The Auto ISO setting on the Fuji X100 had me confused the first time I used it. Initially my reaction was, this can't be right.
Since, I have read many forum posts asking for a better explanation on how it works and why it does what it does, but I failed to locate a clear explanation. After much thought, I finally figured out the logic behind Auto ISO used by the X100. I assume the X100S works similarly.
The Auto ISO setting can be found in a submenu of ISO setting in the Shooting Menu. It is called ISO Auto Control. ISO Auto Control submenu allows you to set the maximum film sensitivity and the minimum shutter speed. Lets have a look at this more closely.
The maximum and minimum ISO values are orders and the camera will never go beyond set limits for these. The minimum shutter speed is just a request.
The Maximum Sensitivity is the highest ISO value you are willing to accept. Remember that higher sensitivity means higher noise (or grain) in the output. The X100 handles noise pretty well, up to ISO 1600, but different photographers have different tolerance for how much noise appears in their picture. So, set this value according to your noise tolerance.
The Minimum Shutter Speed is the slowest you wish to shoot. If you shoot handheld, for example, you may decide to set this to at least 1/30th of a second. On a tripod, you may be willing to shoot slower. This also depends on whether your subject is moving or stationary. The faster the shutter speed, the less blur in your shot, and a faster shutter will also freeze your subjects motion, while a slower shutter will capture any movement.
Finally, there is another hidden setting, the minimum ISO you want to use. This setting is not on the ISO Auto Control menu. Rather, the minimum ISO used by the camera is the fixed ISO value you choose in the ISO Setting menu. Usually this is set to ISO 200.
Now, once you turn ISO Auto Control ON, you are instructing the camera to work out the best ISO sensitivity, automatically, in order to attempt and achieve your chosen exposure, above your requested minimum shutter speed, while at all times staying within the minimum and maximum ISO limits. Phew!
Fully automatic operation (P)
To see it in action, Turn off the flash, then move the shutter speed and aperture control dials to Auto (A). Then use the exposure compensation knob alone to set your desired exposure and shoot. The camera tries to calculate the aperture and shutter speed for achieving your desired exposure at the minimum ISO value first. If the calculation results in a shutter speed slower than you requested, the camera increases the ISO sensitivity up one notch and tries to work out the aperture and shutter speed again for your desired exposure. It does this repeatedly until either the shutter speed meets or exceeds what you've requested under Auto ISO, or it hits the maximum ISO limit you've selected under ISO Auto Control.
What does this mean?
Under fully automatic operation, if your desired exposure can be achieved meeting or exceeding the minimum shutter speed set under Auto ISO, the ISO will be set to the best value, between the minimum ISO value and the maximum ISO value (inclusive), so as to achieve a shutter speed faster or equal to your set minimum. If the minimum shutter speed cannot be achieved even after hitting the maximum ISO value, the camera will give up and use the calculated shutter speed at the maximum ISO value so your desired exposure is still obtained. The camera will never exceed the maximum ISO value you've set. The camera will never lower the ISO below the minimum ISO value you've set. The camera will always try and achieve your desired exposure. The camera may offer a slower shutter speed than the minimum you requested.
So far so good. Things become more interesting however when you move out of fully automatic operation.
Aperture priority mode (A)
Here you set the F-stop of the lens manually. The camera behaviour is similar to what is described above, only that the camera will not attempt to change your set Aperture during exposure calculation. It will only have shutter speed and ISO sensitivity to play with. Again, It will start at the lowest ISO and see if it can achieve your desired exposure at the fixed aperture and a shutter speed faster than the requested minimum. If not, it will up the ISO one notch and try again. The camera will never exceed the maximum ISO value you've set. The camera will never lower the ISO below the minimum ISO value you've set. The camera will always try and achieve your desired exposure. The camera may offer a slower shutter speed than the minimum you requested.
Shutter priority mode (S)
Shutter priority mode is the most interesting. The camera no longer has any control over shutter speed and it will use the shutter value you've set on the dial. However, it will still attempt to work out the best ISO for your requested exposure. The camera will first attempt to adjust the aperture and then use the ISO value to achieve your requested exposure. If the image is underexposed, the aperture and ISO will be increased. If the image is overexposed the aperture and ISO will be reduced.
What does this mean?
This means the minimum shutter speed you requested under ISO Auto Control is ignored. If you set the shutter speed knob to fast, it is likely the image will be underexposed and so the camera will try and raise the ISO to compensate. As a result you will get higher ISOs with faster shutter speed. Conversely, if you set the shutter speed knob to low and the image is likely to be overexposed, the camera will lower ISO to compensate. So you will get lower ISOs with slower shutter speeds. This can be confusing at first, but understand that setting the shutter speed knob manually, does not turn off the Auto ISO calculation routine. With the shutter speed unavailable to change, the camera still uses the aperture and ISO value to get your desired exposure. The camera will never exceed the maximum ISO value you've set. The camera will never lower the ISO below the minimum ISO value you've set. The camera will use the shutter speed you've set. The camera will always try and achieve your desired exposure.
Fully manual operation (M)
In fully manual operation the camera behaviour is similar to Shutter priority mode. The minimum shutter speed you requested under ISO Auto Control is ignored. However, the Auto ISO calculation routine is still running and the camera will continue to fudge the ISO value to achieve a balanced exposure. The exposure compensation knob has no effect in this mode, but the camera attempts to achieve balanced exposure nevertheless. If the metering reports a bright object in front of the lens the ISO value is lowered, otherwise the ISO is raised. Again, the camera will never exceed the maximum ISO value you've set. The camera will never lower the ISO below the minimum ISO value you've set. The camera will use the shutter speed and aperture you've set. The camera will always try and achieve balanced exposure.
What does this mean?
In my opinion Auto ISO does not make much sense in fully manual operation. If you are trying to achieve a certain exposed look with exact shutter and aperture values, the camera attempting to balance the exposure by changing ISO is not a good idea. It tries hard to prevent you from overexposing or underexposing your shot, but if that is the look you want to achieve you should remember to turn ISO Auto Control OFF.
Effect of Dynamic Range setting on ISO
If your Dynamic Range (DR) setting is anything other than DR100 (off), it will effect the minimum ISO value used by the camera. DR200 will lock minimum ISO to 400 and DR400 will lock the minimum ISO to 800. If the DR is also set to Auto then the camera will calculate DR before calculating ISO and will use the minimum ISO for the calculated DR.
ISO Auto Control once turned ON causes the camera to try and achieve desired exposure or balance exposure in all operating modes by adjusting the ISO value. The maximum and minimum ISO values are gold standard and the camera will never go beyond set limits for these. The minimum shutter speed is a request. The camera will make a best attempt to exceed or meet the minimum shutter speed, but ignore this value if it interferes in achieving desired exposure.
Auto ISO can lead to unpredictable results in long exposure shutter priority modes like B and T and in fully manual operation. It makes a lot of sense in the P and A modes where you focus on the composition and depth of field and let the camera worry about achieving the correct exposure.
Cover Photo Credit - Yukio Uchida, X-Photographer