Video from First Principles

Take one second of a gimbal shot of a dancer turning across a sunlit lawn and step through it on a computer. It comes apart into 30 still photographs: an arm a few centimetres higher in each, a hem a little further round. Nothing in the file moves. Your eye assembles movement when stills arrive fast enough.

Each picture was exposed for 1/60 s, so the camera gathered light for half the second and waited for the other half. That split decides how motion looks, whether stage lights throb, and how much daylight you must throw away.

01.Frames and Frame Rate

A video is a run of still images, called frames, replayed at the pace they were recorded. That pace is the frame rate, counted in frames per second (fps). At 30 fps each frame owns $\tfrac{1}{30}$ s, about 33.3 milliseconds (ms), so a three-minute dance holds $180 \times 30 = 5{,}400$ frames. The camera writes its rates as a number and a p, for progressive, meaning every frame is a whole picture: 24p, 25p, 30p, 50p, 60p, and faster rates for slow motion (p. 1004).

Why 24, 25, 30 and 60

The numbers come from history, not physics. 24 fps is the rate cinema settled on when films gained sound, and it still reads as film to most viewers. 25 and 30 come from television, which tied its picture rate to the mains electricity: 50 Hz (hertz, cycles per second) in Europe, 60 Hz in North America. 50p and 60p double them, for smoother movement or for footage you plan to slow to half speed.

The North American rates sit one part in a thousand below their names, a legacy of colour television. The camera's 60p, 30p and 24p run at 59.94, 29.97 and 23.976 fps, while 50p and 25p are exact (pp. 201-202, 1004). My everyday frame size and rate are settled in Video Formats and Codecs.

02.Motion Blur and the 180-Degree Rule

Each frame is a photograph with its own shutter speed, and in video too short an exposure makes motion look like a flip book. Suppose, as an illustration, a dancer's hand crosses the frame at 2 metres a second. At 30p it travels $2 \times \tfrac{1}{30} \approx 0.067$ m, about 6.7 cm, from one frame to the next. At 1/60 s it moves about 3.3 cm while the shutter is open, so each frame holds a soft streak covering half the gap, and the eye joins the streaks into movement. At 1/500 s the streak shrinks to 0.4 cm, nothing fills the gaps, and the hand seems to hop.

The habit that follows is the 180-degree rule: expose each frame for half of its interval. With $f$ the frame rate and $t$ the exposure in seconds,

$$t = \frac{1}{2f}$$

so 30p asks for $\tfrac{1}{60}$ s, 24p for $\tfrac{1}{48}$ s and 60p for $\tfrac{1}{120}$ s. The name comes from film cameras, whose shutter was a spinning disc: a cut-out of 180 degrees, half a turn, let light through for half of each frame. A faster shutter gives a deliberate staccato look, so treat the rule as a starting point.

Lights on 60 Hz Mains

US mains alternates 60 times a second, so many lamps pulse 120 times a second, once every 8.3 ms. An exposure of 1/60 s, 16.7 ms, holds exactly two pulses wherever it starts, so every frame collects the same light. That is why my 24p shutter is 1/60 s and not 1/48 s, which at 20.8 ms spans two and a half pulses, so a frame's light depends on where it begins. At 24p, 1/60 s is a 144-degree shutter, $\log_2 \tfrac{60}{48} \approx 0.32$ of a stop shorter than 180 degrees, a stop being a doubling or halving of light. Nikon's own fallback in a 60 Hz area is mode S or M at 1/125 s, 1/60 s or 1/30 s (p. 600), and at 60p I use 1/125 s, though at 8 ms it spans 0.96 of a pulse rather than a whole one. Shutter Types, Rolling Shutter and Flicker covers banding and the camera's remedies.

Shutter Angle

Shutter mode offers Shutter speed, the default, and Shutter angle, which shows the exposure in degrees, as a film camera would, in modes S and M with the same dials (pp. 629, 770). Nikon's Reference Guide gives 15 steps from 5.6 to 360 degrees without listing them (p. 770). Nikon's firmware 1.10 notes name 180 degrees as one, and between the two ends I name no other step.

The conversion is the fraction of a turn multiplied by the frame interval. With $\theta$ the angle in degrees,

$$t = \frac{\theta}{360} \times \frac{1}{f}$$

At 30p, 180 degrees gives $\tfrac{1}{59.94}$ s. At 60p it gives $\tfrac{1}{119.88}$ s, about 8.34 ms, which by this arithmetic sits nearer one 8.33 ms lamp pulse than 1/125 s does. At 30p, 360 degrees is the whole interval, $\tfrac{1}{29.97}$ s, and 5.6 degrees is about $\tfrac{1}{1{,}927}$ s.

