Sensor Cleaning and Weather

The tide is out and the wind is blowing straight off the sand. For two hours the Nikkor Z 180-600mm f/5.6-6.3 VR has been following waders along the waterline, and by late afternoon there is a film of salt on its hood. You want a wider view before the light goes, so you take the long lens off in the car park, with the wind still gusting, and put on the Nikkor Z 24-70mm f/4 S. A few pictures of your companion at f/4, then a dozen frames of the sky over the estuary at f/16.

That evening the sky frames all carry the same soft grey blot, up and to the left of centre. The portraits taken ten minutes earlier with the same lens show nothing, and wiping the lens changes nothing.

The blot is a speck of dust that came in during the lens change and settled in front of the sensor. Why it hides at f/4 and shows at f/16 is a small piece of geometry, and the same geometry explains how to look for dust and when to stop cleaning. The second half of the chapter covers what Nikon actually says about rain, sand, cold, heat, condensation and storage.

Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.What a Speck of Dust Does to a Picture

The sensor is a grid of photosites, tiny light-measuring wells, one for each pixel. The Z6III's sensor measures 35.9 by 23.9 millimetres (p. 997) and records a full-size picture 6,048 pixels wide (p. 998). Dust does not land on the photosites themselves. It lands on the outer surface of the sensor, which Nikon calls its front (p. 832), a short distance in front of them. The lens focuses exactly on the photosites, so the speck is out of focus, and what reaches the picture is a soft shadow.

The size and darkness of that shadow depend on the shape of the light. Light reaches one point of the sensor from the whole opening of the lens, so it forms a cone that narrows to that point. The f-number $N$ is the focal length divided by the diameter of the opening, so a larger f-number means a smaller opening and a slimmer cone. For a distant subject the cone narrows from the width of the opening to nothing over about one focal length, so by similar triangles its width a distance $d$ in front of the photosites is $d/N$.

Here is the arithmetic for the scene above. Let $a$ be the speck's diameter, $d$ the distance from the surface it sits on to the photosites, and $N$ the f-number. Nikon does not publish $d$ for this sensor, so take $d = 1$ mm and $a = 20\ \mu\text{m}$ (twenty thousandths of a millimetre) as illustrative values. Assume the speck is round and opaque, and that light fills the cone evenly. Treat $d$ as if the gap were air. Any glass in that gap bends the light inward and narrows the cone by its refractive index (the factor by which light travels more slowly in glass than in air), which changes the sizes below but not the sixteen-fold ratio worked out after them. The cone's width where the speck sits is

$$c = \frac{d}{N}.$$

A point on the sensor is shaded whenever its cone touches the speck, so the shadow is a disc of width $w = c + a$. Near the middle of that disc the whole speck lies inside the cone, and the fraction of light it removes is the ratio of the two circles' areas:

$$\delta = \left(\frac{a}{c}\right)^2 = \left(\frac{aN}{d}\right)^2,$$

which holds while $a$ is smaller than $c$. Here $\delta$ is the darkening at the centre of the shadow, as a fraction of the light that would otherwise arrive. To turn widths into pixels, divide by the pixel pitch $p$, the distance from one photosite's centre to the next:

$$p = \frac{35.9\ \text{mm}}{6048} \approx 5.94\ \mu\text{m}.$$

At f/4 the cone is $c = 1000\ \mu\text{m}/4 = 250\ \mu\text{m}$ wide at the speck. The shadow is $w = 270\ \mu\text{m}$ across, about $270/5.94 \approx 45$ pixels, and its centre is darker by $\delta = (20/250)^2 = 0.0064$, which is 0.64 percent. At f/16 the cone is $c = 1000\ \mu\text{m}/16 = 62.5\ \mu\text{m}$ wide. The shadow shrinks to $w = 82.5\ \mu\text{m}$, about 14 pixels, and darkens by $\delta = (20/62.5)^2 = 0.1024$, about 10 percent. In photographic stops, where one stop is a halving of light, that is $\log_2\!\big(1/(1-0.1024)\big) \approx 0.16$ of a stop.

As long as the speck is narrower than the cone at f/16, the ratio between the two darkenings does not depend on the values assumed for $a$ and $d$. Divide one $\delta$ by the other and both cancel:

$$\frac{\delta_{16}}{\delta_{4}} = \left(\frac{16}{4}\right)^2 = 16.$$

Going from f/4 to f/16 makes the same speck sixteen times darker and packs it into a much smaller patch. Under one percent spread over forty-odd pixels disappears into the texture of a face. Ten percent in fourteen pixels shows in a plain sky, and more so once the contrast is raised in editing. Figure 1 draws both cases.

