ISO and Auto ISO

A dancer walks into a single blue spotlight. You are at the side of the stage with the Tamron 70-180mm at f/2.8 and 1/500 s, the camera settles on ISO 12800, and the frame that comes back has a fine, crawling grain across her face. The obvious suspect is the ISO number. It is the wrong suspect.

ISO 12800 is 128 times ISO 100, and since $128 = 2^7$, that is seven doublings, or seven stops. The number is reporting that the spotlight delivered about $\tfrac{1}{128}$ of the light a normal exposure at ISO 100 would need at that shutter speed and aperture. The grain is what so little light looks like. Seen that way, Auto ISO becomes the most useful exposure tool on the camera. Page numbers in parentheses point to Nikon's Z6III Reference Guide, edition 09, which matches firmware 2.00.

01.What ISO Does

Counting Light

A camera sensor is a grid of millions of tiny light-collecting sites called pixels. Picture each one as a bucket left out in the rain. While the shutter is open, light arrives as individual particles called photons, and a share of the photons landing in a pixel each free one electron, which the pixel stores. When the exposure ends, the camera measures the charge in every bucket and turns it into a number. For simplicity, the counts in this chapter are of photons actually captured.

Three things decide how much light each bucket catches: how bright the scene is, how long the shutter stays open, and how wide the lens aperture is. That trio belongs to Exposure Modes. Nothing about the ISO setting changes how many photons reach the sensor.

Turning Up the Gain

ISO decides what happens to the count afterwards. The camera amplifies each pixel's signal by a factor called the gain. Cameras can amplify in the analogue electronics, in arithmetic after the conversion to numbers, or both, and Nikon does not publish which the Z6III uses at each setting. Suppose a grey wall needs a reading equal to 10,000 captured photons to look mid-grey at ISO 100. At ISO 400 the gain is four times higher, so a pixel that caught only 2,500 photons gives the same reading. The sensor did not become more sensitive. It caught a quarter of the light, and the camera multiplied the result by four.

Nikon calls the setting the camera's sensitivity to light (p. 159), a name inherited from film. The Glossary wording is the accurate one for a digital camera: ISO is how strongly the camera amplifies the signal from the sensor.

02.Why Dark Pictures Are Noisy

Light Arrives in Grains

Put five identical buckets out in steady rain for a minute and count the drops. They will not match. If the average is 100 drops, a typical bucket lands about 10 above or below it, because drops fall at random. Photons do the same, so neighbouring pixels on an evenly lit grey wall catch different counts.

When events arrive independently and at random, the count in a fixed time follows Poisson statistics, and its spread is the square root of the average. Spread here means the standard deviation, the typical distance between one pixel's count and the average of them all. If a pixel expects $N$ photons, the spread is about $\sqrt{N}$. Raindrops averaging 100 give $\sqrt{100} = 10$.

Now the worked example. In good light each pixel expects 10,000 photons, so the spread is $\sqrt{10{,}000} = 100$. The useful measure of quality is the signal-to-noise ratio, or SNR, the signal divided by the noise: $10{,}000 / 100 = 100$. The noise is 1% of the signal and invisible. Cut the light to a quarter, two stops darker, and each pixel expects 2,500 photons, with a spread of $\sqrt{2{,}500} = 50$ and an SNR of 50. The noise is now 2%. At 100 photons the spread is 10 and the SNR is 10, so every pixel is off by a tenth of its value, and that is grain anyone can see.

The signal is $N$ and the noise is $\sqrt{N}$, so

$$\text{SNR} = \frac{N}{\sqrt{N}} = \sqrt{N}$$

A quarter of the light halves the SNR, because $\sqrt{1/4} = 1/2$, and each stop lost divides it by $\sqrt{2} \approx 1.41$. This is photon shot noise, and it belongs to light itself, not to any camera. The electronics add read-out noise of their own as they measure each pixel, mostly visible in deep shadows, but Nikon publishes no figure for it on the Z6III.

