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Fresnel lenses work as a ring of crystalline prisms that reflect refracted light, which in order to work properly require space between the screen and the lens itself. This results in a larger and heavier viewfinder than Pancake lenses.

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The focal length of a lens indicates its ability to converge light, and it’s measured by the distance between the lens center and focal point. The shorter the distance is, the more sharply it bends the light, and the larger field it’ll be able to squeeze into its image circle. As M12 lenses are made up of several pieces of glass (or plastic), the term “effective focal length (EFL)” is used to present this ability.

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The lens optical format decides the size of the image circle where the whole field will be squeezed into, and the sensor format decides the rectangular area to catch the image circle. In most camera setups, the sensor rectangular area crops a section inside the image circle.

Another method is to use a zoom lens. On a certain image sensor, to increase or decrease the focal length means to zoom in or out on the image. Unlike lenses with a fixed focal length (also known as prime lenses), the zoom lens comes with moveable elements inside, so the elements can move closer or further to alter the effective focal length.

The focal length of a camera is the distance from the center of the lens to the focal points. If the focal points are closer to the lens center, it means the lens has more optical power to converge the light. The stronger the power, the wider field, and there is a mapping relationship between the Field of View and the focal length. Since the light is captured from the scene and cast to the image sensor in the same way, the triangles formed while the light enters and exits the lens are similar. Therefore, we can use the estimated field size and distance to calculate the focal length.

If until now, the use of Fresnel lenses in Virtual Reality viewers had been standardized, we are seeing more and more viewers incorporating Pancake lenses in their designs. Why are Pancake lenses gaining ground? We take a look at it below.

The light cast to the image sensor forms a circle of light, also known as the image circle. Most times, we want the image circle large enough to shed light on the whole image sensor, otherwise, we will see dark areas on the edges of the image. However, for some fisheye lenses, we want the whole image circle inside the image sensor frame so that we never miss any detail of the whole 180-degree hemisphere.

Field of View (FoV) usually refers to the field an image sensor captures. It is not a constant value because further scene results in a larger captured field. Therefore, FoV is usually represented in angles of view – the larger, the wider. The lens casts an image circle while the sensor captures a rectangular section, so the native FoV of the lens is different from the field the sensor captures. Most times, the sensor crops a rectangular section inside the image circle. For the fisheye lens, the image circle is totally inside the rectangle so that the sensor captures the whole cast field.

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What we are sure of is that, day by day, scopes are improving in usability and comfort.s are improving their usability and comfort for the user, with Virtual Reality glasses becoming lighter and more compact.

The first one is simple: just change the lens for other lenses with different focal lengths. To make that easier in an optical test, Arducam has released lens kits that packed the lens of different focal lengths into a small box.

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To capture the best frame on embedded cameras, the key is to capture an appropriate Field of View (FoV). FoV is highly dependent on the lens focal length because the image sensor catches the field squeezed into the image circle by the lens. A shorter focal length means the lens can squeeze more fields into its image circle within a certain distance. As focal length represents the ability to converge light, lenses with the same focal length will similarly bend the light, resulting in a similar field of view on a certain format of sensors.

Although the design of the French physicist, Augustin-Jean Fresnel, has achieved a better aperture and a short focal length, getting rid of the weight and volume of predecessor lenses, it has not overcome one of the limitations in its use applied to Virtual Reality glasses, the glare effectknown as the glare caused by excessive brightness accompanied by discomfort by reducing the visual quality of the user.

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One of the most critical components of a Virtual Reality glasses are its lenses. They are the ones that provide us with a good or bad experience in the virtual world, they allow us to access it by connecting our eyes to the screen and adjusting to our comfort, correcting the angles of the light rays that reach us from the screen so that the experience is pleasant.

Pancake lenses are much more compact, reduce the volume of the viewfinder considerably compared to Fresnel lenses, and also eliminate the glare effect (described above), they provide better image quality and lower cost.

Sensor format refers to the size and aspect ratio of an image sensor, usually with fractions such as 1/4”, 1/3”, 1/2.5”. The 1/4” image sensor has a diagonal of 4.5mm (the 1/4” of 18mm), and an aspect ratio of 4:3. The lens optical format describes the lens image circle with a similar measurement, except that the sensor format is measured by the diagonal while the lens optical format represents the lens image circle’s diameter. Therefore, if the lens optical format equals the image sensor format, the image circle is just large enough to produce a decent image without poorly exposed dark pixels on the corners.

In an ideal case, the lens optical format would equal the sensor format, where the sensor’s diagonal equals the image circle’s diameter, so the diagonal field of view of the sensor (DFoV) equals the native FoV of the lens.

Finally, the optical efficiency: in the case of Pancake most of the light is blocked and, therefore, there could be a color difference between one viewfinder or another depending on the lenses, although in the case of PICO 4 they manage to achieve a saturation rate of 85%. These features are also shared by the business version of Pico's new PICO 4 Enterprise viewer, the PICO 4 Enterprise, which also features this type of lens.

The first to incorporate Fresnel lenses were viewers such as HTC Vive and the Oculus Rift CV1, one of the most popular viewers that include them are the Oculus Quest 2. A first step towards lighter headsets with more elegant designs, which in addition to the aesthetic component, improve vision, minimizing the space between pixels and thus mitigating the door effect, thanks to the use of these lenses.

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If you are a lens maker, you can design the lens elements with differently curved surfaces to alter the focal length. If you are not, you cannot directly change the focal length, because once a lens element is manufactured, its specs are fixed. Although you cannot change the focal length on the elementary level, you still have two ways to change the effective focal length of the optical system.

An object is seen by reflecting the light from a light source (such as a bulb or the sun) or being the light source. The camera lens can transmit and cast the light from the object to an image sensor, which will then produce an image.

It’s also common to have a lens with a larger optical format than the image sensor format. However, the field outside the sensor rectangle will be missed, resulting in a smaller field of view. With lenses of the same optical format, the smaller the sensor is, the smaller the field of view we will get. If the optical format is smaller than the sensor format, dark unexposed surroundings will occur, but it’s helpful to capture the whole field of the fisheye lenses.

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With the incorporation of Pancake lenses in Virtual Reality glasses, the space between the lenses and the screen has been reduced, without affecting the user's comfort. These lenses are known for their use in the world of photography and are characterized by their lightness. That's why Pico's new premium all-in-one viewfinder, the PICO 4Pico's new premium all-in-one viewfinder, the PICO 4, weighs only 295 grams - as light as a soda can. Unlike Fresnel lenses, which in a viewfinder such as the Meta Quest 2, also known as the Oculus Quest 2, weigh 503 grams.