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Filmmakers push technology in certain directions, but technology pushes back at us, too. A breakthrough in one area can hold us back in another; a technology that makes one style of filmmaking easier can impede another. And sometimes, what was seen as a breakthrough with hindsight looks merely like a shift in convention.

Interestingly, the guide number concept is derived from an area relation (which at first glance would seem to support the premise of your question, but as we'll see, there's no need to use square factors). The amount of light incident on an object is inversely proportional to the square of the distance between the light source and the object (the inverse-square law): \$I \propto 1/s^2\$.

Attempting to clarify my question, I am not confused by the 2x/2 series or doubting the benefit making it relative to focal length. My question is only in regard to the names we apply to this series. In my example scheme it doesn't matter if we call it "ƒ/1.4" or "Av1"; the use is interchangeable. So I wonder what convenience is imparted by using the fractional diameters at all?

Perhaps deep focus would never have been talked about as a concept if it hadn't been associated with great cinematographers who felt a particular need for greater depth of field. Perhaps we should see it not as a style, but rather one approach out of thousands to shoot a scene. No cinematic style or convention has value in itself; it's how you use it to tell a story that matters.

Replay: Once upon a time, Deep Focus ruled the cinema screen. Why did it fade in popularity? And do recent consumer and professional technical trends mean that it is now overdue a revival?

Deep focusshot example

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If the lens is 50mm in focal length (FL), the 25mm aperture would be f/2, and the 35mm aperture would be f/1.4 (actually 35.7mm).

The key to understating the f-stop system, which the industry accepts as the fundamental increment of exposure, is a 2X increment. This is a doubling of halving of the light energy of exposure. In modern times, it is sometimes necessary to make finer adjustments. When needed, we can refine the f-stop, and make 1/2 or 1/3 or even 1/6 increments. Let me add, except in a laboratory situation, it is impossible to control a photo process and keep it at 1/3 f-stop tolerances.

Example 1: The hyperfocal distance \$H\$ is the focal distance that theoretically maximizes total depth of field. For a lens of focal length \$f\$ set to a f-number \$N\$, then given a circle of confusion limit $c$, the hyperfocal distance is defined as

Deep focus inCitizen Kane

It might have been more important when large and medium format cameras were more prevalent than they are now and depth of focus considerations were much more dominating imaging decisions than they are now.

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(if the source is not larger than the FOV's like a wall, but it is a point source instead like a street light; then the increase/decrease in size and light follows the inverse square law)

$$\begin{align} H &= \frac{f^2}{Nc} + f \\ &= f\left({f\over Nc} + 1\right) \\ &= f\left({D\over c} + 1\right) \\ &\approx {fD\over c} \qquad(\text{because }D \gg c) \end{align}$$

For a given intensity \$I\$ on the subject, we set our camera's exposure settings to correctly expose the subject. Because we're talking about flash photography, let's assume ISO and shutter speed aren't really free variables available to us for exposure control (e.g., let's leave ISO fixed at 100, and shutter speed at, say, 1/200). That leaves aperture available for adjustment for correct exposure of the object.

It is in relation to FL because a longer FL has a narrower field of view (FOV); it collects less light and spreads it over the same area (image circle). A longer FL must have a larger aperture area in order to have the same f# and transmit the same amount of light, achieving the same exposure.

So, what happened then to deep focus? Well, in one word, television. Detailed vistas didn't work on the small, low resolution TV screens, but close-ups did. Multi-camera TV studios encouraged cutting from angle to angle to tell a story and the language of TV began to influence cinema, partly because filmmakers were aware that the small screen was where their movies would end up.

Bokeh is a term unheard of in film and video a decade ago. It's a Japanese word for 'blur' that still photographers began to use in the late 90s to discuss the quality of the out-of-focus regions of an image. When still cameras and lenses began to be used for shooting video, the concept of ‘bokeh' came with them.

Regarding units and dimension: Note that \$N\$ is a unitless quantity, defined as the ratio of two distance-measures (i.e., millimeters divided by millimeters) that are implicitly understood to be arranged at right angles to each other. If \$N\$ were instead a ratio of focal length to entrance pupil area, the units of \$N\$ would be in [length-1], such as "per meter" or "per millimeter". Net exponents of distance in the denominator is a particularly unwieldy thing for humans to think about and get their head around, in physical models.