What Amplification Can and Cannot Fix

Go back to the dim wall at 2,500 photons and set ISO 400. Every reading is multiplied by four. The average becomes 10,000, as bright as the picture taken in good light, but the spread also grows fourfold, from 50 to 200. The SNR is $10{,}000 / 200 = 50$, exactly what it was before.

Amplification multiplies signal and noise by the same factor, so it restores brightness but never the ratio between them. In the opening scene, if a normal ISO 100 exposure would have given a mid-tone pixel 10,000 photons, the spotlight frame at ISO 12800 gave it about $10{,}000 / 128 \approx 78$. The spread is $\sqrt{78} \approx 8.8$, so each mid-tone pixel is off by roughly 11%. The ISO number only reported it.

What a Higher ISO Really Costs

If raising ISO adds no shot noise, what does it cost? Room at the top. The file has a largest value it can hold, and a pixel pushed past it clips to pure white. Suppose that ceiling equals 40,000 photons at ISO 100. At ISO 400, any pixel that caught more than 10,000 photons hits it. Each doubling of ISO costs one stop of room for highlights such as a white costume under a spotlight.

The working rule I hold to follows. Collect as much light as the picture allows, with the slowest shutter speed that still freezes the subject and the widest aperture that still gives the depth of field you need, then let ISO supply the missing brightness. Turning ISO down at the same shutter speed and aperture only gives a darker file with the same shot noise.

03.The Camera's ISO Range

For photographs the Z6III runs from ISO 100 to ISO 64000 (p. 159), a factor of 640, or $\log_2 640 \approx 9.3$ stops. Extended Lo and Hi settings sit below and above it (pp. 159-160).

The Lo settings run from Lo 0.3 to Lo 1.0, about 0.3, 0.7 and 1 stop below ISO 100, and Lo 1.0 is roughly ISO 50. They are for bright light, when you want a wider aperture or slower shutter speed than ISO 100 allows, but Nikon warns that highlights may be overexposed and advises ISO 100 or higher for most pictures (p. 160). Lo spends highlight room to buy a longer exposure. In my view it is a stand-in for a neutral density filter, the dark glass that lets a slow shutter speed blur moving water in daylight.

The Hi settings run from Hi 0.3 to Hi 1.7, about 0.3, 0.7, 1 and 1.7 stops above ISO 64000, with Hi 0.3 roughly ISO 80000 and Hi 1.7 roughly ISO 204800. Nikon flags them as especially prone to noise (p. 160), and the arithmetic says why: at Hi 1.7 a pixel gets about $\tfrac{1}{2048}$ of the light of an ISO 100 exposure, since $204{,}800 / 100 = 2^{11}$. Active D-Lighting, the setting that protects highlight and shadow detail in high-contrast scenes, has no effect at Hi settings, even when Auto ISO chose them (p. 474).

Setting ISO by Hand

Hold the ISO sensitivity button on the top plate and turn the main command dial (p. 159). The A Tour of the Body shows where both sit. The photo shooting menu offers the same value under ISO sensitivity settings > ISO sensitivity (p. 160), with an extra Auto choice only in the AUTO shooting mode (p. 467). The default is ISO 100 (p. 446).

Each click moves ISO by a third of a stop at default settings (p. 159). A third of a stop multiplies ISO by $2^{1/3} \approx 1.26$, so three clicks multiply it by $\left(2^{1/3}\right)^3 = 2$, taking ISO 100 to ISO 200. Custom Setting b1, ISO sensitivity step value, offers 1/3 step, the default, or 1 step (pp. 624, 652). Switch to whole stops while on a value the coarser scale lacks, and the camera moves to the nearest one it has (p. 652). I leave b1 at 1/3 step, because Auto ISO does most of the work and whole stops are coarse by hand.

04.Auto ISO Sensitivity Control

How the Camera Decides

Auto ISO hands the gain to the camera. You set a starting ISO, and whenever that value cannot give a correct exposure at the shutter speed and aperture in use, the camera changes it (p. 161). Correct means what the meter asks for, which Metering explains. In the photo shooting menu, ISO sensitivity settings holds Auto ISO sensitivity control, ON by default, plus Maximum sensitivity, Maximum sensitivity with flash and Minimum shutter speed, which are available only while it is on (pp. 446, 467).

