Mastering Manual Mode: Understanding aperture, shutter speed and ISO to get correct exposure

Getting a correctly exposed photograph is like filling a container with water, not a difficult task and no need to be intimidated right? Here’s what I mean:

  • Light = Water
  • Aperture = The tap (how far open or closed it is)
  • Shutter speed = How long the tap is open for
  • ISO = The size of the container you need to fill
  • A correctly exposed photo = A full container


Depending on which variable is important to us, we can adjust the others accordingly to achieve a certain result. For example if we really need our container filled up quickly we can open the tap all the way, if that’s not fast enough we can use a smaller container as well. Remember, in this analogy it doesn’t matter what the container is, all we want is a full container. Both of those changes will reduce the time it takes to fill the container, in this particular example that’s what is important to us.

Sometimes the situation will dictate how wide open we can have the tap. In situations where the tap can only be opened slightly we can either wait a bit longer or, use a smaller container again. Often it’s the size of the container that we don’t want to change, in that case we use a combination of how wide open the tap is and how long it has to be open for.

So as you can see it’s possible to change any of the three variables to get the same end result, a full container. In other words, a correctly exposed photo. Unfortunately each of the variables comes with a downside or trade off. That’s why once you understand how to achieve correct exposure it’s important learn the pros and cons of each variable and how you can use them creatively.

Where to Start

Canon Camera Set to Manual Mode

I’m going to assume this is your first step into using any of the manual modes on your camera (Aperture Priority, Shutter Priority or Full Manual). As such this is a very basic starting point to getting a CORRECTLY EXPOSED IMAGE in manual mode.

I would suggest learning to shoot in manual the same way you would imagine filling a standard bucket. You would open the tap all the way and just wait for the bucket to be full. On your camera this equates to setting your ISO to 100. Open the aperture as wide as possible, the smallest “f number” (something like f4.5 or f5.6 most likely).

Now all that’s left is to determine how long to leave the tap open, shutter speed. As we can’t see the sensor “filling up with light” like we can a bucket with water, the camera has a handy tool that does this for us. The light meter. It actually goes one better than observing and tells us how full our bucket will be before we even turn on the tap!

An in camera lightmeter
Balanced Lightmeter

A balanced light meter like the one above is what we are aiming for. Go ahead and point the camera at your subject, now adjust the shutter speed until the light meter is balanced. If the indicator is to the left of centre, it means the bucket isn’t going to be full. The photo will be too dark (underexposed). If the indicator is to the right of centre, it means the bucket will be overflowing. The photo will be too bright (overexposed).

Now that you have a starting point to get correct exposure look out for a future blog post where I’ll explain when you would want to change the various settings to achieve certain results.