I.e. a FL 2x longer has a FOV 1/2 the size, gathering 1/2 the light (infinite source), and transmitting it through an aperture 2x the size. Which results in 2x the light gathered being recorded at the image plane for the same light density/exposure (2X.5=1). I.e. 100mm vs 50mm at same f#.

Now while the presence of \$f^2\$ in the first hyperfocal distance equation (that includes \$N\$ in the denominator) might appear to be a result of some dependence on area, it's really just an artificial creation because of the simple algebraic substitution \$N = f/D\$. In other words, as long as the aperture diameter \$D\$ is much larger than the circle of confusion diameter \$c\$, the hyperfocal distance is linearly proportional to both \$f\$ and \$D\$, and inversely proportional to \$c\$. The equation has nothing to do with the area of the aperture that would be generated by rotating the cross-section of the thin lens of diameter \$D\$ through \$\pi\$ radians.

As video became cheap and commonplace, filmmakers with restricted budgets wanted the films to look classier. They might have shot on video, but they wanted their work to look like the movies. So various tricks were tried: shooting single field so 60 or 50 field per second video looked like 24 or 25fps film, turning sharpening circuits down, using low contrast filters, even adding fake grain. It was only at the end of the 20th century that it occurred to them that perhaps the most characteristic thing about the 35mm movie look was that not everything was sharp - unlike video, where everything was in focus (particularly amateur video shot on camcorders with tiny chips). Shallow DoF became associated with a ‘cinematic' look. It is worth pointing out that until then, shallow DoF was never seen as a particularly crucial asset. Super 16mm was beginning to be used for the cinema (such as The Draughtsman's Contract in 1982, Lock, Stock and Two Smoking Barrels in 1998 through to The Hurt Locker in 2008) and no one seemed bothered by the lack of bokeh. On the contrary, it was seen as making the job of the focus puller a little easier.

Deep focus also involved a few tricks. Citizen Kane is loaded with complex composites where background action was shot separately and matted in. Deep focus also made extensive use of split diopters - basically a close-up lens cut in half that can be slid partially in front of the lens to enable close focus on an object or face one side of the frame while allowing the background to stay in focus. Split diopters are still in use to today. Brian De Palma, director of Carrie, is particularly fond of them.

As screens at home get bigger and resolution higher, now is the time for a revival of deep focus. Movies shot on 65mm that made the most of a huge, detailed canvas, like Lawrence of Arabia or 2001, are no longer a joke on TV. With digital technology, the sort of complex matte shots used on Citizen Kane are now relatively easy. As sensors get faster, it gets easier to achieve the small apertures needed for deep focus. And as television producers adapt to bigger screens and higher resolution, we have seen the return of the wide shot. Look at a high-end US TV series like Fargo or Better Call Saul and you will find it is full of beautifully composed, detailed wide shots - deep focus is back with us. Maybe soon, shallow DoF will soon connote low-budget filmmaking, rather than cinema.

A similar aperture numbering system called the U.S. system (Uniform System) was used by the first Kodak cameras (until around 1920s). That system originated in England (1880s). Not 1, 2, 3, 4, but those stops were numbered 1, 2, 4, 8, etc, starting from todays f/4 equivalence. It was more useful than 1, 2, 3, 4 because it represented exposure increase inversely (doubling U.S. number is one stop less, doubling f/stop Number is two stops less). And exposure was considered important to photographers.

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Perhaps a better analogy for argument is the debate over what is the better circle constant, \$\tau\approx 6.28\$ vs. \$\pi\approx 3.14\$ (Tau Manifesto). The debate is really a non-debate; as long as the correct factor of 2 is used in the right places, it doesn't matter. One notation might lead to a better understanding of the geometry or physics being described by the equations, but in the end, the math doesn't change. Just the notation and more or fewer factors of 2. Just like aperture diameter vs. area.

What is deep focus in filmreddit

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Deep focus (or deep depth of field, as we would call it today) was dependent on innovations in technology. Faster filmstock and powerful carbon arc lamps enabled Toland to shoot between f8 and f16 using (mostly) wide-angle lenses. A couple of years earlier none of this would have been possible.

In Charles Laughton's Night of The Hunter (1955) the figure of Robert Mitchum on a horse in the background is actually a midget on a Shetland pony (yes, really).