So look for dust at a small aperture, expect skies and landscapes at f/11 and beyond to show dust that wide-aperture portraits hide, and never take a clean frame at f/4 as proof of a clean sensor.

02.Is the Sensor Covered When the Camera Is Off?

The Reference Guide describes nothing that closes over the sensor at power off. The setup menu's sensor items are cleaning, a dust reference photo and pixel mapping (pp. 831-834), and although the camera has a mechanical focal-plane shutter (p. 1000), the guide never says that it closes at shutdown. The inspection procedure points the other way. It has you turn the camera off, remove the lens and let light fall on the sensor (p. 989), which only works if nothing covers it. The lens-mounting steps say the same thing, warning you off the sensor once the body cap comes off (p. 97). This chapter therefore assumes the sensor is open to the air whenever the mount is empty, with the camera on or off.

The cover is the BF-N1 body cap from the box (p. 35), which keeps dust out when no lens is fitted (p. 970). The VR lock mentioned in Nikon's launch announcement holds the sensor in place against unexpected shock, which is about movement, not dust.

03.The Camera's Own Cleaning

The Z6III cleans its sensor by vibrating it to shake dust loose (p. 831), either on demand or automatically at power off (p. 987).

Start cleans at once (p. 831). Hold the camera base down for the best effect, and leave the controls and the power alone until the setup menu returns (p. 987). Automatic cleaning offers Clean at shutdown, which cleans at every switch-off, and Cleaning off (p. 831). Clean at shutdown is the default (p. 806).

Two cautions apply. Touching the controls during a shutdown cleaning interrupts it, and several cleanings in quick succession can make the camera refuse for a short while to protect its circuitry (p. 988). If cleaning does not shift the dust, Nikon sends you to a Nikon-authorized service representative (p. 987).

Leave Clean at shutdown on, but notice its timing. It runs as the camera turns off, before the next lens change, so dust that arrives during the change stays until the next switch-off. After a lens change in a place like the estuary car park, run Start by hand before the sky pictures, not after.

04.Checking for Dust

A dust check is a deliberate picture of nothing. Nikon's specifications give f/16 as the smallest aperture of both the Nikkor Z 40mm f/2 and the Nikkor Z 85mm f/1.8 S, so either works. Set f/16 and photograph an evenly lit plain surface, blurred so that it shows no texture of its own. Nikon's arrangement for its dust reference photo does this well: the lens about ten centimetres from a white surface, with focus at infinity (p. 832). Add a stop or so of exposure compensation, because the meter tends to render white as grey.

Inspect the frame at full magnification, section by section, as Playback and Culling in the Camera describes, or on a computer with the contrast raised hard. Nikon's troubleshooting list also sends you to the front and rear (mount-side) lens elements before the sensor (p. 915). To tell them apart, change lenses and repeat. A mark that stays put is on the sensor. One that moves or vanishes went with the lens.

05.Image Dust Off and Pixel Mapping

Image Dust Off is a feature of NX Studio, Nikon's computer program, that reduces the effect of dust on NEF (RAW) pictures using a reference picture of the dust (p. 832). To take the reference, set the photo/video selector to photo, choose Start (or Clean sensor and then start, which cleans first), fill the display with a well-lit, featureless white object about ten centimetres away, and press the shutter-release button halfway and then fully. In autofocus the camera sets infinity itself (pp. 832-833). Save Clean sensor and then start for a reference that will serve only pictures not yet taken (p. 833). Nikon recommends an FX lens of 50 mm or longer, zoomed fully in, and a DX lens cannot be used. One reference serves other lenses and apertures (p. 833).

The catch is timing. A reference recorded before a sensor cleaning cannot be used with pictures taken after it (pp. 833, 919). Read literally, that includes each automatic cleaning at shutdown and each Start run by hand, so a reference belongs only with pictures taken between the same two cleanings. That consequence is my reading, not Nikon's words, so take the reference within the same stretch between cleanings as the pictures it will fix. The software side belongs to NX Studio and Tethered Shooting.

Pixel mapping is for unexpected bright spots, not dust (p. 834). The geometry tells them apart. A dust shadow is dark, soft, many pixels across and changes with the aperture, while a faulty pixel is a pin-sharp light dot that does not. Heat inside the camera can also produce scattered bright pixels or fog, and Nikon's remedies are switching off between uses and pixel mapping (p. 915). Pixel mapping needs a Z mount lens or an FTZ II or FTZ adapter fitted and a power source that will not fail, such as a fully charged battery. Do not operate the camera or cut the power while it runs, and it may be unavailable while the camera is hot inside (p. 834). Long Exposure and Night covers where bright pixels show up most.

