Metering

A little egret stands at the edge of a dark pond, and you have it at 600 mm on the Nikkor Z 180-600mm f/5.6-6.3 VR. You choose spot metering, put the focus point on its chest and press the shutter. The bird on the monitor is the colour of wet cement. That evening a dancer in black velvet steps into a pool of stage light, you meter her costume the same way, and the velvet comes out a sooty charcoal. Nothing broke. The meter did its job, which was to turn both into the same middle grey.

Both pictures can be rescued before they are taken, with a number you can work out in your head. A stop is a doubling or halving of light, and the egret needs about two and a third stops more light for the egret, two stops less for the velvet. It starts from one idea about what a meter cannot know. Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.What a Meter Measures

Light Arriving, Not Tone

Start with two objects in one picture. A grey rock sits in full sun. A white egret stands in shade, where the light is one fifth as strong. Suppose the rock reflects 18% of the light that falls on it and the feathers reflect 90%. The rock sends back $0.18 \times 1 = 0.18$ units of light and the egret sends back $0.90 \times 0.2 = 0.18$ units. To anything that only counts the light reaching the camera, they are the same object.

The light leaving a surface toward the lens is its luminance, and it depends on two separate things: the illumination, meaning how much light falls on the surface, and the reflectance, meaning the fraction of that light the surface sends back. Reflectance is what we mean by a thing's tone. With $L$ for luminance, $E$ for illumination and $R$ for reflectance (a fraction between 0 and 1):

$$L \propto E \times R$$

The symbol $\propto$ reads "is proportional to": double either factor and $L$ doubles. A meter receives $L$ and nothing else, so it cannot tell a pale subject in dim light from a dark subject in bright light.

The Z6III meters through the lens with the image sensor itself (p. 1001). Through the lens, shortened to TTL, means the meter sees the same light that forms the picture, after the glass and any filter on the front. The camera turns that reading into a shutter speed, aperture and ISO according to the exposure mode, which Exposure Modes covers.

Why the Meter Aims at Middle Grey

A meter that cannot know tone has to assume one. The simplest reflected-light meter assumes that the area it reads, taken as a whole, should come out at a middle tone, roughly halfway between black and white as the eye judges brightness. That tone is middle grey, and the grey cards sold to stand in for it reflect 18% of the light reaching them. Nikon does not publish the exact target its meter is calibrated to, so the 18% below is a reference for arithmetic, not a specification of this camera.

For ordinary scenes the assumption is a good bet. Most frames mix light and dark things, and their average lands near the middle. When one tone dominates the area being read, the bet fails in a direction you can predict. Pointed at something white, the meter makes it grey by giving it too little light. Pointed at something black, it makes it grey by giving it too much.

The Egret and the Costume, Worked Out

A stop is a doubling or halving of light, and one EV, short for exposure value, is the same step on the camera's scale. Exposure compensation tells the camera to make the picture brighter or darker than the meter's verdict, set in EV, and belongs to Exposure Modes.

Take the egret at 90% against a middle grey of 18%. The ratio is $90/18 = 5$, so a meter reading only the bird gives it one fifth of the light it needs. How many stops is a factor of five? Count doublings: 18 to 36 is one stop, 36 to 72 is two, and 72 to 90 is about a third of a stop more. The exact answer is the power of two that equals five, which the base-2 logarithm gives. With $R_s$ for the subject's reflectance and $R_g$ for middle grey's, the number of stops $n$ is

$$n = \log_2\!\left(\frac{R_s}{R_g}\right)$$

For the egret, $n = \log_2 5 \approx 2.32$. A positive answer means the subject is brighter than middle grey, so the meter underexposes it by that many stops and the cure is that much positive compensation. Compensation moves in 1/3 EV steps unless you change b2 (pp. 624, 653), so the nearest setting is $+2\tfrac{1}{3}$ EV. For a white bird I would stop at $+2$. The brightest feathers sit near clipping, the point where pixels fill up and record flat white with no detail, and a third of a stop under is easier to recover than feathers gone blank.

The costume runs the other way. Suppose the velvet reflects 4.5%. Then $n = \log_2(4.5/18) = \log_2(1/4) = -2$. The subject is darker than middle grey, the meter overexposes it by two stops, and the cure is $-2$ EV.

These reflectances are illustrative, so the histogram stays the final judge. The arithmetic still gives the direction and rough size of the correction, and condenses to a rule: meter something white and add about two stops, meter something black and take about two away, or meter a mid-tone in the same light and leave compensation alone.