The appeal is that the angle ignores frame rate (p. 770). Move from 30p to 60p and 180 degrees stays 180 degrees, whereas in Shutter speed 1/60 s stays put and becomes a 360-degree shutter. The price is that Shutter angle disables High-frequency flicker reduction and g11 (p. 770). Nikon is also silent on how 360 degrees squares with the floors on p. 769: $\tfrac{1}{29.97}$ s at 30p runs a tenth of a percent past 1/30 s, and $\tfrac{1}{23.976}$ s at 24p about 4% past 1/25 s. I keep Shutter speed, because my speeds come from Nikon's flicker list and I want High-frequency flicker reduction ready when an LED (light-emitting diode) wall bands.

03.Switching the Camera to Video

The photo/video selector's video position puts the camera in video mode (pp. 102, 107). The mode dial keeps its positions, and video brings its own recording menu, i menu, button roles and Custom Settings group g. Flash cannot be used (p. 107).

Press the video record button to start. A recording indicator, a red border, the time recorded and the card's estimated remaining time appear (p. 108). Press it again to stop. A take also ends when you turn the selector, change shooting mode or remove the lens, and when the battery, the clip limit or the camera's temperature calls time (p. 109), the last three being the subject of Heat, Power and Recording Limits.

What Carries Over From Photo Mode

The video menu keeps its own copies of photo settings. Four default to Same as photo settings and follow the photo menu until changed: three in the table below and Vibration reduction (p. 579). Four more, Storage folder, Vignette control, Diffraction compensation and Auto distortion control, are shared both ways (pp. 581, 595-597). The rest hold separate values. Flicker Frequency reaches the other way and also governs the photo display (p. 600). Defaults below are from pp. 578-579. FX is the full-frame image area and DX the smaller crop.

04.Exposure in Video

The Exposure Modes That Work

Video uses the dial's AUTO, P, S, A and M, which differ in what you may change while recording (p. 209): nothing in AUTO and P, the shutter in S, the aperture in A, and all three in M. Outside M the camera sets ISO: in P, S and A up to Maximum sensitivity, 51200 by default (pp. 578, 588), and in AUTO from ISO 100 to 51200 (p. 1005). In S and M the shutter runs from 1/25 s to 1/16000 s, with a slowest speed that depends on the frame rate (pp. 209, 213). Exposure compensation spans $-3$ to $+3$ EV in video (pp. 151, 1005), where EV, exposure value, counts stops. In M it moves only the exposure indicator unless auto ISO is on (p. 152).

A Manual-Exposure Workflow

I film in M because nothing changes unless I change it, and I set things in the order the physics dictates. The shutter comes first, since frame rate and mains have chosen it: 1/60 s at 24p and 30p, 1/125 s at 60p. Aperture comes next, for the depth of field you want, the zone around the subject that looks sharp. ISO then sets the brightness, with Auto ISO control (mode M) at OFF so it stays put on stage. Outdoors in daylight I turn it ON, so the ISO follows the light, and hold AE lock (hold) when the camera swings past a bright sky. If the picture is still too bright at the lowest ISO, a neutral density (ND) filter is next. Last, fix the white balance and check exposure with zebra stripes or a waveform, described with N-Log, Nikon's flat recording for grading, and HLG (Hybrid Log Gamma), a high dynamic range picture, in N-Log, HLG and Exposure Tools.

Aperture is the one setting I change mid-take, with power aperture on Fn1 and Fn2. It works only in A and M, and Nikon warns that it can make the display and footage flicker and can be heard (pp. 109-110, 756), so keep changes slow and rare.

Auto ISO in Brief

Auto ISO lets the camera move ISO to hold the exposure. Outside M it is always on, and in M it follows Auto ISO control (mode M), ON by default (pp. 578, 588). Switched OFF, mode M uses the value in ISO sensitivity (mode M), 100 by default (p. 578). I turn it off for dance, because stage light changes on purpose and a camera that chases it undoes the lighting designer's work. ISO and Auto ISO covers the rest, including the ISO limits in HLG and N-Log.

Fine ISO Control

On (1/6 EV) moves mode M video ISO in sixth-stop steps from ISO 100 to 51200, set through ISO sensitivity settings > ISO sensitivity (mode M) (p. 768). The default is Off (p. 629). A sixth of a stop brightens the picture by $2^{1/6} \approx 1.12$, against $2^{1/3} \approx 1.26$ for a third. It helps when matching two cameras. I leave it Off, since I set ISO once per venue.

