There are three key elements in photography work together with each and every photo you take that create an “exposure” – Aperture, Shutter-Speed and ISO.  It is paramount that every photographer fully understands how these three components work with each other, and what happens to one when you change the other and vice versa.  Today we are going to explain what ISO is, what role it plays in the exposure triangle and some tips and ideas on how you can use ISO for creative purposes and how to use ISO for strictly technical reasons related to proper exposure.

What is ISO?

ISO (or ASA) is a term originating from the film photography days which referred to film sensitivity to light.  The lower ISO film number (100,200,400 etc…) the lower film grain or noise one would get on their final image.  The exact same idea applied today, in the digital world – the only difference is, instead of light sensitivity of the film, it is how sensitive to light the camera’s imaging sensor is.

Why wouldn’t I always keep my ISO at the lowest number?

Being that ISO refers to sensitivity to light, there is a direct correlation between ISO and shutter-speed, specifically – the higher the ISO number (thus more sensitive to light) the faster the shutter-speed you can use for a proper exposure.  If you find yourself getting blurry photos, one thing to check is your shutter-speed and ISO settings as we discussed in this tutorial.

So if you are out in the middle of the day taking photos, it is most likely that there is plenty of daylight, in these cases make sure your ISO is as low as possible which will give you the most clean (noise free) images but also the most dynamic range and depth of color in your photos.

Photography Basics ISO Explained (2 of 4)
ISO 200, 85mm, f/2.2

In these cases make sure your ISO is as low as possible which will give you the most clean (noise free) images but also the most dynamic range and depth of color in your photos.

Sometimes you might want to emulate an older looking photo taken on film with a grainy look to it, if so, then using high ISO numbers will give you this look.  Of course you can always add grain/noise after the fact in post-processing to get the same look.

Photography Tutorials Photography Tips
ISO 6400, 50mm, f/1.4

Questions to ask yourself when considering changing ISO

  1. Is my shutter-speed too slow?
  2. If yes – can I go to a wider aperture to get a faster shutter-speed?
  3. If no – then my only option is to raise ISO to achieve faster shutter.

In the above scenario, please note the subjective balance the photographer must make between using wider apertures vs higher ISO.  Its a tradeoff and your subject will likely be the factor in your decision making if you are to go one route or the other.

For example:

The Problem – Lets say I am doing a family portrait session and there are 5 people in my group shot, its middle of the day with bright harsh sunlight so we are working in the shade in even lighting under some trees, since we are in the shade though it isn’t as bright and at f/4 and ISO 400 my shutter speed is only 1/50th on my 50 f1.4 lens.  I know that 1/50th is pushing the limits and I will easily have subject blur if any of them move at all OR even if I don’t hold the camera still enough so the decision and trade-off is – lower my aperture to allow more light in or crank up the ISO.  The pitfalls of these though are: wider aperture = less depth of field, and raising ISO will add grain and noise to the image.

The Solution – Since my subjects consists of multiple people, depth of field and having everyone in focus is more of a priority than noise/grain in the photo, therefore, I’d raise the ISO to 800 or 1000 to give me a shutter speed of at least 1/100 and still lets me shoot at f/4.  Now – if I was doing portraits of a single person, I’d then favor using a wider aperture like f/2.8 or even f/1.8 to allow me to increase my shutter speed without raising my ISO.

Remember that you can help reduce noise and grain in post-processing after the fact but you can never recover subjects that are out of focus!

Examples of when you may “Need” to use high ISOs

  • Wedding Ceremonies in Churches – no flash is allowed
  • Concerts – no flash & lots of movement
  • Birds – usually long telephoto lenses which require extra fast shutter-speeds
  • Sports – fast movement & telephoto lenses = fast shutter speeds
  • Using your “zoom kit-lens” – is your only lens is the zoom kit-lens that came with your camera, these don’t have wide apertures and therefore to compensate you may have to use higher ISO numbers.
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ISO 4000,  28mm,  f/2
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ISO 2000,  85mm,  f/2.8

I hope that this quick tutorial offers some key insight into an easy concept that can really confuse people.  Review your settings and find out how to fix them!