The Range the Meter Can Read

The meter works from $-4$ to $+17$ EV, figures Nikon gives for ISO 100 and an f/2.0 lens at 20 °C (p. 1001). Here EV measures scene brightness: zero is the light that exposes correctly at f/1.0 and one second at ISO 100, and each step up doubles it. Those 21 steps span a factor of $2^{21}$, about 2.1 million. The dark limit belongs to an f/2.0 lens, so by my arithmetic the 180-600 at f/6.3, which passes $(6.3/2)^2 \approx 9.9$ times less light, about 3.3 stops, reaches only to about $-0.7$ EV. The exposure indicators show how far the settings sit from the meter's verdict (p. 145). When the camera cannot reach a correct exposure with the settings available, the indicators and the shutter speed or aperture display flash, and Nikon's remedies are a change of ISO, shutter speed or aperture, or an ND filter (pp. 145, 924).

02.The Four Metering Modes

No mode can measure reflectance. Spot and centre-weighted act as averaging meters over the area they read, aiming it at middle grey, and matrix and highlight-weighted follow rules Nikon does not publish. What changes from mode to mode is which part of the frame is read and how heavily each part counts. The Z6III offers four modes for stills (p. 487), drawn in the figure over one stage scene.

Matrix Metering

Matrix reads a wide area and does more than average it. Nikon says it weighs the distribution of tones, colour, composition and subject distance, aiming for a result close to what the eye sees (p. 487). What it does with those inputs is not published.

Matrix is the default for stills and video (pp. 447, 579), Active D-Lighting works best with it (p. 473), which Picture Controls and Colour covers, and Nikon recommends it for HDR overlay (p. 519). It is the right mode for most pictures. Its weak spot is every meter's: it cannot measure reflectance, so a frame of snow or black stage can still pull it toward grey. How far it corrects for such scenes on its own is undocumented, which is a reason to check the histogram, not to avoid the mode.

Nikon itself steers you away in one case. With a filter whose filter factor is above $1\times$, matrix may disappoint, and Nikon suggests centre-weighted instead (pp. 487, 970). The filter factor is the number a filter divides the light by: 2 costs one stop, 8 costs three. Nikon's list includes circular polarisers and neutral density filters.

Centre-Weighted Metering

Centre-weighted reads the whole frame but gives 75% of the weight to a central area the size of a circle 12 mm or 8 mm across (pp. 656, 1001), leaving 25% for the rest, and b5 sets the size. Nikon suggests it for subjects that dominate the composition (p. 487).

How big is that circle? The sensor is 35.9 by 23.9 mm (p. 997), an area of $35.9 \times 23.9 \approx 858$ square millimetres. A circle of diameter $d$ has area $\pi d^2/4$, so the 12 mm circle covers $\pi \times 36 \approx 113$ square millimetres, 13.2% of the frame. These shares assume the full FX image area, and a cropped image area makes the circle a larger share of the picture. Three quarters of the decision rides on about an eighth of the picture.

Suppose a subject fills that circle with luminance $L$ and the background is three stops darker, at $L/8$. If the camera combines the zones in Nikon's 75 to 25 proportion, averaging the amounts of light themselves rather than their stops, the simplest reading of the specification though not one Nikon spells out, the meter reads

$$0.75L + 0.25 \times \frac{L}{8} \approx 0.78L$$

The meter treats $0.78L$ as middle grey, so the subject comes out $\log_2(1/0.78) \approx 0.36$ stop brighter than middle grey. A plain average would count the subject only by its 13.2% share, reading $0.132L + 0.868 \times L/8 \approx 0.24L$, and the subject would come out $\log_2(1/0.24) \approx 2.06$ stops brighter. For a mid-toned subject that is the whole case for the mode: about a third of a stop of error against two.

Spot Metering

Spot reads a circle 4 mm across, about 1.5% of the frame, and ignores the rest (p. 487). The figure checks out: $\pi \times 2^2 \approx 12.6$ square millimetres is 1.46% of the sensor, so Nikon's millimetres are measured on the sensor. The circle is centred on the current focus point and moves with it. Under Auto-area AF, the AF-area mode in which the camera chooses where to focus, there is no point of yours to follow, so spot reads the centre focus point instead (p. 487).

Spot is the most precise mode and the least forgiving, because it renders whatever sits under the circle as middle grey, the egret problem exactly. It wins when you know the tone under the circle: a mid-toned face in a spotlight, a brown bird against bright sky, or a white subject once you add the compensation you worked out. It is not offered for video (pp. 212, 603). With a flash on the shoe it switches the flash to standard i-TTL fill-flash (p. 418), which the flash part of this guide covers.

