Therefore, to maintain a PPD above 60, theoretically, the maximum horizontal FOV of this optical engine can only reach 13° (800/60≈13).

Earlier this year I posted an article on some of my favorite practical light sources, and since then I have been utilizing practical lighting more than ever. As such, I am writing this Part Two post, which focuses on even more accessible and universal practicals that can benefit just about any set.

In daily life, we encounter various physical display screens. Simultaneously, we experience a clear sense of "closer appears larger, farther appears smaller" when observing these screens.

As mentioned earlier, after determining the FOV in HUD design, we can also select suitable optical engines based on the concept of PPD. The resolution of the optical engine screen is often a constant value. For example, for a 3.1-inch TFT, the resolution is 800px*480px, meaning its pixel count in the horizontal width is 800px.

In the previous two articles on the principles of 3D depth perception (Part 1 and Part 2), we introduced the principles of 3D depth perception. In this article, we will discuss the definition and application of FOV.

Typically when I use fluorescent ceiling lights as practicals, I’ll simply unscrew or remove certain bulbs as a means to shape and control light. Doing so allows for an even textured lighting to fall throughout the scene, giving the actors a lot of wiggle room to move freely during their performance. In addition to their quality of light, fluorescents also look fantastic on camera. Low angle shots that reveal the ceiling will look great as the fluorescent tubes glow in the background and add depth to the shot.

Filllight

Many offices, parking garages, and industrial buildings have fluorescent ceiling lights, and they can make for some of the most useful practical light sources out there. The quality of light from these types of ceiling lights is very similar to what you might achieve with a kino-flo, but of course with the added benefit that they can be shown in the frame.

Assuming you’ve got a relatively sensitive low-light camera, chances are you can get away with using only candle light if that’s the look you’re after. That said, you may not want to rely solely on candle lighting for your scene, as that can cause other headaches on set. An alternative option would be to use a small space light to bring up the ambience in the room, and then use candles to shape the rest of the light in the scene. The quality of light and natural flicker effect from real candles simply can’t be replicated perfectly with any other tool on set or in the color suite.

Practicallighting in film examples

Practicals are of course “regular” lights that can organically appear in a shot. Examples might include a street light, a desk lamp, or even a fire. Virtually any light source that’s not a traditional film light can be considered a practical light, though some are more effective than others. Below is a short list of my three favorite practical lighting sources, and how to use them most effectively.

In a Head-Up Display (HUD), we determine an FOV based on the vehicle's performance requirements and installation space. The distance (d) between the driver's observation point (eyebox) and the virtual image remains constant. Therefore, according to the formula, the width (w) of the virtual image does not change. So, if we project the virtual image farther while keeping the FOV constant, does the virtual image get larger?

In years past, it was relatively uncommon to use practical lighting on a film set as a means of achieving proper exposure. In fact, it was much more common to simply use practical lights as a method of motivating light that was actually coming from a traditional film lighting source. Today however, practical lights are more useful than ever as the increased sensitivity of digital cinema cameras allows for practicals to be used in a more powerful way.

FOV, or Field of View, refers to the angular extent of the observable area from the viewpoint of an observer. In daily life, it denotes the angle formed between the contours of an observed object and the line connecting the observer's eye to the center of the pupil.

Conversely, if the PPD when viewing a screen is less than 60, it will result in a "grainy" sensation on the screen, as depicted in Figure (7). This is also why HUD's angular resolution is designed to be at least 60 PPD.

Practicallighting examples

In other words, even if the virtual image is projected 100 meters away and its width (w) increases tenfold compared to when it's projected 10 meters away, the driver's perception of the virtual image won't vary significantly, as shown in Figure (4).

Film lighting

In conclusion, FOV is used in HUD design to represent the size of the virtual image, while PPD is used to calculate the requirements for optical engine pixels.

In summary, the size of the observed object (w) and the distance from the eye to the observed object (d) are indirect determining factors. Only FOV can directly determine our visual perception of object size.

For commonly used physical display devices such as smartphone screens, iPads, and computers, we often evaluate screen clarity based on resolution. The higher the resolution, the more pixel information contained, and the clearer the screen appears.

Objectively speaking, yes. Because with the constant FOV, increasing d naturally leads to an increase in the width (w) of the virtual image. However, there won't be a significant perceptual change. This is because our FOV remains unchanged, meaning the proportion of the virtual image on our retina also remains unchanged.

In a Head-Up Display (HUD), the Field of View (FOV) refers to the angular size of the virtual image, specifically the solid angle formed by the edges of the virtual image and the center of the pupil. At this point, FOV represents the maximum visible range of the virtual image in the horizontal or vertical direction (for example: 10°x5°). For HUDs, FOV is a crucial metric as it directly impacts the perceived image size by the driver.

Thus, in HUDs, FOV is typically a fixed value, and the perception of image size by the human eye is directly proportional to FOV, independent of the projection distance.