Extended Shutter Speeds

At the default, OFF (p. 629), the slowest shutter in S and M is tied to the frame rate: 1/60 s at 60p, 1/30 s at 30p, 1/25 s at 25p and 24p (p. 769). ON lowers that floor to 1/4 s from 60p down to 24p, leaving faster rates alone (p. 769). A 1/4 s exposure at 30p spans $\tfrac{1/4}{1/30} = 7.5$ frame intervals, so frames repeat, and Nikon recommends a tripod (p. 769). Slow motion and Shutter angle disable it (pp. 203, 770). I leave it OFF, since dance needs 1/60 s or faster.

Rolling Shutter in Brief

The sensor reads each frame row by row from the top, so a subject moving during the readout lands in a different place at the bottom than at the top. Nikon warns that fast subjects and sideways pans can look distorted (p. 110). A partially stacked sensor reads faster, shrinking the effect without removing it. Shutter Types, Rolling Shutter and Flicker and What a Partially Stacked Sensor Changes give the detail. On a gimbal, pan smoothly rather than whip.

05.Neutral Density Filters for Daylight Video

Here the 180-degree rule meets the sun. At 30p you are held at 1/60 s, and in SDR (standard dynamic range, the ordinary picture) the lowest ISO in mode M is 100 (p. 588). Stopping down to f/16 throws away the shallow focus a fast lens was bought for and softens detail through diffraction. What remains is the ND filter, grey glass that dims every colour equally.

Take, as an illustration, a cloudless midday that meters f/16 at 1/125 s and ISO 100, roughly what the sunny-16 rule of thumb predicts, and suppose you want f/2.8 on the Tamron 28-75mm and 1/60 s. The light admitted scales with the inverse square of the f-number, so opening from f/16 to f/2.8 adds

$$2 \log_2 \frac{16}{2.8} \approx 2 \times 2.515 \approx 5.03 \text{ stops}$$

and slowing from 1/125 s to 1/60 s adds $\log_2 \tfrac{125}{60} \approx 1.06$ stops, about 6.1 in all. You need a 6-stop filter, a factor of $2^6 = 64$, sold as ND64 or as density 1.8, since $\log_{10} 64 \approx 1.81$.

Each stop you open the lens is a stop more filter: about 5 stops at f/4 on the Z 24-70mm f/4 S, about 7 at f/2 on the Z 40mm f/2. Real daylight varies with season and haze, so the sum only shows where in a filter's range you will live. Nikon cautions that matrix metering can go astray through any filter with an exposure factor above $1\times$, NDs included, and suggests Center-weighted metering (p. 970), but in M with auto ISO off the meter only guides an exposure I set by hand, checked against the Zebra pattern on my video i menu.

My variable ND spans roughly 1 to 9 stops. Variable NDs have a wide-angle limit and a tell-tale dark cross, both set out in Long Exposure and Night.

06.Zooming While Recording

Two kinds of zoom work during a take. A power zoom (PZ) lens moves its glass with a motor, more smoothly than a hand can. Hi-Res Zoom moves nothing: it enlarges part of what the sensor captures, and Nikon says it does so without losing resolution, even on a prime (p. 217).

Hi-Res Zoom

Set it ON, from its default of OFF (p. 579), and the multi selector or the sub-selector, pressed left or right, zooms during the take, with an icon and a zoom bar on screen (pp. 217, 620). It tops out at $1.4\times$ in 4K, the 3840 by 2160 frame, and at $2.0\times$ in 1920 by 1080 (p. 217), and it can sit on the video i menu (p. 746). On the Z 85mm f/1.8 S in 4K, $1.4\times$ frames like an $85 \times 1.4 = 119$ mm lens. Nikon does not explain the method, but the numbers fit a crop from a wider readout: the widest video frame is 6048 pixels across (p. 200), and $6048 / 1.4 = 4{,}320$ pixels still exceeds the 3,840 that 4K needs. That readout would allow about $6048 / 3840 \approx 1.58\times$, though, so it does not explain the $1.4\times$ cap.

It refuses several settings.

• It takes only the ProRes 422 HQ, H.265 and H.264 file types, so neither RAW type (p. 217).
• In FX it allows 4K and 1920 by 1080 from 60p to 24p. In DX it allows 1920 by 1080 from 120p to 24p, plus slow motion (p. 217). Slow motion in FX blocks it (p. 204).
• The AF-area mode, the part of the frame autofocus (AF) watches, is fixed at Wide-area AF (L) with no focus point shown (p. 217). Subject detection normally works in that mode (p. 128), but Nikon does not say whether it runs while the point is hidden.
• Electronic VR, the camera's digital vibration reduction (VR), is fixed at OFF (p. 217).
• Digital flicker reduction is unavailable with it at 4K 30p, 25p and 24p and at 1920 by 1080 60p and 50p (p. 601).