Highlight-Weighted Metering

Highlight-weighted gives the most weight to the brightest parts of the frame, to keep detail there (p. 487). Nikon's example is spotlit performers on a stage, the scene in the figure. A plain average sees mostly dark stage and raises the exposure to pull it toward grey, which pushes the lit dancer toward white. Highlight-weighted refuses that trade: it protects the bright areas and lets the shadows fall dark, which is what a stage looks like anyway.

The same logic suits a white egret on dark water. It fails where the brightest thing is not the subject, so a face against bright sky comes out dark while the sky keeps its detail. Nikon does not say how close to clipping it places the highlights, so treat the first frame as a starting point.

03.The Metering Menu Items

Metering for Stills and for Video

The stills item offers all four modes and starts at Matrix metering (p. 447). Metering is also a default entry in the i menu, the quick-settings screen the i button opens (p. 625), and a button can take a Metering role, held while you turn a command dial (p. 712). Both belong to later parts of the guide.

Video has its own item, also starting at Matrix metering (p. 579), with three choices because spot is not available while filming (pp. 603, 1004). The two items are separate, so spot chosen for stills does not carry into video. Zebra stripes and the other filming exposure aids belong to the video part of this guide.

Matrix Metering Face Detection

With Matrix metering selected and b4 at ON, the default (p. 624), the camera adjusts exposure for the faces of people it detects (p. 655). At OFF it ignores them. A face is the tone viewers judge most harshly, so letting it steer makes sense. It works only in matrix, and Nikon describes it only for human faces, not animals. I leave it ON. The only case for OFF is a frame where faces are present but are not what you are exposing for.

Center-Weighted Area

b5 sets the circle that centre-weighted favours: Small is 8 mm, Standard is 12 mm and the default, and Average weights the whole frame evenly (pp. 624, 656). The 8 mm circle covers $\pi \times 16 \approx 50.3$ square millimetres, about 5.9% of the frame. Average turns the mode into the plain averaging meter that missed by two stops in the worked example.

I keep Standard, because a subject small enough to need the 8 mm circle is better served by spot. The shooting display can draw the circle through a Center-weighted area indicator in d20 and d21, and shows none when b5 is Average (pp. 687-688).

Fine-Tune Optimal Exposure

b6 shifts the meter's idea of correct, separately for each of the four modes, from $-1$ to $+1$ EV in steps of one sixth of a stop, all starting at zero (pp. 624, 657). Set matrix to $+\tfrac{1}{3}$ and every matrix exposure from then on is a third of a stop brighter than the meter would choose.

Leave it at zero. Unlike exposure compensation, b6 shows no icon, and the only way to see the amount is to open b6 (p. 657). The one other trace is an Optimal exposure tuning line in the playback shooting data, shown only when b6 is not zero (p. 228), and the shooting data pages appear only once Shooting data is ticked under Playback display options (pp. 223, 783). A value set months ago silently shifts every frame, and you will blame the meter. Being per mode, it also changes your bias when you switch modes. Everything b6 does, exposure compensation does in the open. Nikon does not say whether b6 applies to video.

04.Locking the Exposure

Back to the egret. Suppose a stand of reeds beside it is a middle tone in the same shade. Meter the reeds, freeze that reading, swing back to the bird and shoot. The reeds come out mid-grey and the egret, in the same light, comes out white with no compensation. Freezing a reading so it survives a change of framing is autoexposure lock, or AE lock.

The Sub-Selector Centre

Many older Nikon cameras had a button marked AE-L/AF-L for this. The Z6III has none, and the Reference Guide never uses the label. The job belongs to the centre of the sub-selector, the small joystick on the back (p. 50), whose default role is AE/AF lock (p. 626). Vertical multi selector center and Lens Fn button share that default (p. 626), for an optional battery pack or a lens with that button.

The sequence is short (p. 149). If autofocus is on, half-press the shutter-release button to focus. Press and hold the sub-selector centre, and an AE-L icon appears: focus and exposure are frozen. Keep holding, recompose and shoot. The lock lasts only while you hold (p. 706). With spot, the frozen reading comes from the circle on the current focus point, and with centre-weighted it leans toward the centre circle (p. 150). The camera stores the measured brightness (p. 1001), so while holding you can still change shutter speed in S, aperture in A or the program shift in P (p. 150). The metering mode cannot change during a lock (p. 916).