A new Z6III already runs Auto ISO from ISO 100 with a ceiling of ISO 64000 (p. 446). The shooting display shows ISO AUTO and the control panel ISO-A, and when the camera moves away from your starting value it shows the number it chose (p. 161). In playback, a picture's ISO appears in red if Auto ISO was on (p. 225). To switch it, hold the ISO sensitivity button and turn the sub-command dial, which flips between ISO AUTO and plain ISO (p. 161).

Here is how the pieces combine in mode A, where you set the aperture and the camera sets the shutter speed. Say you are at f/8, starting at ISO 100, with a minimum shutter speed of 1/250 s and a maximum sensitivity of ISO 12800, and you walk from sunshine into a dim arcade. At first the camera holds ISO 100 and lengthens the exposure, through 1/1000 s and 1/500 s. At 1/250 s the shutter speed stops falling and the ISO climbs instead, one stop for each stop of lost light. Only once ISO 12800 is used up does the shutter speed drop below 1/250 s (p. 467).

Maximum Sensitivity

Maximum sensitivity is the ceiling, from ISO 200 to Hi 1.7, with a default of ISO 64000 (pp. 161, 446). If your starting ISO is already higher, the starting value becomes the ceiling (p. 162).

What the ceiling does depends on the mode. In P and A, reaching it makes the camera slow the shutter and let in more light, so a lower ceiling really does mean less noise, paid for with more risk of blur. In M you have fixed shutter speed and aperture, so the light is the same whatever the ceiling. Once Auto ISO hits it, the frame just comes out darker, with the same shot noise. In M the ceiling is a brightness limit, not a noise control. S behaves like M once the aperture runs out. The camera sets the aperture there, and once it is wide open and ISO reaches the ceiling, frames come out darker. That is my inference from how Nikon describes Auto ISO (p. 161).

From ISO 12800 to 64000 is a factor of 5, about $\log_2 5 \approx 2.3$ stops, so the SNR at 64000 is lower by $\sqrt{5} \approx 2.2$. In A, I would rather let the shutter slow than accept that, so my ceiling there is ISO 12800. In M, I raise it to 25600, since a darker file at a lower ISO is no cleaner, and keep it below 64000 so that dark frames warn me the light has outrun my shutter speed. Whether brightening it later beats the higher ISO depends on read-out noise, which Nikon does not publish.

Maximum sensitivity with flash is a separate ceiling for flash pictures, and its default, Same as without flash, reuses the main one (pp. 446, 467). With flash, a Minimum shutter speed outside the range allowed by Flash sync speed (e1) and Flash shutter speed (e2) gives way to the e2 value (p. 162).

Minimum Shutter Speed

Minimum shutter speed is where Auto ISO starts raising ISO instead of letting the shutter slow further. It works only in P and A, the modes in which the camera picks the shutter speed (p. 467). You can fix it from 1/8000 s to 30 s, or choose Auto, the default (pp. 446, 467).

Auto sets the minimum from the lens's focal length, choosing faster speeds for longer lenses, where a small tremor of the hands becomes a large movement in the frame (p. 467). That makes it the right choice for a zoom such as the 180-600. To fine-tune it, highlight Auto, press right on the multi selector, and move it towards faster or slower minimums (p. 467). Faster suits moving subjects, because a speed chosen for steady hands does nothing about a running dog.

If even the maximum sensitivity cannot give a correct exposure, the shutter speed drops below your minimum anyway (p. 467). In the C15, C30, C60 and C120 release modes of High-Speed Frame Capture +, which shoot bursts of JPEGs at 15 to 120 frames a second, the minimum is ignored while Auto ISO is on, and ISO stops at 64000 even if a Hi setting is chosen (p. 157).