06.When Vibration Is Not Enough

Nikon recommends that manual sensor cleaning be left to a Nikon-authorized service representative (p. 989). The owner's procedure it gives is short: camera off, lens removed, look at the sensor in good light, and clear any dust with a plain hand blower. A blower with a brush on its tip is ruled out, because the bristles can harm the sensor. What a blower will not move goes to service, and the sensor is never touched or wiped (p. 989). Nor should it be pressed, poked with tools or hit with strong air currents (pp. 51, 992).

That gives an order of escalation, from least risky to most:

• Vibration. Run Start two or three times, waiting if the camera pauses the command.
• A hand blower. A few gentle squeezes, never compressed gas or a brush. Holding the mount angled down lets loosened dust fall out, which is my advice rather than Nikon's.
• Software. Retouch the spot with an imaging application's clean-up tools, as Nikon suggests (p. 990), or use Image Dust Off on NEF files.
• Service. A Nikon-authorized service representative.

Wet swabs, sensor-width spatulas used with cleaning fluid, are not on that list, and they contradict Nikon's instruction never to touch or wipe the sensor. A swab kit is in my bag, and it sits outside the list: a deliberate departure from Nikon's instructions, used knowingly and at your own risk, for a day when service is out of reach. The guide describes no swab procedure, and this chapter does not supply one.

Prevention beats all of these. Most foreign matter arrives during lens and cap changes, though in rare cases lubricant or fine debris from inside the camera reaches the sensor too (p. 990). Nikon also recommends a paid inspection every one to two years and a service every three to five, more often for professional use, with the lenses brought along (p. 990).

07.Lenses, Mounts and Contacts

Nikon's order for lens glass is a blower first, then, for fingerprints, a small amount of lens cleaner on a soft cloth, wiped gently (pp. 986, 992). An aerosol blower must be held vertical so it does not spray liquid onto the glass, and alcohol, thinner and other volatile chemicals are never used (p. 986). The blower comes first because a cloth pulled over grit drags the grit across the coating.

The rear element counts as much as the front, since Nikon lists both as sources of smudges (p. 915). Keep the lens contacts clean and untouched (pp. 97, 993), and clean the camera mount, the lens mount and the body cap so that their dust is not carried in at the next change (p. 990).

A fluorine coating on the front element helps water, grease and dirt wipe off. Nikon states it for the Nikkor Z 24-70mm f/4 S and the Nikkor Z 180-600mm f/5.6-6.3 VR. Tamron lists a water- and oil-repellent fluorine coating on all three G2 zooms without naming the surface, and the 40mm and 85mm pages read for this chapter do not mention one. A coating eases cleaning but does not change the blower-first order.

08.The Monitor, the Control Panel and the Eye Sensor

For the rear monitor, blow off dust, then wipe smudges lightly with a soft cloth or chamois and never press, because pressure can damage it or make it malfunction (pp. 986, 993). The control panel on the top plate gets the same care. Static from the cloth can occasionally make a display light up or go dark, which Nikon says is not a fault and soon clears (p. 992). My monitor and control panel each carry a ULBTER glass protector, so the cloth meets the protector. Nikon says nothing about protectors, so the no-pressure rule still applies to the screen beneath.

The viewfinder takes the same blower-then-cloth treatment as a lens (p. 986). The eye sensor just above the eyepiece (p. 49) can stop working properly if dust or lint covers it, and Nikon's fix is a blower (p. 912). A Tour of the Body shows where it is.

09.What Nikon Says About Weather

Nikon's statements about the body are comparisons, not ratings. The US press release says the Z6III is "sealed against dust and moisture to the same level as the Z8", and both it and the global announcement give operation down to −10 °C (14 °F). Both add that the optional MB-N14 power battery pack has the same dust and drip resistance as the camera. No Nikon source read for this chapter, the Reference Guide and the product page included, gives a numerical water or dust rating.

The Reference Guide is blunter. Keep the camera dry and do not handle it with wet hands (p. 37), because water inside can rust the mechanism, sometimes beyond repair (p. 991). The operating environment is −10 to 40 °C at 85 percent humidity or less, without condensation (p. 1009). My judgement is to treat the sealing as protection against an accident, a shower or spray off a wave, not as permission to stand in the rain for an hour.

The lens makers word their claims with similar care, and Table 1 paraphrases each maker's own page.

No maker gives a number, and two Nikon pages say outright that resistance is not guaranteed in all situations or conditions. Each entry is a design intention, not a promise about a particular storm.

