N-Log, HLG and Exposure Tools

A dancer turns on a lawn at the end of the day with the sun low behind her. Suppose a meter read three patches of the scene: the sky over her shoulder at 2,048 units of brightness, her shadowed face at 32 and the folds of a black skirt at 2. The numbers are illustrative and only their ratios matter. Expose the face correctly in the camera's everyday mode and the sky would likely become a flat white sheet. Record the same moment in Nikon's log mode and the sky should keep its colour, but the monitor shows a grey picture, as if seen through fog.

What differs is the rule that turns light into numbers, and the Z6III offers three such rules, called tone modes: SDR, HLG and N-Log. In any of them the monitor picture can mislead you about exposure, so the chapter ends with the measuring aids: View assist, zebra stripes and the wave-form monitor. Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, for firmware 2.00.

01.Dynamic Range in Stops

A stop is a doubling or a halving of light, so 2 units to 4 is one stop and 4 to 8 is another. The lawn's skirt-to-sky ratio of 1,024 is ten doublings, because $2^{10} = 1024$. The face sits six stops below the sky, since $2048 / 32 = 64 = 2^6$, and four stops above the skirt, since $32 / 2 = 16 = 2^4$.

Counting doublings is what a base-2 logarithm does. Write $L_{\max}$ for the brightest tone you want to keep, $L_{\min}$ for the darkest, and $D$ for the span between them in stops:

$$D = \log_2 \left( \frac{L_{\max}}{L_{\min}} \right)$$

For the lawn, $D = \log_2 (2048 / 2) = \log_2 1024 = 10$ stops. The dynamic range of a camera or a file is the same kind of span, from the brightest tone it records before turning pure white to the darkest it records before detail drowns in noise.

Nikon's one figure for Z6III video is 13+ stops, for N-Log recorded as H.265 10-bit (MOV) at ISO 800, which Nikon calls the lowest possible, though in mode M N-Log also offers Lo 0.3 to Lo 2.0 below it (pp. 216, 588). That is a brightness ratio of $2^{13} = 8192$ to 1, so on paper the lawn's ten stops fit with three to spare. How the span divides above and below the face depends on exposure, and Nikon does not print where N-Log places middle grey. Nikon prints no stop figure for SDR, saying only that HLG covers a wider range than SDR (p. 199), so I will not invent one.

02.Why a Flat Recording Keeps More

The Numbers Are the Limit

The H.265 and ProRes 422 files hold colour as YCbCr (p. 198), one brightness channel plus two that record how far the colour departs from grey. A 10-bit file stores each pixel's brightness as a whole number from 0 to 1023, which is $2^{10} = 1024$ possible values. What matters here is how those values are shared among the stops of a scene, because a stop that receives few of them loses its texture.

Under a linear rule the number is proportional to the light. Full white is 1023, so the brightest stop runs from 512 to 1023 and takes 512 values, and each stop below takes half as many as the one above. With $b$ for the bit depth and $k$ for the stop's position counted down from the top, starting at 1, stop $k$ receives $N_k$ values:

$$N_k = 2^{\,b - k}$$

With $b = 10$, the top stop gets $2^9 = 512$, the fifth gets $2^5 = 32$ and the tenth gets $2^0 = 1$. One value cannot hold the folds of a skirt.

Spreading the Numbers Evenly

A logarithmic curve, or log curve, gives every stop a similar share. In an idealised case, 13 stops spread evenly over 10 bits would get about $1024 / 13 \approx 78.8$ values each. Nikon does not print the N-Log curve, and real curves leave values unused at both ends, so 78.8 only illustrates the principle. The principle itself is Nikon's: log recording converts light to numbers through a log function (p. 216).

The price is the look. Shadows are lifted toward the middle and highlights sit below full white, so on an ordinary screen the picture looks grey and low in contrast. That flat image is a container, and its contrast is restored in editing, a process called grading.

SDR, short for standard dynamic range, does the opposite. Its gamma curve, a power-law curve of the same family as the sRGB curve in Picture Controls and Colour, plus the contrast of the chosen Picture Control, makes the picture look finished straight from the camera. The cost is headroom. Metering uses 18% grey as its reference, and BT.709, the high-definition television standard of the International Telecommunication Union, or ITU, defines its curve (item 1.2 of BT.709-6) for light running from 0 at black to 1 at white. A face exposed at 18% therefore has only $\log_2 (1 / 0.18) \approx 2.5$ stops above it. Picture Controls bend the top of that curve by amounts Nikon does not publish. I doubt they recover the further 3.5 stops the lawn's six from face to sky would need, but that is my inference. A sky stored as pure white stays white whatever is done to the file later.