Auto ISO in Mode M

Mode M with Auto ISO is the setup I recommend for anything that moves. You fix the shutter speed and aperture, which decide sharpness and depth of field, and the camera sets ISO for a correct exposure at those values (p. 145).

A heron flies past at 1/500 s and f/6.3, and Auto ISO settles on ISO 800. You change to 1/2000 s to freeze the wings, a quarter of the exposure time and two stops less light, so the ISO rises two stops to 3200 and the picture stays as bright. The SNR falls by $\sqrt{4} = 2$, the honest price of the faster shutter speed. Auto ISO only settled the brightness side of that price.

Exposure compensation tells the camera to make the picture brighter or darker than the meter suggests. Hold the exposure compensation button and turn a command dial, from $-5$ to $+5$ EV for photographs (p. 151), where EV, exposure value, is counted in stops. In plain mode M it only moves the exposure indicator (p. 152). With Auto ISO on, the camera shifts the ISO to follow it, so the picture really changes (p. 152). If the heron comes out dark against a bright sky, $+1$ EV raises the ISO from 3200 to 6400, with shutter speed and aperture untouched.

The trap is that compensation survives switching the camera off (p. 152). A $+1$ EV left over from the heron brightens tomorrow's sunrise by a stop.

Zooming the 180-600 from f/5.6 at 180 mm to f/6.3 at 600 mm loses about a third of a stop, since $(6.3 / 5.6)^2 \approx 2^{1/3}$, and Auto ISO absorbs it. With Auto ISO off in mode M, the b7 Custom Setting, Keep exp. when f/ changes, can absorb it, but only if you change it from its default, Exposure maintenance off, to Shutter speed or ISO sensitivity (pp. 624, 658).

Where Auto ISO Is Limited

• HLG tone mode, which records stills for high dynamic range (HDR) displays using the Hybrid Log-Gamma standard, makes ISO 400 the lowest value, keeps Maximum sensitivity at ISO 800 or above, removes the Hi settings and gives noisier pictures than SDR, the standard dynamic range of ordinary displays (p. 462).
• Bracketing in mode M varies ISO when Auto ISO is on (p. 504).

05.Auto ISO Setups for Real Shooting

These values are my judgement, not Nikon's, with the reasoning so you can adapt them.

Birds in Flight

Mode M, 1/2000 s or faster, f/6.3 on the 180-600 at the long end, Auto ISO on with a ceiling of ISO 25600, and $+0.7$ to $+1$ EV against a bright sky. Shutter speed matters most, because wingtips move far faster than the body. The aperture is already as wide as the lens allows at 600 mm, so ISO is the only thing that should move. Against dark trees, take the compensation back to zero or below. When frames start coming out dark, the ceiling is telling you to choose a slower speed.

Stage Dance

Mode M, f/2.8 on the Tamron, 1/500 s, Auto ISO on with a ceiling of ISO 25600, and about $-0.7$ EV under spotlights. f/2.8 is the lens's widest, and on a dark stage every photon counts. In my judgement 1/500 s freezes most dance short of a leap, and for leaps I would go to 1/1000 s and accept one more stop of ISO. Auto ISO follows the light from cue to cue. The negative compensation is there because the meter sees mostly black stage and lifts everything, so skin and white costumes clip. Metering covers modes that handle this more directly.

Street

Mode A at f/8, Auto ISO on with a ceiling of ISO 12800, and Minimum shutter speed fixed at 1/250 s. f/8 keeps a person a few steps away and the street behind both reasonably sharp. The fixed 1/250 s guards against walking subjects, whereas Auto, set by focal length to guard against shake, settles on a slower speed with a short lens. Here the ceiling is a real noise decision, because in A the camera slows the shutter once it is reached, and in a dim alley I would rather have 1/125 s and slightly soft walkers than a higher ISO's extra shot noise.

06.High ISO NR

High ISO NR, where NR stands for noise reduction, offers High, Normal, Low and Off, with Normal as the default (pp. 447, 476). The first three work at every ISO, more strongly as ISO rises. Off is not quite off: the camera still reduces noise where it judges it necessary, always less than Low would (p. 476).

