Understanding Exposure - Aperture / ISO / Shutter Speed

Exposure is the term used for an image that is lit correctly, if there is not enough light in the scene it’s UNDER-EXPOSED, if there’s too much light, it’s OVER- EXPOSED.

There are 3 things that determine accurate exposure…

1: ISO

2: APERTURE

3: SHUTTER SPEED

Accurate Exposure is about juggling these 3 things to get the right balance between them all, for you to capture a nicely exposed image.

1: ISO

 ISO is the sensitivity of light, the higher the ISO number the faster your sensor will react to the light, the lower the ISO number, the slower it responds.

For Example:

50 ISO is a very SLOW reacting ISO setting, this might be something you’d use in bright outdoor situation. 3200 ISO is a very FAST reacting ISO setting, this might be something you’d use in a night time situation.

ISO also determines how GRAINY or NOISIE an image is, for example the higher the ISO number, the GRAINIER your image becomes.

A good habit to get into, is to select an ISO that’s most appropriate for what you’re shooting and stick to it. Don’t chop and change.

If you’re in a controlled lighting environment and the light isn’t changing set it and forget it.

Before digital cameras when shooting film, you’d put in a roll of ISO 400 for example, you’d have to shoot the whole job on ISO 400. Nowadays you can change whenever you feel like it but for consistency, I’d recommend you decide on an ISO and go with it. Why juggle 3 balls when you can put one down?

Of course, if you are shooting in an environment that has a huge variation in lighting conditions its ok to change your ISO as needed.

2: Aperture

Aperture is the size of the opening in the lens that allows light to pass, The bigger opening, the more light. The smaller the opening, the less light.

This also dictates your depth of field, the BIGGER the opening, the SHALLOWER the depth of field, the SMALLER the opening, the DEEPER. I’ll go through this in more detail a little further into the course.

3: Shutter Speed

Shutter Speed is the time the recording medium is exposed to light. You can have a slow shutter speed that allows a lot of light to come into the camera or you can have a very fast shutter that only allows thousandths of a second to come in. This really depends on how much available light there is.

For example, if you are shooting outside on a bright sunny day, you might be shooting at a very fast shutter speed like 2000’s of a second, whereas if you were shooting a night city scape for example you might be shooting at a slow speed like 10 or 15 seconds, much more in some instances.

The best way to understand exposure is with the Tap Principle.

The below is a Kitchen Tap,

Let’s just for a second look at this tap as a source of light, the longer I keep the tap open the more light is going to come out.

Let's say that the CUPS are an indication of how much water I need for an accurate exposure, if they are filled to the brim I have perfect exposure, if they are not full they are UNDER EXPOSED, overflowing OVER EXPOSED.

The TAP HANDLE is the SHUTTER SPEED and the APERTURE, the longer I have the tap open the more time the water has to come out, the more pressure I have the faster the water comes out, SHUTTER SPEED & APERTURE.

If I put this cup under the tap I need to open and close the tap for exactly the right amount of time and the right amount of pressure to fill the cup exactly to the top, too much and ill have an over exposed image, not enough and my photo will be under exposed.

The size of the cup can be used to demonstrate the ISO…

The BIGGER the cup the SLOWER the ISO, if fills slower.

The SMALLER the cup the FASTER the ISO, if fills much faster.

You can see now how and what you need to control to fill up the cup exactly and you can also see what you have to juggle to get your exposure perfect.