10.Rain, Spray, Sand and Salt

Rain and snow can also confuse subject detection in auto capture, and falling snow can trigger a capture (p. 577), as Auto Capture explains.

After the beach, Nikon's order is to blow off dust, take off sand or salt with a cloth barely moistened with water, and then dry the body completely (pp. 986, 992). One page says distilled water (p. 986) and the other fresh water (p. 992), and distilled satisfies both. Nikon also warns that grit or anything else that gets inside can make the camera malfunction, and that the warranty excludes such damage (p. 986), which is the best reason to clear sand before it works into the seams around the controls.

11.Cold, Heat and Condensation

The body's operating floor is −10 °C (p. 1009), the same as the EN-EL15c battery's in this camera (p. 1010). Battery capacity drops in the cold, and the battery-level display shows the drop (p. 995). Nikon's advice is to keep a charged spare warm and swap it in, and a cold battery that warms up may recover some charge (p. 996), which Batteries and USB Power takes further. Do not handle the camera with bare hands in extreme cold, which risks frostbite (p. 38), and expect the control panel's response time and brightness to vary with temperature (p. 912).

The ceiling is 40 °C (p. 1009). Keep the camera out of a closed car on a hot day and away from heaters (p. 993), and never leave the lens pointing at the sun. A lens concentrates sunlight like a magnifying glass, and Nikon warns that the focused light can start a fire or damage internal parts (p. 39), and that intense light can damage the image sensor (p. 991). Heat from long video recording belongs to Heat, Power and Recording Limits.

Condensation is the weather people make for themselves. Carry a camera from −5 °C outside into a 21 °C room, and the warm, moist air touching the cold metal and glass cools until it cannot hold all its water. The excess settles on the camera, outside and in (p. 991). Nikon's remedy is to seal the camera inside a bag or a plastic box before it crosses between cold and warm air, in either direction, and to leave it there until it has reached the new temperature (p. 991). The water then forms on the bag. Taking the card out first lets you start on the pictures while the camera warms, which is my suggestion, not Nikon's.

12.Storage

Nikon asks for a dry, well-ventilated place, to keep mould and mildew away, with no naphtha or camphor mothballs, no strong electromagnetic fields such as televisions and radios, and no heaters nearby (pp. 986, 993). For longer storage, confirm the camera is off and remove the battery (p. 986). Store the camera in a plastic bag with a desiccant, a moisture-absorbing sachet, and replace the desiccant at regular intervals. A leather camera case stays out of that bag, because sealing leather in plastic can make it deteriorate (p. 993). Avoid humidity above 60 percent and temperatures above 50 °C or below −20 °C (p. 986). Once a month, take the camera out, put the battery in and release the shutter a few times (p. 993). Batteries and USB Power covers storing the batteries themselves.

13.Recommended Settings

14.Beginner Mistakes

Checking for dust at a wide aperture. At f/4 the illustrative speck darkens its patch by less than one percent and vanishes. Check at f/16, against a plain blurred surface.

Reaching for a brush, a swab or a can of air first. Nikon allows a gentle blower and nothing else on the sensor, and warns against powerful air currents (pp. 989, 992). Run the vibration and the hand blower before anything stronger, and send the rest to service.

Changing lenses in the wind with the camera on. Turn the camera off (pp. 97, 98), get out of the dust if you can (p. 990), and have the next lens ready so the mount is open for seconds.

Wiping a lens before blowing it. The cloth drags grit across the coating. Blower first, every time (p. 986).

Pressing on the monitor while cleaning it. No pressure, a soft cloth, and no alarm if a display flickers from static (pp. 992, 993).

Carrying a cold camera straight into a warm room. Seal it in a bag first and let it warm up inside the bag (p. 991).

Putting the camera away for the season with the battery in. Battery out, camera in a plastic bag with fresh desiccant, and a monthly outing (pp. 986, 993).

15.Related Chapters

• A Tour of the Body: the lens mount, body cap, eye sensor and the warning not to touch the sensor.
• First-Day Setup: charging, cards and the settings to make on day one.
• Playback and Culling in the Camera: zooming in to inspect a dust check.
• Long Exposure and Night: bright pixels, pixel mapping and condensation on night shoots.
• Auto Capture: how rain and snow affect subject detection.
• Heat, Power and Recording Limits: heat during long video recordings.
• Batteries and USB Power: batteries in the cold and in storage.
• Cards and Getting Files Off the Camera: looking after the cards that hold the pictures.
• NX Studio and Tethered Shooting: applying Image Dust Off to NEF files.
• Troubleshooting: spots, smudges and other faults in pictures.