03.SDR, HLG and N-Log

The Z6III has no separate menu item for the video tone mode. You choose it inside Video file type by highlighting a file type and pressing right on the multi selector (p. 199), and Video Formats and Codecs covers that item. A new camera records H.265 8-bit (MOV) (p. 578), which is SDR only (p. 199).

SDR

SDR's colour space is BT.709 (p. 199), and a colour space is the agreement that tells a screen which red, green and blue a set of numbers stands for. BT.709 is also called Rec. 709, for Recommendation 709. The look comes from Set Picture Control, where Flat leaves a little room to adjust contrast later without committing to log.

HLG

HLG, Hybrid Log Gamma, is a form of HDR, high dynamic range video, made for screens that show brighter highlights than an ordinary display, and Nikon names HDR broadcasting as one use (p. 215). The curve is defined in the ITU's HDR standard, BT.2100, not by Nikon, and its name describes it: the darker part is a square-root curve, a conventional gamma shape, and the brighter part a log curve, so highlights get extra room and the footage plays without a grade. The Z6III tags HLG footage with BT.2100 (p. 199).

Only H.265 10-bit (MOV) offers HLG (p. 199). An HLG icon appears in the shooting display, HLG quality replaces Set Picture Control, and video Active D-Lighting is fixed at Off (p. 215). Nikon warns of more noise than in SDR or N-Log, seen as scattered bright pixels, fog or lines, and says accurate colour needs HLG-compatible screens, computers and software (p. 215).

HLG Quality

HLG quality acts only when HLG is the tone mode (p. 592). It offers Quick sharp, Contrast, Saturation and Hue, and Quick sharp moves Sharpening, Mid-range sharpening and Clarity together, for fine, medium and broad detail, with each of the three also adjustable alone (p. 592). All four start at 0 (p. 578), and the item can sit in the video i menu (p. 745). I record SDR, so mine stay at 0, and a reader moving to HLG would do well to start there too, since the camera's screens may not show HLG highlights and saturated colours faithfully (p. 215).

N-Log

N-Log is Nikon's own log curve, offered by the four file types in Table 1, and an N-Log icon appears in the shooting display (p. 216). In H.265 10-bit (MOV) it carries BT.2020 (p. 199), the ITU's ultra-high-definition standard, which reaches more saturated colours than BT.709. N-Log is meant for colour grading, which needs compatible third-party software (p. 216). Five items that shape the finished look switch off: Set Picture Control and Active D-Lighting, noise reduction through High ISO NR, and the two portrait tools, Skin softening and Portrait impression balance (p. 216).

In both HLG and N-Log the monitor may flicker or look grainy and autofocus may struggle, which Nikon says is no fault (pp. 215-216). Subject detection can also weaken with HLG or N-Log in the 10-bit ProRes 422 HQ and H.265 types (p. 130), which Video Autofocus and Stabilisation takes up.

04.Grading and LUTs

Grading, the editing step defined above, does more than undo the flatness: Nikon notes that one recording can be graded in several ways (p. 216).

The usual starting point is a LUT, short for look-up table, and the name is literal. A one-dimensional LUT for a 10-bit channel is a list of 1,024 rows, each saying what one input value becomes. If row 300 holds 180 and row 700 holds 820, darks get darker and brights get brighter, which is how a table adds contrast. Those entries are illustrative. A colour LUT maps a red, green and blue triple to a new triple. Listing every 10-bit triple would take $1024^3$, about 1.07 billion rows, so it stores a coarser grid, often 33 points per side for $33^3 = 35{,}937$ entries, and the software interpolates between them.

Nikon's N-Log LUTs produce a picture for Rec. 709 screens, and the RG points to the Nikon Download Center for them (p. 216). Nikon mentions LUTs only for N-Log (pp. 199, 216). For HLG it asks instead for the HLG-capable screens and software described above (p. 215). HLG over HDMI and monitor-side LUTs belong to HDMI and External Monitors.

