Relationship between ISO, aperture and shutter speed

January 18, 2014  •  Leave a Comment

Have you ever wondered how to achieve the perfect picture that has the correct exposure? It should not be a complicated matter, but the truth is it might give you a headache when trying to work out how the different settings on your camera work together to achieve this. Fortunately, all you need to know is your ISO (or film speed), your aperture and your shutter speed. How do I achieve that I hear you ask? Well, let's find out what each one is first.

ISO

ISO is a term used during the film days to determine light sensitivity of film. The history is very interesting but all we need to know is that it represents sensitivity to light. The larger the number the more sensitive to light the film was. This has been translated to digital in a similar manner, where sensor sensitivity to light increases as you increase the value of the ISO. ISO 100 is less sensitive than 400. In times of film you had to choose the ISO speed prior to shooting, so if you knew that that day was cloudy you might have chosen an ISO speed of 400 for that day, whereas if the day was very sunny an ISO of 50 or 100 would serve a better job. In digital photography, high ISO speeds can be achieved with the press of a button. These are set by the manufacturer and are arbitrary in the sense that the manufacturer determines what "normal" should look like based on how similar it looks to film of the same ISO speed with the same exposure.

But I digress, what you should keep in mind is that high ISO speeds introduce more noise into your pictures, so low ISO is preferable. However, newer cameras have very good noise reduction algorithms that make it possible to shoot at high ISO speeds (even to 3200 or 6400 for some cameras) without major noise. You will have to determine what your camera can handle and what you find acceptable.

Things to remember:

  • At higher ISO you will get more noise in the picture. This varies with each camera, so find out how much noise you are willing to accept.

Aperture

Aperture is the iris of the lens. When you shine a bright light into your eyes, your iris responds by closing to the smallest hole possible. This happens to minimise the light passing through so that the scene does not become too bright. On the other hand, if you try and look into a dark cave your iris dilates to allow as much light as possible so you can see somewhat into the darkness. Lenses work the same way. A lens needs to be opened up in dark places to allow more light into the sensor (so your image is not too dark) and closed down in bright days (so your image is not too bright). The opening of the lens is determined by a number called the f-number (mostly referred to as f-stop). This is where most people get confused, as the f-number is inversely related to the lens opening. That means that at f/2.8 your lens is open wide while at f/11 your lens is open only a little. This comes from the fact that the number is a direct relationship of focal length over diameter. What we need to remember is that a smaller aperture is a bigger f-number.

Now bear with me for this brief simplified mathematical lesson. The f-numbers follow a sequence of numbers. Each number is one stop or f-stop starting from f/1. The number is determined by the square root of 2 to the power of a sequential number (i.e. 0,1,2,3,4 etc.). The best way is to memorise the numbers (this will be much faster). Here is a good chart. For a quick reference look at my table below. You just need to know the full stops to begin. When you rotate the dial to select the next f-number the dial will "stop" at a value that is either a full, 1/2 or 1/3 of an f-stop. To give an example: if you have a lens with a minimum possible f/2.8 and turned the dial to the next  position and it "stopped" at 3.2 then you are climbing 1/3 of a stop. This is because a full stop would be at f/4 (My table below shows the full stops).

The reason I am mentioning stops is because I am going to use them later to unify the changes that need to be adjusted for aperture, ISO and shutter speed. 

Things to remember:

  • Aperture also determines the depth of field (DOF). What that means is that there is a certain space of distance that things are in focus and anything outside that space is not in focus (I will write a special article about this so stay tuned. For now you can watch this video that has an excellent explanation). In simple terms, a larger aperture (small f-number) gives you small DOF while small aperture has more things in focus.

Shutter speed

Shutter speed is the eyelid of your camera. When you blink, you are closing your eye at a particular speed that allows you to see continuously without prolonged periods of blindness. The camera has a similar mechanism called a shutter, only the camera is continuously blind until we press the release and tell it to snap a picture. Then it opens its shutter very quickly to see the image and closes it back up again. The duration of how long the shutter is open is called the shutter speed. The shutter speed is faster the larger the number is. That is because you divide everything by 1 second. So for example 1/1000 of a second is faster than 1/500 of a second. The amount of light entering the sensor can, therefore, be influenced by changing that number.

