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How to understand the optical lens

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September 01, 2021

How to understand the optical lens

To understand the capabilities and limitations of a optical lens, it is essential to understand the difference between the eye and the optical systems available to us in photography or film.
How a optical lens sees
The eye is in constant motion. Our two eyes together cover a field angle of about 200°.

Without realizing it, facing a scene covered by our eyes, we "pick out" the essential points that hold our attention.

The whole scene remains blurred, only the area where our gaze is fixed becomes clear.

In the same way, by the play of the pupil, our eye regulates the luminosity for the place we are observing.

A optical lens in a photographic system, captures indiscriminately the whole scene.
The viewer of the resulting photograph can focus on any part of this image.

It is therefore necessary to recreate on this photograph the subjective impression that the photographer had at the time of the release.

No lens, at present, is able to cover all the needs of photographers!
Not even zooms, despite their flexibility.

A good knowledge of the optical system, of its capacities, of its shortcomings, allows the photographer to face all situations efficiently.
How does a optical lens work ?
A lens is made of one or more lenses.

These lenses "channel" the light rays to reproduce an image with a minimum of defects, on the sensor of the camera:

the film in film photography;
the digital sensor on a digital camera.
Lens and angle of view
The different fixed focal length lenses cover angles of view from 5 to 180°.
Each fixed focal length lens covers a specific angle of view.

The lenses are classified into three families in relation to their focal length and the angle of field that follows:

Normal focal length corresponding to an angle of view of about 45°;
Long focal length (telephoto) angle of view less than 45 °;
Short focal length (wide angle) angle of view greater than 45 °.
The zoom lenses, with variable focal lengths can replace, in a small volume several lenses and thus cover a wide variety of angles of view.

Lens and focal length
The focal length is measured from the center of the lens (where the light rays cross) to the image plane (where the image is formed).

The concept of focal length can not be separated from the image format produced by the camera.

The normal focal length of a lens is equal to the diagonal of the image format produced by the camera.

A normal focal length lens for a 24×36 camera has a focal length of 50 mm.

For a 4.5×6 camera, this same 50 mm lens will correspond to a wide angle lens.

Lens, image circle and useful image circle
A lens is built with circular lenses.
The image produced is also circular.

The largest circular image produced by the optical lens determines what is called the image circle.
The edges of the image are always of poor quality.

A smaller part is usable, hence the notion of a useful image circle.

The image format is therefore a rectangle or a square cut in the useful image circle.

In theory, a lens that can cover a large format (6×9 for example) is usable with all smaller formats.

This feature is mainly used with chamber lenses or optical benches, which allows a professional photographer to use two chamber formats, with a single range of lenses.

Similarly, with medium format lenses such as Hasselblad, thanks to the interchangeable backs, it is possible with lenses designed for 6×6, to make photographs in 4.5×6 cm, or even 24×36 mm.

Unfortunately, medium format lenses are not compatible with 24×36 cameras.

For more info, please visit DZOptics website.

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September 01, 2021
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  1. P h o t o g r a p h

    y 1 0 1 : U n d e r s t a n d t h e l e n s