The uniqueness of the focal ratio is that it intertwines two optical factors. The longer the focal length, the more light that is lost. Double the focal length and the light loss is 4X. The other factor is the diameter of the entrance pupil doubles its diameter, and the lens will gather 4X more light. The f-number system balances both phenomena.

Why the odd crazy number set? 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32 – 45 – 64 Each number going right is its neighbor on the left multiplied by 1.4 (square root of 2). Each number going left is its neighbor on the right divided by 1.4. Why? If suppose you have a circular lens with a diameter of 2 inches. This is its working diameter. Its area, the dimension that captures light is 3.14 square inches. Now suppose you wish to double its light gathering power. To do so, you must increases area 2X. What will be the revised diameter? Answer 2 multiplied by 1.4 = 2.8 inches.

Curious, then, that the least cinematic part of filmmaking - the talking head interview - began to set the aesthetic standard for filmmaking. For most of the time, particularly in documentaries, we do want to see the details, we do want the audience to find elements and make connections (and not just look at the things we are pointing at). Also, if we are capturing the world as it happens (which, for me, is what documentaries are all about), we don't have time to change lenses or pull focus. Pulling focus on a documentary also implies we know already what is the most important part of the frame (and usually we don't). Shallow DoF lends itself to composed, studied images; it is a impediment for a handheld, moving camera. It has changed the way filmmakers approach documentary.

I found the Wikipedia page on the APEX system and see—unsurprisingly—that the naming shown above was proposed at least by 1960, as aperture value. What that page doesn't seem to provide is a robust explanation of why the proposal never took root.

I.e. a 25mm aperture diameter (entrance pupil) has an area of 490mm; and 1.4 x 25mm = 35mm with an area of 962mm... approximately double the area/light/exposure.

What is deep focus in filmexamples

The f-stop is actually a ratio. This is important because a ratio is dimensionless (look up ratio if you doubt). Actually f-stop is the accepted jargon for focal ratio. This value is derived by dividing the focal length of the lens by the working diameter of the entrance pupil (aperture). Thus a 100mm with a working diameter of 12.5mm = focal ratio 100 ÷ 12.5 = f/8 (written with a slash). By the way, an 8000mm lens with a working diameter of 1000mm is also an f/8. Both produce the same exposing energy if set to f/8 and pointed at the same vista.

By the end of the 20th century, the standard shooting formats for TV was the 2/3" chip video camera and the 16mm film camera (this in the UK - 35mm remained the standard film format for TV in the USA). As we all know, the size of the sensor affects Depth of Field - not directly, but because with smaller sensors (or gates, in film terms), we use shorter focal length lenses for the same angle of view and DoF is dependent on focal length, camera-to-subject distance and aperture. This meant that, compared to 35mm, we had reasonably deep focus cinematography whether we wanted it or not, but TVs were still small and there was no great desire for detailed wide shots. The term 'deep focus' fell out of fashion and, in the 21st century, filmmakers become more concerned with the opposite - welcome to bokeh.

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Classic ƒ numbers are entrance pupil diameter as a fraction of focal length. This seems like a slightly strange choice as exposure is proportional to area rather than diameter. Naively I would think it easier work with these as exposure stops from reference ƒ/1:

The guide number encapsulates this dependency. Because f-number \$N\$ is inversely proportional to aperture diameter, the constant exposure relation is now a product rather than a ratio: \$N\cdot s\$. And importantly, the dependence on squares of distances is not necessary. We can just use linear flash-to-subject distance and linear aperture diameter.

Deep focusdefinition

If the distance were changed by a factor of \$k\$, then the light intensity falls by \$k^2\$. In order to keep the photometric exposure the same, we need to compensate by increasing the aperture area by \$k^2\$, or the aperture diameter by a factor of \$k\$. Thus, for constant exposure, the ratio of flash-subject distance to aperture diameter needs to remain constant.

Aperture f-numbers are near approximations. And the 1.4x increment is approximately equal to doubling of the aperture area.

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But the f/stop system rapidly gained favor starting just before 1900, because it also factored in the lens focal length (f-number = focal length / working diameter). The working diameter is as seen as magnified through the front lens element (entrance pupil). It is named f/stop in reference to the division of focal length f by diameter.