Other Ways to Hold a Reading

Other controls can take other lock roles (p. 706). AE lock only freezes exposure while held and leaves focus free. AE lock (hold) locks with one press and releases with a second or when the standby timer, the idle period after which the meter and displays switch off, ends. AE lock (Reset on release) also locks with one press and lets go when you take the picture, press again or the standby timer ends (p. 706). A button can also switch metering mode while held, for instance to Spot metering, with the previous mode back on release (p. 707). Not every control accepts every role (p. 704). For video, AE lock (hold) keeps its lock when recording starts (p. 751).

If you focus with the AF-ON button, the back-button focus of the autofocus part, I would give the sub-selector centre AE lock only, so freezing the exposure does not also freeze focus on a moving subject.

Shutter-Release Button AE-L

c1 decides whether the shutter-release button itself locks exposure. At Off, the default, it never does (pp. 624, 659). With On (half press), the half-press that focuses also freezes exposure while held (p. 150), so in AF-S, where the half-press also holds focus (p. 135), focus-and-recompose with spot needs one finger. The cost is that every half-press is a lock, and if the light changes while you wait, the exposure is stale.

On (burst mode) locks exposure only while the button is fully down (p. 659). As I read Nikon's description, the meter follows the scene during the half-press and then holds one exposure across a burst. That suits a bird taking off past water, reeds and sky, whose changing tones would otherwise swing the meter between frames. It works against you if the bird flies from shade into sun mid-burst. I leave c1 at Off and lock deliberately.

05.Checking the Meter Against the Histogram

Every method here is a prediction, and the histogram checks it. A histogram is a bar chart of the picture's pixels sorted by brightness: dark on the left, bright on the right, with the height showing how many pixels have each brightness (p. 227). For the egret on dark water, a good exposure shows the water as a hump near the left and the bird as a hump close to the right edge without touching it. The spot-metered grey egret puts the bird's hump in the middle. A hump crammed against the right wall means clipped highlights. Positive compensation slides the chart right and negative slides it left (p. 227), so it tells you which way to turn the dial and roughly how far.

Add the Histogram indicator to one of Displays 1 to 4 in d20, or to the viewfinder through d21, for a live version (pp. 686-688). Starlight view hides it (p. 675). For playback, tick Highlights and RGB histogram under Playback display options, since both start unticked (pp. 783, 787). Highlights blink over areas that may be overexposed (p. 225), and zooming in limits the histogram to the visible part (p. 227), which lets you check the egret alone. Nikon calls the camera's histogram a guide that can differ from editing software (p. 227), so use it to catch a stop, not to argue over a sixth.

06.Recommended Settings

For stage dance with the Tamron 70-180mm f/2.8 Di III VC VXD G2, I use Highlight-weighted metering, the mode Nikon built for spotlit performers, in the video menu too when filming a stage. The shadows go dark, and on a stage they should.

For wildlife and birds with the 180-600mm, matrix handles most scenes. For a white bird on dark water I switch to highlight-weighted, or spot on the bird with about $+2$ EV of compensation.

For portraits with the Nikkor Z 85mm f/1.8 S, matrix with b4 ON lets the face lead. For landscape, matrix until a polariser or ND filter goes on, then Center-weighted metering as Nikon advises (p. 970). For street, matrix, because there is rarely time for anything else. For gimbal dance video outdoors, matrix with AE lock (hold) on a spare control, so a reading frozen before you press record carries into the take.

07.Beginner Mistakes

Trusting the meter on a white or black subject. Snow, white birds, black costumes and dark stages all come out grey. Work out the correction, about two stops either way, and confirm it on the histogram.

Forgetting that spot follows the focus point. Move the point and you move the meter. Under Auto-area AF spot reads the centre point instead (p. 487).

Setting b6 and forgetting it. It shifts every frame with no icon. Leave it at zero, and if pictures seem consistently off, look for Optimal exposure tuning in the playback shooting data, with Shooting data ticked under Playback display options (pp. 228, 783).

Letting go of the lock too early. With AE/AF lock the lock ends when you release the sub-selector (p. 706). Hold until the shutter fires, or use AE lock (hold).

Using matrix through a polariser or ND filter. Nikon suggests Center-weighted metering with filters that cut light (p. 970).

Looking for spot metering in video. It is not there (p. 603). Highlight-weighted is the nearest tool for a bright subject on a dark ground.

08.Related Chapters

• Exposure Modes: P, S, A and M, and exposure compensation, the correction this chapter keeps calling for.
• ISO and Auto ISO: the third setting the meter's verdict is spent on.
• Picture Controls and Colour: Active D-Lighting, which works best with matrix metering.
• Menus and Controls: reaching the custom settings menu and the sub-selector.
• Settings Finder: every metering item with its path and default.
• Glossary: EV, stop, metering and AE lock in one line each.