Auto-ISO and blown highlights

With a digital camera, you can not only use shutter speed or aperture to set the exposure of your images, but also ISO. Some photographers use manual shutter speed and aperture settings, and set the camera to Auto-ISO to let it find the correct exposure. While this often works fine, there is a caveat. The danger of this technique is not at high ISO, as many people think, but at the lowest ISO values. Yes, you can get rather high ISO values with this method and that may cause some noise in your images. But noise can be filtered and a little noise is often not that ugly anyway. It resembles the days of grainy film. A much worse problem can occur at the lowest ISO values.

Changing the shutter speed or the aperture changes the amount of light that falls on the sensor, but changing the ISO value does not. Changing the ISO only changes the way the sensor signal is processed. Every camera has a ‘base’ ISO, usually around 200 ISO. If the amount of light that falls on the sensor is too bright to shoot at your chosen shutter speed and aperture at this base ISO, the sensor wells will become saturated (‘clipped’). You will get blown out highlights. Yes, because of the Auto-ISO setting the camera will simply use an even lower ISO (100 ISO or 50 ISO). But because that does not change the amount of light on the sensor but only the signal processing, it won’t solve the problem of the saturated sensor wells. That harm is already done. Your overall image will indeed become darker and may even look well exposed, but the blown out highlights will remain.