The simplest noise reduction averages each pixel with its neighbours. Averaging four independent pixels, each with a typical error of 10, gives a result with a typical error of only $10 / \sqrt{4} = 5$, because the random errors partly cancel. The same averaging blends away real detail, such as the barbs of a feather, so stronger settings remove texture with the grain.

For raw shooters the choice stays open. The camera records the setting with each picture (p. 230), and in-camera raw conversion lets you choose High ISO NR again (pp. 265, 270). RAW, JPEG, HEIF and Tone Mode explains raw files. My choice is Normal in general and Low for birds and anything whose texture is the point. The video menu's own High ISO NR belongs to Picture Controls and Colour.

07.ISO for Video

Outside mode M, the camera always uses Auto ISO for video (pp. 162, 588). The video recording menu's ISO sensitivity settings item holds three settings (pp. 578, 588).

• Maximum sensitivity, the Auto ISO ceiling, from ISO 200 to Hi 2.0, default ISO 51200. It applies in P, S and A, and in M when Auto ISO is on there.
• Auto ISO control (mode M), ON by default. OFF uses the fixed value below.
• ISO sensitivity (mode M), from ISO 100 to Hi 2.0, default ISO 100. With the N-Log tone mode, Nikon's flat recording meant for colour grading afterwards, the choices become Lo 0.3 to Lo 2.0 and ISO 800 to 51200.

The standard video range is ISO 100 to 51200, with Hi steps up to the equivalent of ISO 204800 (p. 159). Electronic VR, the camera's digital stabilisation for video, caps video at ISO 51200 (p. 209). HLG keeps Maximum sensitivity at ISO 800 or above and the mode M value at 400 or above, without Hi (p. 215). N-Log keeps Maximum sensitivity at 1600 or above, drops Hi and removes the video High ISO NR (p. 216). Custom Setting g10 Fine ISO control (mode M) adds sixth-of-a-stop steps (p. 768). The video part of this guide goes further.

For gimbal dance video I recommend mode M with Auto ISO control (mode M) at OFF and a fixed ISO chosen for the venue. A performance is lit to have bright moments and dark ones, and Auto ISO would fight that, lifting a fade to black and pulling down the moment a white costume fills the frame.

08.Recommended Settings

Portraits with the Nikkor Z 85mm f/1.8 S suit mode A at a wide aperture with Auto nudged faster. For landscape on a tripod I would switch Auto ISO off at ISO 100, so a long exposure stays long.

09.Beginner Mistakes

• Turning ISO down to avoid noise. At the same shutter speed and aperture, a lower ISO gives a darker picture with the same shot noise. The fix is more light: a slower shutter speed or a wider aperture.
• Expecting Minimum shutter speed to work everywhere. It acts only in P and A, and gives way once Maximum sensitivity is reached. In S and M, the speed you set is the minimum.
• Treating the ceiling the same in every mode. In A a lower Maximum sensitivity trades noise for blur. In M, and in S once the aperture is wide open, it only makes frames darker.
• Leaving compensation behind. It survives switching off, and in M with Auto ISO it changes the picture. Check it before every outing.
• Not knowing Auto ISO is on. It is on from the factory. Look for ISO AUTO in the shooting display.
• Treating Lo as a cleaner setting. Lo 1.0 lets in a stop more light for the same brightness, so mid-tones gain a little, but highlights clip a stop sooner. Nikon recommends ISO 100 or above for most pictures (p. 160).

10.Related Chapters

• Exposure Modes: P, S, A and M, and how shutter speed and aperture set exposure.
• Metering: how the camera judges a correct exposure, the target Auto ISO chases.
• Picture Controls and Colour: Active D-Lighting and video noise reduction.
• RAW, JPEG, HEIF and Tone Mode: raw against JPEG and HEIF.
• A Tour of the Body: where the ISO sensitivity button and command dials sit.
• Glossary: ISO, stop, noise and the Lo and Hi settings.