Stay in control
The mantra I am following sums up to “When you got the time, why not stay in control; when in a hurry, or with rapid changing scenes take the aids that your camera offers.” This blog post will not deal with ISO as a concept, but only why you shouldn’t default to Auto ISO.
I will not use AUTO-ISO
when I am in studio conditions where lighting conditions are static, in situations with harsh backlit, in situations where my composition includes harsh shadows and highlights or simply in situations where I got time to set my settings manually.
I do prefer to be in full control, setting all the settings manually, avoiding the camera making creative decisions based on its auto-ISO algorithm.
Another downside is the risk of loss of detail fidelity and dynamic range as you amplify by boosting the ISO. The higher ISO sensitivity, the more conspicuous is the noise. But ISO also correlates 1:1 with how much the exposure increases or decreases. Basically, the camera will watch your shutter speed and if it drops below the “Minimum Shutter Speed”, it will automatically increase the ISO to a higher number, to try to keep the shutter speed above this setting. The general rule is to set your shutter speed to the largest focal length of your lens so you avoid motion blur in the picture. Even with our great, modern sensors we risk the camera taking radical decisions boosting the ISO to something crazy like 6400 giving 1/3*focal length; when it likely would have be fine at 1/focal length and 1600.
Situations when AUTO-ISO makes sense
I use Auto-ISO when I am in a hurry. It allows for adapting to changeable light conditions, while sticking to your preferred aperture and shutter speed. You’ll be able to use preferable shutter speeds aperture more of the time than if ISO sensitivity was fixed. Another great way to use it is in full manual; combined with auto ISO can be very creative.
For Sony Alpha A99v is Auto ISO automatically set between ISO 100 and ISO 6400, when the exposure mode is set to P/A/S/M. I recommend changing this to avoid too radical values, The ISO range 100 to 1600 works a lot better for me.
When [AUTO] is selected, press >, select [ISO AUTO Maximum] or [ISO AUTO Minimum], then select the desired range.
The exposure triangle in the middle of this diagram illustrates the relationship between ISO, shutter speed and aperture. If you change one parameter, you will have to compensate one or more other parameters in order to keep balance and maintain correct exposure. Increasing the f-number decreases the size of the lens’ diaphragm, resulting in a reduction in the amount of light hitting the image sensor, but also increasing the depth of field in the final image. Reducing the shutter speed affects the capture of motion, but it also increases the amount of light hitting the image sensor, so everything is brighter. Increasing the ISO lets you shoot in lower light situations, but you increase the amount of digital noise due to amplification in the photo.