You often hear professional photographers, including me, saying you need to master your camera, be the sole decision maker when it comes to taking an exposure. That often refers to keeping your camera dial on the Manual, Aperture priority or Shutter priority mode. But what about the ISO? All modern DSLR will let you choose a specific ISO value (100,200,400 etc.) or the AUTO option. Should I discard this latter option if I want to be a true photographer? Will I go to a noisy Hell if I let my brainless camera choose the ISO for me? The choice remains yours but here are my few thoughts on the matter:
If you are an aficionados who swears on the Manual mode and pretends to be able to assess the Shutter speed, the Aperture and the ISO just by looking at the scene with his/her own eyes and gets it perfect all the time (and I would love to meet you) this approach will not be reliable enough to capture an action rapidly when the light condition is constantly changing (i.e. sport, wedding…).
This is the very reason why the Aperture and Shutter Priority modes are favoured by many. You are still keeping the driver seat by telling the brainless camera what factor is the most important to you whether it is the Depth of Field or the Time while still specifying the light sensitivity (ISO).
Let’s face it the ISO is the least creative factor of the 3 (Aperture, Shutter, ISO). If we could, we would always use the lowest ISO possible (50, 100 or 200). What if our camera could figure out when increasing the ISO is a must? Well, it can!
Indeed, it can your camera has a metering system built-in that is quite reliable. So all you need to do is set your ISO to AUTO. But how will it behave? How can we make sure the meter will not increase the ISO either too early or too high leading to an unusable exposure? That is where camera differs from one another. Some will let you specify what the slower shutter speed can be before the ISO is increased (Nikon D90). Some will let you choose only the highest ISO the AUTO mode can use (Canon 60D). Some will do both (Canon 5D iii).
Cameras that do not let you specify the minimal Shutter speed will have an algorithm like ISO 100 will be increased if the Shutter speed is inferior to 1/(2x Focal length). I suspect this algorithm was chosen to take in consideration the potential crop factor of the sensor and the rule of thumb to avoid camera shake (Shutter speed >= 1/(crop ration x focal length)).
But the drawback of such algorithm is if one uses a lens with an Image Stabilisation system (IS for Canon or VR for Nikon). With my 60D (x1.6 crop ratio) and my 24-105L IS USM, i know I can shoot hand-holding down to 1/10 sec with a focal length of 105mm if the IS is active. Therefore the ISO threshold according to the algorithm is higher (1/(2x105mm) sec) than I can cope with.
I have discovered this feature after an event i was covering where I centralised my attention to the action I was capturing and while in Aperture priority mode I had not realised that for some of my shots the Shutter speed went lower that what I can handhold the camera for and it resulted in camera shake. Therefore once in front of my computer I wondered I was faced with the same situation how I could make sure to avoid camera shake.
Maybe Auto ISO is the more useful when used in Manual mode where one would want to freeze every action by enforcing a Shutter speed like 1/250sec while keeping the shallowest Depth of Field with an F2.8 or F4. Then the ISO will adjust as needed. I have personally used that with wildlife, and the result was quite good.
Some may argue that the Auto ISO parameters only let you specify the extremes ( when raising above 100 or how high you can raise). That is true, but personally, I know that with the 5DIII I am satisfied with an ISO as high as 6400. So Given the context for which I would need to speed-shoot, having my exposures changing from 100 to 6400 does not matter to me.
In conclusion, I have tested this feature the camera offers. I know it exists and I know I can use it in some uneasy occasions. The majority of the time I would always set my ISO.
Now you know it too, and you are free to test it and make your opinion about it.