ISO sensitivity is one of the three parameters, together with aperture and shutter speed, used to determine the exposure of a shot. Raising the ISO sensitivity lets the sensor react more quickly to light, thus allowing for smaller apertures or faster shutter speeds. The ISO sensitivity scale is linear: using an ISO 200 setting, for example, will have the sensor react with twice the speed than with an ISO 100. If other parameters are kept fixed, raising ISO sensitivity from 100 to 200 corresponds to a light increment of 1 stop.
The possibility of choosing an appropriate ISO sensitivity with the flick of a switch gives photographers a degree of freedom: you can maintain a fixed exposure level changing the ISO sensitivity and compensating with a corresponding aperture change or shutter speed change. It's so handy that I assigned the Fn button of my camera to ISO sensitivity, so that I can change it with just one click.
Auto ISOTo make photographers' life easier, many digital cameras offer an Auto ISO mode: the camera will automatically raise the ISO sensitivity in insufficient light conditions. Auto ISO on recent Nikon cameras works as follow:
- The currently (manually) selected ISO sensitivity is treated as a minimum.
- You choose a maximum ISO sensitivity.
- You choose a minimum shutter speed.
- When light condition is such that a shutter speed slower than the selected minimum is required, the camera will automatically increase the ISO sensitivity.
- When the maximum ISO sensitivity is reached, the camera won't increase it further and will fall back to changing other parameters, depending on the mode you're shooting.
Auto ISO with a Hot-Shoe Mounted FlashThe behaviour of the Auto ISO algorithm is consistent when using the camera pop-up flash in slow mode as well. Unfortunately, things are weirder when using a hot-shoe mounted flash, such as an SB-400. In this case, the camera increases the ISO sensitivity up to four times the value currently selected and won't raise it any more, even if you selected a greater maximum ISO.
Let's suppose you chose a value of 1600 ISO as maximum sensitivity and the current sensitivity is set to 100. When using a hot-shoe mounted flash, the camera will progressively increase the sensitivity up to four time the selected ISO, in this case 400, and will not increase it any more.
As far as I know, this behaviour is not documented in the camera manual and a quick Google search confirms this behaviour is known on other Nikon cameras as well.
Once you learn it, it's something you can live with. In fact, I often manually increase the ISO sensitivity so as to increase the maximum sensitivity the camera will choose. Since I'm not often using sensitivities as high as 6400 ISO, that's just a couple of button clicks away.
However, since it's undocumented behaviour, I do consider that overriding the it Auto ISO settings is no good. After all we shoot manually, although with the help of partially automated task such as this, because we're supposed to know what we're doing.
Could I quickly switch Auto ISO on and off, I'd surely shoot with manual ISO when using an external flash. Unfortunately, the Nikon D5100 won't allow you to do this easily, the quickest way being customizing your own menu; but to be fair, the reduced control customization capabilities is by far the only complaint I have about the D5100.