Ambientlight

FOV includes vertical field of view (as shown in Figure (1)(A)), horizontal field of view (as shown in Figure (1)(B)), and diagonal field of view. Typically, unless specified otherwise, the default FOV is generally the horizontal field of view. FOV represents the range of the observable area perceived by the human eye. It is a concept of angle, not a single, fixed focal point[1].

Calculating FOV is straightforward. Figure (3) illustrates the principle of FOV calculation: FOV = arctan(w/2d) * 2, where w refers to the width of the observed object, and d represents the distance between the observer's eye and the screen.

Practicallighting vs motivated lighting

Many well renowned Directors and DPs have turned to practical lights over the years as a means to stylize and enhance their work. David Fincher for instance has been using them for years, and this scene from Fight Club (which mixes practical and film light sources) is a great example of that:

However, in reality, considering factors such as distortion correction and optimization processing, a more reasonable PPD should be set to above 80. Therefore, the maximum FOV supported by this optical engine is 10° (800/80=10). This illustrates that if a optical engine's screen has a higher resolution, it can support a larger FOV.

Typically, a person's retinal size is limited, and correspondingly, the field of view (FOV) of human vision is also limited. For instance, the maximum horizontal FOV for a single eye can reach up to 160°, while for both eyes, it can reach a maximum of 200°[2]. The maximum FOV when both eyes overlap is approximately 120°. Within this overlapped 120° FOV, stereopsis, enabled by binocular disparity, allows the perception of objects in 3D. Generally, the FOV for both eyes ranges between 90° and 120°.

[1]Alfano, P.L.; Michel, G.F. (1990). "Restricting the field of view: Perceptual and performance effects". Perceptual and Motor Skills. 70(1): 35–45.

At this point, we can introduce another commonly used resolution parameter related to angles, which is PPD (Pixel Per Degree). PPD represents the number of pixels that the human eye can perceive per degree of field of view. As mentioned earlier, the value of α is typically 1/60°, meaning there are 60 pixels per degree. When there are 60 PPD, it reaches the limit of human eye resolution. The better the visual acuity of the human eye, the smaller the minimum resolution angle α, and the larger the PPD required to reach the limit. As shown in Figure (6).

Headlights are extremely powerful light sources and can be hugely useful in a wide variety of shooting scenarios. My favorite way to use them is to backlight my actors with them, and then use a bounce to push some fill light back onto their faces. They can also be used as a key light, but remember that if the car isn’t in your frame, you’ll at the very least want to establish it in a master shot. Otherwise the extremely bright lights will look out of place and may disorient the viewer.

When we approach a display screen, the distance (d) between us and the screen decreases. Meanwhile, the width (w) of the screen remains constant. According to the formula, our FOV for this screen increases accordingly, as illustrated in Figure (5). This results in the screen's proportion on our retina increasing, thus creating the sensation of "closer appears larger, farther appears smaller."

For HUD, what we see is a virtual image, and PPI is not used as the unit of resolution, but PPD is used for description. This is because PPI is generally used to describe the clarity of physical display screens, whose width w remains fixed, and observations are typically made at a standard distance (such as the 25cm nominal viewing distance for smartphone screens). In HUD design, however, the projection distance usually varies, leading to changes in the width w of the virtual image. In such cases, considering the imaging of the virtual image on the retina, using the concept of PPD is more accurate.

Also, don’t be afraid to add diffusion to your car headlights to soften them and bring down the overall light levels. Even if the lights appear in your frame, the diffusion won’t likely be noticeable at all, especially if you’re shooting at a relatively shallow depth of field.

Ever since Stanley Kubrick used candle light to illuminate Barry Lyndon, filmmakers everywhere have been trying to emulate that look. In Kubrick’s day though, he needed to film on a custom F0.7 lens in order to get enough exposure to shoot this way. Filmmakers today have things much easier.

In VR products, a 90° FOV is often considered the minimum requirement for an immersive VR experience. A 120° FOV is commonly regarded as the standard for achieving a partially immersive experience, while a 180° FOV is considered the standard for achieving full immersion in VR. Different FOVs can evoke different sensations, as shown in Figure (2).

A "Retina Display" with 326 PPI means that at a normal viewing distance (≥25cm, where 25cm is the normal viewing distance), when the display screen's resolution reaches 326 PPI, the human eye can no longer distinguish individual pixels on the screen. In other words, at this point, the pixels on the display screen are at the limit of eye resolution on the retina, hence termed "Retina Display." Referring to the previous discussion, w represents the pixel size on the display screen, d represents the normal viewing distance of 25cm, and the corresponding FOV is the minimum resolution angle α of the human eye. α represents the minimum FOV that the human eye can distinguish, with a minimum resolution angle of 1/60 degrees for standard vision 1.0[4].

PPI is frequently used to describe the pixel resolution of such devices. PPI stands for Pixels Per Inch, describing the number of pixels per inch on the device. Certain smartphone brands introduced the concept of "Retina Display," with a resolution of ≥326 PPI[3].