f/16, because at this limit, i.e. f/16 aperture, the phenomenon of bending of light rays begins as they pass through the aperture. Too small, and reducing the aperture less than this amount reduces the clarity of the image.

As for lenses with a long focal length, the aperture can be reduced below this amount without harming the clarity of the image.