As I’ve mentioned in my other tutorials, there are three basic fundamentals to exposure. The first two, aperture and shutter speed, have already been covered.  They primarily influence depth of field and sense of movement, respectively.  ISO is the last piece of the puzzle, and we’re going to spend a few paragraphs going through this with enough detail to understand the basics but not enough detail to bore you to death!

So first things first – what is ISO?  ISO was a way that they used in the film days to denote the speed of a given color negative film.  Okay….but that still doesn’t give me anything.  What is it?

I define ISO as your camera’s sensitivity to light.  That’s it.  For a given ISO setting, your camera will alter it’s sensitivity to the light that’s available.  Now that’s something we can get our head around.  Let’s say an ISO setting of 100 is “normal.”  You would use this in an outdoor setting.  An ISO setting of 800 effectively makes your camera more sensitive to light.  So you would use ISO 800 if you were shooting indoors at a party or in a museum.  ISO 1600 would be used in even darker environments, because it makes your camera even more sensitive to the available light.  I use 1600 ISO in a dark setting; inside a medieval church for instance.  Below you see a quick sampling of 6 images that I took. The exposure is noted – I shot these in Manual mode so that when I adjusted the ISO value the aperture and shutter speed stayed constant.

As the ISO in the above images was decreased, the sensitivity to light is also decreased and as a result the exposure gets darker.  Taken at ISO 100, the image is almost black.  I chose the ISO increments above because the math is simple and straightforward.  ISO 200 is twice as sensitive to light as ISO 100.  ISO 400 is twice as sensitive to light as ISO 200.  I think you get the picture…

I tend to view ISO as an enabler for the rest of the exposure.  What do I mean by that?When I capture an image I think more about my aperture and shutter speed settings, rather than my ISO.  I then use the ISO setting to allow me to achieve the desired aperture and/or shutter speed.  This gives me more creative control.  Let’s face it – ISO is pretty boring.  It’s more useful to use aperture and shutter speed to give your images that creative edge.  For example, many people will use ISO to ensure they don’t suffer from camera shake.

Now – ISO has an important property that can have an impact on the image quality.  That is noise.  Noise in a photo is the result of the camera’s sensitivity being set too high and the camera’s sensor is basically not able to cope.  So you get these little speckles in the photo, that tend to be more prevalent in the dark or shadow areas of the image.  This is a massive, massive challenge for many cameras, and camera manufacturers do their utmost to reduce this “noise” in their cameras.  Photographers pay a lot of money for cameras that have “High ISO performance.”

I took some pictures below to show you noise in images.  But to be honest after reviewing these I’m not sure it will get the point across.  These were taken with a 5D Mark III and it’s well known for having low levels of noise at high ISO’s.  Nevertheless you can have a look and do a comparison for yourself – if you click on the image it will give you a larger version.

To give you a final example of noise in an image I cropped the ISO 25600 image even closer and here you can definitely see the “freckles” associated with noise.

Now noise isn’t always a bad thing.  I’ve used it to good effect in some images; it can give an older look to a photo and the grain can add interest, particularly to B&W photos.  Many software programs like Lightroom, Photoshop and Aperture all have the ability to reduce noise in images.  For any given camera a low ISO will always give you best image quality.  All things being equal, shoot at ISO 100 (or 50 if your camera has it).

I tend to shoot using Auto ISO.  This is a setting that some cameras have that allows you to just go out and shoot in Av or Tv mode (Aperture or Shutter priority) and then the ISO auto-adjusts to ensure that the exposure will be correct and that the shutter speed will be fast enough to mitigate blur.  I can use Auto ISO now because I’m very pleased with the quality of images up to ISO 1600; with my 50D I could begin to see image degradation around ISO 800.

Since ISO increases your camera’s sensitivity to light, it invariably will have an impact on your flash strength as well.  So for a given image where you need a stronger flash, instead of increasing the power of your flash you can bump your ISO by an interval or two.

The table below summarizes some of the things we’ve discussed and is a reference for general use.

ISO Setting Shooting Situation
100-200 Outdoors, well lit environment
400-800 Cloudy day, indoor shooting
800-1600 Indoor low light environments, dusk
1600+ Dark shooting conditions

When we get to the section on Manual exposure I’ll get into far more detail on how I use all three aspects of exposure – Aperture, Shutter Speed, and ISO – to create images that will separate your photos from the norm.