05.ISO Floors and Exposure Habits

ISO is the camera's sensitivity setting, and each doubling of the number is one stop. HLG and N-Log both raise the floor of Maximum sensitivity and change the lowest fixed ISO in mode M. In SDR, video ISO runs up to 51200 and beyond, to the equivalent of ISO 204800 (pp. 159, 1005).

Maximum sensitivity is the ceiling for auto ISO, and ISO sensitivity (mode M) is the fixed value in mode M with Auto ISO control (mode M) set to OFF (p. 588). With N-Log the mode M choices become Lo 0.3 to Lo 2.0 and ISO 800 to 51200 (pp. 588, 1005), and HLG's range is ISO 400 to 51200 (p. 1005), figures ISO and Auto ISO also gives.

The RG never says what the N-Log Lo steps equal or what they cost, and it ties the 13-stop figure to ISO 800 as the lowest setting possible (p. 216). I read ISO 800 as N-Log's working floor, so anyone trying Lo should first check what it does to the highlights. That reading is my inference, not Nikon's.

Going from ISO 100 to ISO 800 is $\log_2 (800 / 100) = \log_2 8 = 3$ stops, so at the same shutter speed and aperture a sunny N-Log scene needs three stops more neutral density, or ND, filtering than SDR at ISO 100. An ND filter is a grey filter that cuts light evenly across colours. Video from First Principles works out about 6 stops for midday sun at f/2.8, 1/60 s and ISO 100, so N-Log there needs about 9, the darkest end of my variable ND, where Long Exposure and Night explains the uneven darkening such filters can show. HLG's mode M floor of ISO 400 costs $\log_2 (400 / 100) = 2$ stops. The RG does not say what happens to a stored ISO below a new floor, so check the readout after each switch.

Exposing Log and HLG

Nikon prints no target brightness for skin or middle grey in either mode. It warns that the N-Log preview lacks contrast (p. 216) and that in HLG the camera's screens may misrender highlights and strong colours (p. 215), so the picture itself is a poor exposure guide. Expose by measurement instead: decide which highlights must survive, watch the zebras on them, and read the wave-form monitor for the rest. In N-Log, avoid dark faces too, because lifting one in the grade lifts its noise, which is general practice for log footage rather than Nikon's advice.

06.View Assist

Set to ON, g13 View assist raises the contrast of the N-Log preview at the cost of simpler colours, and an icon appears in the shooting display (pp. 216, 771). Recorded colours are untouched, and the boost also applies when N-Log footage plays back on the camera (p. 771). The default is OFF (p. 629), and the View assist role in g2 Custom controls toggles it from a button (p. 751). Nikon describes it only for N-Log, so for SDR and HLG I treat it as doing nothing. It aids framing, but judge exposure with the next two tools.

07.Zebra Stripes

Zebras are stripes the camera draws over every part of the live picture whose brightness falls in a chosen band. g14 Zebra pattern has five settings (p. 772). Pattern tone range chooses Highlights, Mid-tones or the default, Zebra pattern off. Pattern offers two stripe designs, Pattern 1 by default, and Zebra pattern color offers Black, the default, plus Gray, Red, Green and Blue (pp. 772, 629). The last two settings define the bands.

Highlight Threshold

Highlight threshold runs from 120 to 255, with 250 as the default (pp. 773, 629). A lower number marks more of the picture, and 255 marks only the areas at risk of overexposure (p. 773). A top value of 255 suggests an 8-bit scale from 0 to 255, though the RG states neither the bottom of the scale nor the signal it measures. On that reading 250 sits at $250 / 255 \approx 98\%$ of the scale's top and 120 at $120 / 255 \approx 47\%$. I keep 250, which warns slightly before the top and leaves time to stop down before a spotlit white costume loses its folds.

Mid-Tone Range

Mid-tone range marks a band set by a Value and a Range either side of it (p. 773). The default is 160 $\pm 10$ (p. 629), so the stripes cover 150 to 170, which is $150 / 255 \approx 59\%$ to $170 / 255 \approx 67\%$ on the same reading. Mid-tone zebras are for consistency: put a face at the same level every take and the clips will match. Nikon gives no value for skin, so choose one from your own footage.

When the Stripes Disappear

In manual focus with both switched on, focus peaking, the coloured outlines that mark sharp edges, takes over and the zebras vanish. Nikon's remedy is to set a12 Focus peaking > Focus peaking display to OFF (p. 773). My video i menu holds Zebra pattern in slot 9 and Focus peaking in slot 11, so the swap is quick. In N-Log the RG does not say whether zebras read the recorded log signal or the View assist picture, so check where they fall on your own clips before trusting a threshold.

