ISO, aperture and shutter speed

As I said in my introduction, I am going to take you through my (re-) learning curve with my DSLR.  I am using numerous websites to find the info to explain it briefly and succinctly. I do hope I have the facts correct (if not, you are welcome to correct me – politely, please). We all know “practice makes perfect” and one doesn’t learn to play tennis or golf or any sport by simply reading the “How to …” manual.  So when you think you have a general idea of what I explain below go for it and shoot away – you can delete every photo, your bank balance won’t be affected and no one need know 🙂

  1. ISO:

Question:  How do we say “ISO”?

Answer: “eye-so”.

Question:  What does ISO stand for? Does it matter?

Answer: No… so we’ll move on. (You can find that on many websites if you really are interested!)

Question:  What does ISO do/mean?

Answer: ISO is to do with the light required in a photo or ‘light sensitivity’. The ISO on my camera (Canon ESO 650D) ranges from 100 – 12800. The lower the number the more light is required. A low number would be used in bright sunlight or when using a tripod. BUT… ISO is always balanced with…


Question: What is the “shutter speed” dial/tab/button on the camera?

Answer: It is the fraction on your screen.  Mine ranges from 1/4 to 1/4000.

Question: What does each fraction mean?

Answer: A fraction of a second.

Question: What is the relevance of fractions of a second?

Answer:  The higher the number (fraction) the slower the shutter speed i.e a setting of 1/4 means that the shutter opens and closes very much more slowly than a setting of 1/4000. Or looking at it the other way, the bigger the number on the bottom (4000) the faster the shutter opens and closes.

(Gosh, no wonder one gets confused!!)

Question: When and why do we need different shutter speeds?

Answer: Because life is not static! We can use a slower shutter speed (1/4 e.g.) when photographing landscapes or using a tripod but we will need a fast shutter speed (1/4000 – well, towards that end of the scale) when capturing an action shot.

Question: How do the ISO and the shutter speed interact?

Answer: Hold on… there is a third element to what is commonly called the “Exposure Triangle” (sounds we’re into something secretive now!) and we must look at all three elements together …

(Hey, I remember all this from my pre-digital SLR days!)


Question: What is aperture?

Answer: The definition of ‘aperture’ is ‘hole’ or ‘opening’ or ‘gap’ so in your camera it is the opening through which the light travels when you click the button. The aperture can be compared to the pupil of your eye… when you are in bright sunlight your pupil is smaller as it doesn’t need to let in as much light as when you are looking for an object in a dark room.

Question: What is the label/dial/button that adjusts the aperture on my camera?

Answer: The “F-stop”. On my camera it goes from F4.0 to F25

Question: What do the numbers mean?

Answer: The numbers adjust the size of the aperture/opening. And here is the important thing… the lower the number the larger the opening. (Now that should be easy to remember – lower = larger :-))  And if we looked at the actual opening we would see that the smaller the opening the less exposure. (Sounds confusing but it does makes sense!)

Question: So why do we need to adjust the size of the aperture?

Answer: For the same reason that our pupils adjust their size according to the light.

Question: So how do these three (ISO, Shutter speed & aperture) interact?

Answer: Before I answer that I will re-cap briefly then go and take some shots playing with the different settings…

ISO: Lower = less exposure: 100 = less exposure   12800 = more exposure

SHUTTER SPEED: Slow (1/4) = more exposure   fast (1/4000) = less exposure

APERTURE: Small (f4.0) = less exposure   Large (f25) = more exposure

Have fun & I’ll be back real soon with some of my shots 🙂