Auto ISO is a relatively new addition in the toolkit photographers have at their disposal, and it’s one that can be incredibly useful if you understand what’s happening.

Being able to change ISO settings from shot to shot is one of the biggest advantages with digital photography. If we find ourselves in need of more light, we can increase the ISO and hey presto.. Of course, the price you pay can be some digital noise. But at many times, that is a small price to pay for such great freedom and flexibility.

Let’s start with a  quick primer on exposure and see what ISO really does. Aperture, Shutter Speed and ISO are the three determinants of exposure. But it is really only aperture and shutter speed that give us the creative choices. With them we can control depth of field and movement. But ISO is just a tool. It doesn’t really have a creative use (unless you find digital noise to be attractive). Instead it is a great tool, allowing us to achieve higher apertures or shutter speeds by increasing the light entering the camera. This is where ISO shines and is the best way to think about it and use it.

So if we set ISO manually, we adjust it to let more or less light into the camera and that affects what is possible with our aperture and shutter speed choices in that given light. It’s relatively simple, and as the light levels get lower, we need to use more and more of it. Too much of a good thing however can turn into a negative and very high ISO settings can leave us with noise, particularly evident in the shadow areas, and generally reduced quality images. Typically, we want the ISO levels to be at the minimum possible, while still achieving the aperture and shutter speed we desire.

OK, so far so good. How does Auto ISO come into it?

With Auto ISO, the camera sets the ISO on our behalf. The first thing to do, if using Auto ISO, is to go into your menu settings and set the Maximum sensitivity you will allow Auto ISO to use. Most recent cameras allow this, although some do set it automatically. This requires knowing your camera  and how it responds to noise. Most recent cameras have great noise response up to 1600 or 3200. But over 3200 say, the noise will become glaringly obvious and you might consider that you are paying too high a price for that high shutter speed or more depth of field. So that’s the first thing. Set the maximum ISO Auto ISO will use. I’d suggest 1600 or 3200 depending on your camera.

Now, with a name like Auto ISO, you might think this only works in Automatic shooting modes, like P, AV and TV. And with many cameras, you’d be right. It goes against logic to use it in manual mode, as it’s hard to be both auto and manual at the same time. So let’s look at it in each of the main shooting modes.

P – If you use Program (sorry I still prefer the term Point and Pray :)) then Auto ISO just works. The camera adjusts the shutter speed and aperture, attempts to keep the shutter speed near the inverse of the focal length (more on this later), and increases the ISO as necessary. At least that’s how it works on my Canon 5D MkII. Not being a Nikon shooter, I’m not sure which models use this feature, but I’d imagine most if not all. Let’s not dally here anyway. Who shoots on P anyway!! :)

In Aperture Priority, Auto ISO is a fantastic feature, although the implementation could be better in some models. In this mode, as you know, you choose the Aperture and the camera chooses the Shutter Speed (and the ISO if you’re on Auto). What’s really interesting though, is the fact that the camera queries the lens, and attempts to keep your shutter speed as fast as the inverse of your focal length. People who have read my previous posts or have done on of my classes would know that this is the way to avoid camera shake in handheld photography – keep your shutter speed faster than the inverse of your focal length. But in AV with Auto ISO turned on, the camera pretty much does it for you. I’d like to see a menu option that allowed you to specify Focal length plus one stop for example so it would always keep the Shutter Speed one stop faster than the focal length. In lieu of this, you can also turn on Image Stabilisation if you have it on your lens, and then in this mode, you are pretty much bullet proof against camera movement. I have just read that many Nikons actually allow several rules around this feature, which is basically what I want – the choice of controlling just how much the Auto ISO is used in relation to the focal length and desired shutter speed. Good work Nikon! Come on Canon, catch up!!

OK, what about Shutter Priority (TV / S). In this mode, you choose the shutter speed and the camera sets the aperture as usual. When the aperture starts to run out, Auto ISO kicks in. I suppose that’s logical enough. I rarely use this mode so I’m not quite sure if there’d be a better way..

And as I said Auto ISO doesn’t make a lot of sense in Manual mode. Surprisingly though, in some Nikons, you can use it in Manual, though when you do, it’s not Manual at all ;) Really. Let me explain. In this mode, which is Manual Mode with Auto ISO turned on, but which I’d call Aperture + Shutter Priority because it’s not Manual at all, you set both Aperture and Shutter Speed and Auto ISO is the exposure variable that gives you correct exposure. In this mode, you can even use Exposure Compensation!!! Now this doesn’t work at all in most Canons and in some it does but in a slightly different way. Check your camera to see if you can use this. It’s an interesting way to shoot. I certainly wish my camera had this. In my camera, I can turn Auto ISO on in Manual mode, but it’s not Auto at all, being set at 400. How bizarre is that! Generally if I’m shooting in Manual mode, I don’t want Auto ISO, as it goes against the concept, but this new mode hiding under the Manual mode button does sound very interesting.

OK, for beginners just learning, I’d advise setting the ISO yourself until you are comfortable with the variables and interaction between the three determinants of exposure. Once you’re familiar with what’s going on however, turning it on opens up a whole world of interesting options, well worth exploring.

A final example of when to possibly use it. The first time I used Auto ISO I was at Australia Zoo with Canon, testing (playing really) with a whole bunch of their lenses. I had the 100-400 f/4 on, and I thought about the issues. I was in variable lighting conditions with moving subjects and so manual didn’t seem like a very smart option. AV would let me control the depth of field, but I’d have to watch the shutter speed like a hawk, especially with such a long focal length available to me. Instead I realised that if I used Shutter Priority with Auto ISO, I could set a minimum shutter speed (I chose 1/500th to account for the focal length and possible animal movement) and the camera lowered the aperture as necessary and increased the ISO when needed. Depth of field varied throughout the afternoon, but I had basically no issues with movement or camera shake and I didn’t have to be constantly aware of the shutter speed. Win, win I’d say.

Finally, here are a few links if you want to read more about this interesting subject.

I’d love to hear your experiences of AUTO ISO. Do you use it? Does it make sense to you? In which modes do you use it? Does your camera have good implementation and options, or are they lacking? It’s definitely an area of rapidly changing technology and features, so keep an eye on it. ISO is suddenly getting a whole lot more user friendly and useful.