Beyond PASM:
Manual + auto ISO on Nikon Z8

This page discusses how and why to use Auto ISO on the Nikon Z8 camera. The generalities are also applicable to other camera models and brands, but their implementation details of Auto ISO and related settings may differ in significant ways.

History of automatic exposure

M

Before automatic exposure, cameras only operated in M (manual) exposure mode. Many cameras were already able to meter a correct exposure through‑the‑lens, and suggest it to the photographer. The photographer, however, needed to manually set on the camera the ISO required by the current film, except with some, mostly consumer‑level, cameras that could electrically read the film ISO value from a sort of bar‑code printed on the metal capsule enclosing 35 mm film.

Most cameras could also meter the exposure with the lens diaphragm fully open. This, in turn, required the camera to detect the aperture set on the lens aperture ring of the camera, via a mechanical (and later electric) coupling between lens and camera. Following the camera suggestions was as simple as manually changing exposure time and/or lens aperture, until two visual indicators in the viewfinder overlapped each other.

The photographer was free to ignore the exposure suggested by the camera, and to arbitrarily choose ISO, aperture and exposure time. The photographer was also free to make a mistake and to choose an obviously wrong combination of the three exposure factors.

A

The first cameras capable of automatic exposure had only one automatic exposure mode, i.e. A (aperture priority). In this mode, the photographer manually sets the lens aperture, and the camera adjusts the exposure time accordingly. The photographer has previously chosen also the ISO sensitivity and, if necessary, an exposure compensation factor. Historically, aperture priority was the earliest type of automatic exposure because it was entirely implemented in the camera body and did not require the camera to be able to change the lens aperture.

A mode takes advantage of lenses that allow the camera to meter with the lens aperture fully open, which in turn requires a way for the lens to transmit to the camera its aperture setting, and a way for the camera to stop down the lens aperture during the exposure. Both capabilities were already available in manual‑exposure film SLRs.

On modern SLRs and DSLRs, A mode typically performs both metering and AF with the lens aperture fully open, and stops it down only during the exposure. The Z8, instead, keeps at all times the aperture stopped down to the user‑selected aperture, or to f/5.6 (whichever f/number is lower). This behavior can be overridden with a menu setting that forces the Z8 to behave like a DSLR in this respect, but has the potential of lowering image quality, exposure consistency, and autofocus precision.

S

The second type of automatic exposure is S (shutter priority), i.e. the photographer chooses the exposure time, and the camera automatically adjusts the lens aperture. Also in this case, the photographer has previously chosen the ISO and exposure compensation. The camera must be capable of changing the lens aperture (not just of stopping down the aperture to the value set by the aperture ring), via a mechanical or electronic connection.

S mode required the successive development of two generations of lenses. The first added a variable mechanical aperture actuator in the lenses, operated by a motor in the body via a mechanical coupling in the lens mount, and the second replaced this mechanism with dedicated electrical contacts on the lens mount and an electromechanical actuator in the lens.

Multiple camera brands did away with their lenses' physical aperture rings, mainly as a way to reduce the manufacturing costs. Nikon did this in their AF‑S G series of lenses, but the aperture ring had already begun to become obsolete in the AF‑S lenses, in which the aperture ring must be locked in the fully stopped down position to allow automatic exposure in S mode.

P

The third type of automatic exposure is P (program). The camera chooses both lens aperture and exposure time, based on a "best choice" algorithm. It tries to avoid excessively long exposures that may result in camera shake or subject movement during the exposure, as well as excessively wide or narrow lens apertures. Usually, it also takes into account the focal length of the lens. The photographer still retains control over ISO and exposure compensation. This mode was a straightforward extension of the camera capabilities, and did not require changes in the lenses.

PASM

Many modern system cameras allow the photographer to choose among the four above modes by rotating a mode dial. Each mode is indicated by the initial letter of its name, and the mode dial positions typically read PASM.

The Z8 is among a minority of cameras that do not have a rotating mode dial. Instead, it has a MODE button, which when kept pressed allows the mode to be chosen with the main (i.e. rear) command dial. The actual sequence of modes in the Z8 is PSAM.

Auto ISO

In practice, a digital camera can control exposure by varying three parameters:

• lens aperture,
• exposure time, and
• ISO sensitivity.

In film photography, ISO was a property of the film and largely a fixed property of each film type, although photographers fortunate enough to have their own darkrooms could "push" the chemical development process to change (mostly to increase) the ISO stated by the film maker.