Limiting the Tone Range on a Button

A button with the Pattern tone range role in g2 Custom controls steps through the zebra modes with each press (p. 754), and g15 sets which modes it visits (p. 774). Highlights alternates highlight stripes with off, Mid-tones does the same for mid-tone stripes, and the default, No restrictions, cycles through all three (pp. 774, 629). The RG describes g15 only for that role, and my zebras live in the i menu, so g15 stays at its default.

08.The Wave-Form Monitor

What a Wave-Form Plots

A histogram, the graph Metering uses as a final check, counts the pixels at each brightness but forgets where they are. A wave-form monitor keeps the position. Take the picture one column of pixels at a time, from left to right, and plot each pixel's brightness as a dot directly above that column's place on the horizontal axis, dark near the bottom and bright near the top. The columns together form a trace, denser wherever many pixels share a brightness.

In Figure 1, the window makes a band high on the left of the trace, the wall a band in the middle, and the curtain a band low on the right. The dancer adds a short run above the wall's band, only in the columns she fills, and the run moves with her. The RG does not describe the Z6III's scale or markings, so read it by position: a trace flat against the top is clipped, and one flat against the bottom has lost its shadows.

Setting It Up

g17 Brightness information display chooses Histogram, the default, or Wave-form monitor (pp. 776, 629). Highlighting either and pressing right sets its Size, Large or Standard, its Transparency in three steps, and its Position in any of the four corners (p. 777). Both types start at Standard, 1 (low transparency) and Bottom right (p. 629). The graph shows only where Brightness information is ticked in g19 Custom monitor shooting display, or in its viewfinder twin, g20 (pp. 776, 779). As Viewfinder and Monitor Displays describes, I tick it in one display on each screen.

I choose Wave-form monitor. On a dark stage a spotlit dancer is always bright, so the useful question is whether the brightest thing in the frame is the dancer or the lamp behind her, and of the two graphs, only the wave-form answers it. The defaults give a solid trace in a corner that usually covers stage floor.

09.Recommended Settings

For stage dance with the Tamron 70-180mm f/2.8 Di III VC VXD G2, the table is the whole story: SDR, Highlight-weighted metering as Metering explains, a fixed ISO in mode M, highlight zebras at 250 for white costumes, and the wave-form to tell a dancer from a lamp.

Outdoors on the gimbal the table still holds, with matrix metering and AE lock (hold) as Metering sets out, and the sky is the decision. When zebras cover the sky and the face is right, I give up the sky, because no SDR setting holds both on a lawn like the opening one. That scene is the case for N-Log.

For a reader who grades, the next step is N-Log in H.265 10-bit (MOV) at ISO 800, with g13 View assist ON, three stops more ND than SDR at ISO 100, zebras guarding the highlights and the wave-form placing the faces. For delivery to HDR screens, choose HLG and set the look with HLG quality.

10.Beginner Mistakes

Recording N-Log with no plan to grade. The footage stays grey. If it will be posted as shot, record SDR.

Judging N-Log or HLG exposure from the monitor picture. Neither preview shows the recording faithfully (pp. 215-216). Use zebras and the wave-form.

Looking for HLG under the wrong file type. Only H.265 10-bit (MOV) offers it (p. 199), and the 8-bit default is SDR only.

Losing the zebras in manual focus. Focus peaking takes priority (p. 773), so turn Focus peaking display off while you need the stripes.

11.Related Chapters

• Video Formats and Codecs: Video file type, where the tone mode is chosen, and bit depth.
• Video from First Principles: exposure in video and ND filter arithmetic.
• Long Exposure and Night: variable ND filters and their limits.
• ISO and Auto ISO: fixed ISO for video and the tone-mode floors.
• Picture Controls and Colour: the SDR look and the Flat Picture Control.
• Metering: stops, middle grey and the histogram.
• Viewfinder and Monitor Displays: g19 and g20, where brightness information is switched on.
• HDMI and External Monitors: HLG over HDMI and monitor-side LUTs.
• Video Autofocus and Stabilisation: subject detection with log and HLG.
• The i Menu: placing Zebra pattern in the video i menu.