Like the aperture, the shutter speed can be described in stops. This time it is much simpler though, as all you need to do is divide or multiply by 2 to get to the next stop number.

Things to remember:

  • Slow shutter speeds induce shake (blur) into your pictures, especially at longer focal lengths. A general rule of thumb is that the minimum shutter speed should be 2x the focal length. E.g. at a focal length of 100mm your shutter speed should not go below 1/200 unless you have some image stabilisation

How to use them together

When you want to take a picture of a scene your camera will take all of these values and combine them to figure out what the correct exposure of an image is. In practice most cameras have a light meter embedded into the camera that lets you know if your exposure is correct or not. You can change any of the 3 values mentioned to achieve the correct exposure and balance any 2 values to keep the exposure the same but change the way you capture the image.

Please keep in mind this table to figure out how many stops to change each value to balance your exposure:

Stop numbers for adjusting your exposure
Stop 1 2 3 4 5 6 7
Aperture f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22
Shutter Speed 1/60 1/125 1/250 1/500 1/1000 1/2000 1/4000
ISO 100 200 400 800 1600 3200 6400

 

I think it is easier to talk about this with an example.

Example

Lets assume that you are trying to take a picture of your kids at a party running around. You are in the house and it is a cloudy day. There is not much light available at the scene but you want to get sharp images of your kids with no blur. When you adjust your setting to get the correct exposure either manually (by checking the meter) or in automatic mode (by switching to P mode) you see that the correct exposure is at an aperture of f/2.8 with a shutter speed of 1/60 at ISO 200.

At f/2.8 the DOF is small and you might find that focusing on your child is very difficult when he/she is running around. You most likely will get some other part in focus but not your child like you intended. You need some flexibility in your shot so that you get your child in focus, so you increase the DOF by increasing the aperture to f/5.6. Note in the table above that this is now 2 stops above the optimal exposure (3-1 = 2) or 2 stops less light.

In order to balance this, you need to either A) decrease the shutter speed by 2 stops to 1/15 (slower shutter speed = more light) or B) increase your ISO from 200 to 800. 1/60 shutter speed is already very low and if you have a lens that is less than 30mm that might be fine for handheld pictures but at 1/15 you will find it very hard to keep your hands sufficiently steady so as not to introduce blur. This is even if you have image stabilisation enabled. So the most obvious option is B provided your camera can handle it (most digital cameras these days do fine up to 1600 ISO).

Now that your image is balanced again you might run into another problem. The kids move just too fast and their movements are too blurry in your pictures. The only solution here is to increase the shutter speed to freeze the action. Now you increase your shutter speed from 1/60 to 1/250 and find that it is sufficient to freeze the action but need to balance the exposure again. What to do?

If you can increase the ISO from 800 by 2 stops to 3200 and not get too much noise you are in business, but if like most, your camera cannot handle such ISO speeds and tops at 1600 you need to tweak another setting. Here the choice is obvious: increase the ISO by 1 stop and at the same time decrease the aperture by 1 stop (remember that decreasing the aperture means that the iris of the lens is actually wider) but still retain some DOF. This is a compromise but you will find that this is something common in photography, especially when you haven't planned your shooting. Now you are good to go. Of course, if you can find a way to introduce more light into your picture (open the curtains or use the build-in flash) you might not need to change any more of the features. Also, I want to point out that some cameras have exposure dials or buttons that let you change the exposure by some stops without changing your shutter or aperture settings. This can be very helpful for quick adjustment, especially when you desire to keep the shutter speed and aperture of your choice.

Final Thoughts

In the example above it looks like you would need to do a lot of adjustment. Don't worry too much about it. You have been given a lot of tools to work with but you can practice slowly and learn to adjust. Start with shutter priority mode or aperture priority mode. Play around and don't be afraid of the mistakes. Make sure you understand what went wrong and you will be a better photographer for it. If possible read your camera manual to find out what is possible with your camera. Today's cameras can do remarkable things.

Was this helpful to you?

I hope it was and that everything was clear. If you have any questions email me and I'll be happy to answer them. 

 

 

 

 

 


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