In a digital camera, ISO is largely an artificial concept. With the exception of the now ubiquitous dual‑ISO sensors, which switch between a "normal" and an "enhanced" mode (with a rather small ISO difference among the two), the sensor in reality has a fixed sensitivity, and the camera's ISO setting only affects the way the data read from the sensor is processed after the exposure.

It did not take a great stretch of imagination for camera manufacturers to see that ISO, like the other two parameters that directly affect exposure (aperture and shutter), could be put under the camera's control instead of being solely decided by the photographer. Consequently, most modern cameras are capable of automatically adjusting the ISO setting. This setting is called Auto ISO. It does not have a reserved place on the mode dial, because it can be used in combination with any of the PASM modes.

Many photographers regard ISO as something one sets in advance, depending on the general illumination level of a scene, then uses for multiple shots and changes only if either illumination or the type of scene changes, e.g. if the sun becomes hidden by clouds. It is relatively common for curious photographers to briefly experiment with Auto ISO, and quickly (and erroneously) reach the conclusion that this setting gives unpredictable results, sometimes resulting in noisy pictures, other times in unnecessarily very short exposure times, and should better be left unused because working at a fixed ISO manually chosen by the photographer is far more predictable.

Of the three exposure parameters listed above, aperture and exposure time are the two factors that largely control the appearance of a photographic image, by affecting depth of field, motion freezing, and exposure compensation. ISO does not directly and visibly change the appearance of images, except by adding generally undesirable noise with increasing ISO. Indirectly, however, ISO does affect the other two parameters.

Auto ISO + M

The real usefulness of Auto ISO is in combination with the M mode. It may sound strange that M needs to be used for achieving an automatic exposure, since the large majority of photographers have been trained to think of M as the opposite of automatic exposure. However, by combining Auto ISO with M the photographer gains the capability of manually choosing both aperture and exposure time, while one or both of these parameters remain under the control of the camera in the frequently used P, A and S automatic exposure modes.

Occasional photographers often use A and S as "set and forget" modes, and only change aperture or shutter settings when their current values make a correct exposure impossible. Well-trained professional photographers, on the other hand, always keep an eye on the aperture and shutter chosen by the camera, and adjust either, or both, whenever required to obtain the desired results. Getting into the habit of doing the same with aperture, shutter and ISO in Auto ISO + M is one of the main keys to success with this mode. It is not just a "good habit", but an absolute necessity.

Setting a maximum ISO

Turning on Auto ISO without setting a reasonable ceiling for the ISO chosen by the camera frequently causes disappointing results for photographers experimenting with Auto ISO.

To configure Auto ISO:

1. Go to Photo shooting menu (camera icon) > ISO sensitivity settings. The topmost setting in this menu is ISO sensitivity. On the Z8, this setting works in two ways:
• With Auto ISO set to OFF, this is the ISO used by the camera, e.g. in A, S and P mode.
• With Auto ISO set to ON, this setting is ignored.

I have seen statements saying that, with Auto ISO ON, this setting is the minimum, or floor value for Auto ISO. On my Z8, this is not true. Auto ISO will go down all the way to ISO 64, regardless of the value of this setting.

2. To turn on Auto ISO: Auto ISO sensitivity control > ON. Alternatively, in shooting mode, keep the ISO button pressed and rotate the secondary (front) command dial. This alternates between Auto ISO ON and Auto ISO OFF. The information display, shown by clicking the i button, can also be used to change ISO, switch Auto ISO on or off, and set Maximum sensitivity (see below). When Auto ISO is ON, an ISO Auto label blinks in front of the ISO value displayed on the LCD screen.
3. In the same menu, choose a ceiling value for ISO: Maximum sensitivity > your chosen value. Auto ISO will not exceed this ceiling value. I would suggest ISO 3,200 for open-air bird and wildlife photography in sunny or slightly overcast conditions, or ISO 6,400 under vegetation cover and/or poor light. You can choose a higher value in particularly poor illumination, but high-ISO noise will become more visible.
   • If you notice that, on the spur of the moment, you sometimes forget to check the actual ISO value in the viewfinder before shooting, you may wish to lower the ISO ceiling.
   • If you have a good discipline and always check your ISO, you may set the ceiling to 6,400 even in good light. This will avoid the need to dive into the menus if ambient light should temporarily reduce, requiring a higher ISO ceiling.
4. Optionally choose an ISO value for the Maximum sensitivity with flash setting. By default, it is set to Same as without flash, but in general I would chose a lower value, like 1600. If you are using flash, it makes sense to use a flash intensity sufficient to use a relatively low ISO.
5. Optionally set a value for the Minimum shutter speed setting. By default, it is set to AUTO, which means the camera will choose a shorter shutter time with lenses of high focal length. I leave this on AUTO, because it rarely causes problems. Alternatively, this setting allows the minimum exposure time to be set to a fixed value in the range between 30 s and 1/16,000 s.