I would use Hi-Res Zoom for a slow push-in on a prime when the gimbal cannot walk closer, and leave it OFF otherwise, because it takes away the Subject-tracking AF that Video Autofocus and Stabilisation uses for one performer.

Hi-Res Zoom Speed

g7 sets how fast Hi-Res Zoom travels when driven by the multi selector, or by Fn1, Fn2 or the lens Fn ring once g2 gives them Hi-Res Zoom roles (pp. 218, 762). The default is 0 (p. 628), and the guide prints no range. A lens control ring ignores g7 and follows f13 Control ring response (pp. 218, 740). With power aperture on my Fn buttons, the multi selector would be my zoom control.

Assign Power Zoom

I own no PZ lens, so this item and the next are for readers who do. Use x/w buttons, OFF by default, lets the camera's zoom buttons drive the lens (pp. 628, 763). Power zoom speed (zoom buttons) sets their speed, and that of any power zoom role in g2, separately for Pre-recording (STBY) and Video recording (REC), both 0 by default. Power zoom speed (zoom switch) does the same for the zoom lever on the NIKKOR Z 28-135mm f/4 PZ, both +3 (pp. 628, 763).

Brisk on standby and slow while rolling makes sense, since zoom noise can be recorded and a slower speed reduces it (p. 763).

Power/Hi-Res Zoom Collab

g9 joins both zooms into one movement when a compatible PZ lens is fitted and Hi-Res Zoom is ON (p. 765). Off keeps them apart. On (extend) zooms optically to the long end and then carries on with Hi-Res Zoom, with a marker at the handover and a possible pause there. On (synch), the default, runs both together for a smooth zoom (pp. 628, 765-766). On the NIKKOR Z 28-135mm f/4 PZ, the lens Nikon lists (p. 767), extend would carry 135 mm on to the framing of $135 \times 1.4 = 189$ mm in 4K, or $135 \times 2.0 = 270$ mm in 1920 by 1080. Those two figures are my inference, since Nikon does not say whether the combined zoom keeps the $1.4\times$ and $2.0\times$ caps.

With collab on, g8 sets the speed and g7 is ignored, and a control ring set to Hi-Res Zoom stops zooming (p. 767). A saved zoom position cannot be recalled in the Hi-Res range under extend, or at all under synch (p. 766). Under synch the lens may also move by itself, which Nikon calls normal, and with Save zoom position (PZ lenses) on, the zoom may shift at power-on or on waking (p. 767).

07.Controls While Filming

Three Custom Settings decide what your hands do during a take.

g1 chooses the 12 slots of the video i menu, which is separate from the photo one (pp. 627, 745). Mine keeps Nikon's defaults except Zebra pattern in slot 9 and Focus peaking in slot 11, as The i Menu explains.

g2 sets the video button roles (p. 748). My map, explained in Custom Controls, puts Power aperture (open) on Fn1, Power aperture (close) on Fn2 and AE lock (hold) on the sub-selector centre, AE meaning auto exposure, and leaves the video record button on Record videos. Nikon's default Fn1 role, White balance (p. 627), allows white balance changes mid-take by holding Fn1 and turning a dial (p. 213). My setup gives that shortcut up, which suits a fixed white balance. Giving the Shutter-release button the Record videos role lets a remote start and stop recording (pp. 213, 628).

g3 can lock shutter speed in M, aperture in A and M, and the focus point (p. 758). All three are OFF by default (p. 628), and I leave them off. If someone else flies the gimbal, Shutter speed lock is cheap insurance.

08.Recommended Settings

09.Beginner Mistakes

Letting the shutter climb in daylight. In AUTO, P or A the camera chooses the shutter, and on a sunny lawn it may pick 1/1000 s or faster, so the dance turns staccato. Use M at 1/60 s with an ND filter.

Using 1/50 s for 24p in the US. That speed suits 50 Hz countries, and under 60 Hz lamps it can band. Use 1/60 s.

Changing frame rate and forgetting the shutter. At 60p, 1/60 s is a 360-degree shutter. Reset it to 1/125 s.

Expecting exposure compensation to darken a mode M clip. With a fixed ISO it moves only the indicator (p. 152). Change aperture, ISO or the filter.

10.Related Chapters

• Video Formats and Codecs: file types, frame sizes and the everyday frame rate.
• N-Log, HLG and Exposure Tools: zebra, the waveform, and flat and high dynamic range recording.
• Video Autofocus and Stabilisation: AF-area modes in video, Electronic VR and the gimbal.
• Heat, Power and Recording Limits: clip length, heat and batteries for long events.
• Shutter Types, Rolling Shutter and Flicker: banding, the camera's flicker remedies and rolling shutter.
• Long Exposure and Night: ND arithmetic and the limits of variable ND.