It's easy to understand the appeal of shallow DoF; it makes things look beautiful or, if you like, it aestheticises reality. When the background is soft, the foreground actually looks sharper and attention is concentrated on the subject. In still photography, shallow DoF is great for portraits and in filmmaking, of course, it's much desired for interviews. When we film interviews, we are seldom interested in the background and don't usually have much control over it, we want the background to go soft to concentrate on the interviewee (with smaller sensor cameras, you would do this by moving the camera back and going on to a longer lens, but, of course, this is not quite the same look as the perspective changes).

What isshallowfocus in film

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Also, having unbalanced ratios of distances would pin the number to choice of units. Any values of such an area-based f-number would be explicitly dependent upon choice of units used for focal length. So aperture settings on lenses with fractional-inch based focal lengths would have completely different values than for millimeter-value focal length lenses (and also for centimeter-valued focal length lenses).

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The f-number system is unique in that it is universal. In other words, any lens, on any camera, regardless of focal length or image size, when set to the same f-number, will return an identical exposure. Well, not exactly, but close enough for most every need. In cinematography, a T-stop is favored. This is an f-top that has been calibrated to take into account light loss induced by the color of the glass, influence of lens coating and inaccuracies of the aperture diameter etc. The T-stop is deemed necessary in this usage because it gives improved uniformity, scene-to-scene and lens change-to- lens change. Pictorial still photography is contented with the f-stop.

Math. It's because in many equations regarding simple optics, the ratio \$N = f/D\$ (where \$N\$ is the f-number, and $D$ is the lens (or more often precisely, entrance pupil) diameter) pops up a lot, or the use of the ratio simplifies the expression or understanding of the expression.

What is deep focus in filmphotography

To throw enough light on the set to have stopped down sufficiently to keep a real hand in focus would have imposed unacceptable levels of heat on Ingrid.

ShallowfocusShot

The effective pupil diameter is important as a measure of unsharpness: the entrance pupil forms the base of "unsharpness cones" that have their respective tip (indicating full sharpness) in the focus plane and widen again from there. Double the f number, and you halve the diameter of any bokeh circle visible in the image.

The significance then of the f/stop system is that the exposure of any one stop number, like f/4, is still f/4 on any lens of any size. Two photographers with different cameras standing side by side could use the same aperture number then. In practical terms, it made the later concept of light meters possible (for any camera lens). :) The f/stop number provided significance to exposure in any camera, more about the exposure than just about the lens.

Normalization with respect to fundamental "figures of merit" happens all the time. The first thing to pop to my mind is in relativistic physics. We talk all the time about velocities as some fraction of the speed of light, \$c\$, which is approximately 3 x 108 m/s, or about 186,282 mi/s. We don't talk in absolute values of meters per second or miles per second. But in terms of fractions of \$c\$, it's much more useful.

Deep focus cinematographers became particularly ingenious in the 40s and 50s. In Alfred Hitchcocks' Spellbound (1945), the murderer, in a point-of-view shot, turns the gun away from his psychoanalyst (Ingrid Bergman) and on to himself. To enable both the gun and the subject to stay in focus Hitchcock, created a giant mechanical hand and gun.

In the heyday of movies in the 1940s, a cinematographic breakthrough was proclaimed: deep focus. Most famously pioneered by cinematographer Greg Toland in Orson Welles' Citizen Kane, deep focus meant everything on the film set could be in focus at once. No longer did the filmmaker have to decide what was in focus and what was not. Everything was clear and sharp for the eye to discover. Some critics saw this as liberating the audience; rather than being controlled and directed, the viewer was free to make his or her own connections.

Someone realised that if you first projected your image onto a ground glass screen – say, the screen you might find inside a 35mm SLR film camera - then re-filmed it onto video, something magical occurred; not only did you have shallow depth of field, but you could use inexpensive 35mm still lenses and it looked like the movies. This was the DoF adapter which became obsolete as soon as the DSLR revolution occurred. Large sensor cameras with longer focal length lenses became the economic way to make films.

The effect on exposure is something easily folded into the metering and also accesible via counting stops, but assessing the effect on image geometry depends on proportions and geometry and if you want to do rough estimates, not having to calculate square roots helps.

The formula for hyperfocal distance is merely a special case of the computation of far depth-of-field when the far focus distance is infinity. The geometry that describes the depth of field equations is completely described by similar right-triangles in the cross-sectional plane through the optical axis of the lens, and the thin-lens equation relating the focal length (strength) of the lens and its object-side and image-side focus distances.