I do not recommend to exceed ISO 6,400 in Auto ISO, unless really necessary. Above this ceiling, the Z8 produces an increasing amount of low-ISO noise. However, keep in mind that image stabilization does nothing to correct subject movement, so in really poor illumination a noisy picture is better than no picture at all, or a picture blurred by subject movement. Other photographers may disagree with my setting the ISO ceiling at 3,200 or 6,400, and I am aware that some photographers routinely use an ISO ceiling of 12,800 or thereabouts.

Auto ISO causing underexposure

Underexposure is a common failure experienced by photographers experimenting with Auto ISO.

Simple‑minded auto‑exposure modes like A and S try to achieve a perfect exposure, cost what it may. A (with no Auto ISO) will blithely choose very long exposure times when there is insufficient light, and produce images with subjects blurred by their own active movements. S (with no Auto ISO) will fully open the lens aperture in low light, and if this is not enough the camera will simply underexpose.

In poor light, M + AutoISO will increase the ISO up to the chosen ceiling, and then will underexpose. It will underexpose even if the lens aperture is not fully open and the shutter time is shorter than really necessary, because these settings are controlled by the photographer. The Z8 by default does decrease the brightness of the viewfinder image if the shot will be underexposed, but this may not always be obvious for the photographer, especially with an underexposure of only half a stop or one stop. You must learn to always check the viewfinder display for either of two warning signs:

• The exposure scale at the right edge of the viewfinder (which is displayed only in certain viewfinder modes) shows the amount of over- or underexposure, up to a limit of three stops in either direction. If this exceeds three stops, the exposure scale will also blink slowly.
• The exposure histogram (also displayed only in certain viewfinder modes) will be skewed to the left. The histogram is less precise than the exposure scale, but gives a better idea of the proportion of bright and dark areas in the image. If you know, for example, that the subject is a dark bird on a bright background, you also know that the histogram will have two broad peaks, and that the one you want to move toward the center of the histogram by opening the lens aperture (or lengthening the exposure) is the left one.

If you have the opposite problem and Auto ISO is causing overexposure, you are one of the lucky few photographers who can afford the luxury of shortening the exposure and/or stopping down the lens. You may need to do this even if the Z8 is apparently happy, but displays an unusually low ISO. You should probably not use an ISO below 200, because doing this reduces the dynamic range of the sensor. The Z8 has no floor setting for Auto ISO, only the ceiling discussed above. Likely, this is because only very unusual circumstances would make an ISO floor setting useful.

When to use Auto ISO + M

I use Auto ISO + M for birds and wildlife, and almost always with long telephoto lenses. I am aware that several professional photographers have comparable preferences. More in general, photographers discuss Auto ISO + M as a suitable mode for subjects that move quickly, or for shooting in rapidly changing conditions of illumination, in particular:

• birds and wildlife
• sports
• children
• concerts and poorly lit activities

Auto ISO + other modes

Although Auto ISO can be combined also with other modes than M, I believe that results with these other modes are less predictable, and overall less useful. The combination with M appears to be the most practical way for the photographer to retain control of both shutter and aperture without giving up automatic exposure. In practice, one can regard Auto ISO + M as a third automatic exposure mode, besides A and S, that leaves the photographer in sufficient command of the parameters that control exposure and its visible results. I am excluding P from these modes because, essentially, it gives the camera exclusive control over all exposure parameters.

Conclusions

On the Nikon Z8, Auto ISO can be combined with any of the PASM modes, but only Auto ISO + M is predictable and flexible enough to deserve being regarded as a third type of automatic exposure, in addition to the well known A and S modes. These three modes help the photographer to work creatively without giving the camera full control of all exposure parameters (as the P mode does).

Auto ISO + M is especially suitable to bird and wildlife photography, and in general to photography with long telephoto lenses. It is also suitable to other types of photography of rapidly and unpredictably moving subjects, as well as poorly lit activities where it is important for the photographer to retain control of both aperture and shutter.