RPS Talk - Holy Trinity - Aperture, Shutterspeed and ISO

Talk I was asked to do at Rangiora Photographic Society...

When Kevin asked me to do a talk I initially wanted to talk about aperture as it features in my style of photography, however I realised I could not really talk about aperture without talking about shutterspeed - but you can't talk about shutterspeed without also talking about ISO - so rather ambitiously I thought I would talk about all three!

Now I probably don't have the same thoughts as some about what you need to know in order to produce a good image. Many will think you need to know loads of technical information about aperture, shutterspeed and ISO when in reality what you actually need to know if the effect each has on your images.

Modern digital cameras are based on their film counterparts - unfortunalty while technology has changed, the terms used in the film based days still persist. When shooting form you do need to have a much better technical understanding of the process as you had no way to see the results until you had the film processed. Often what this meant was that if you did not shoot it right you could not reshoot. Now you can immediately see the results on the back of the camera - if it is not right you can reshoot straight away

Getting back to what I am hear to talk about - because if the power you have being able to immediately see the results, knowing that the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane is no longer required. What you actually need to know is that aperture effects how much in the picture is in focus.... but I am getting ahead of myself.

Lets start with talking about getting the perfect sun tan...

What are the thing you need to consider when trying to get the perfect suntan?

* How much light hits your skin
* How long you are out in the sun
* How sensitive your skin is to the sun

We all know that the longer you are out in the sun, the more chance there is of getting burnt. We also know that the brighter the sun is, the shorter the time before getting burnt. Finally if you have fair sun you are more likely to get burnt and you need to control how sensitive your skin is but using sun tan lotion.

So if you want the perfect tan on any given day you can adjust all three as they are all linked.

If I increase the amount of light (maybe by being out at midday) then I need to decrease either the amount of time I spend in the sun or decrease how sensitive my skin is by using lotion.

What if I decrease the amount of time I spend in the sun but still want a perfect tan?

I need to either make my skin more sensitive (by applying less lotion) or I need to increase the amount of light hitting my skin.

So what does this have to do with photography?

Getting the perfect exposure is exactly the same.

Imagine that your cameras sensor is your skin. You can control the aperture to control how much light there is that hits the sensor. You can control the shutter speed to determine how long the sensor is in the light and you can use the ISO to control how sensitive your sensor is to light.

So there are three things to balance in order to get the perfect exposure - each of them control the amount of light in one way or another.

The key is that these things all have different effects on the image - it is these things that we are going to look at.

Aperture controls the amount of light getting through the camera - the consequence of this is that the less light causes more things within the scene to be in focus. If I focus the lens on something, when you look at the aperture numbers you can think of them as the lower the number, the less things in front or behind the focus point are in focus, the higher the number, the more things in focus. This is usually referred to as the depth of field or depth of focus.

You don't really need to understand are more of the technical reasons behind this phenomena, just the effect. When you want lots of things in focus, set a high number, when you want less in focus set a low number.

Lets say we don't want lots of things in focus, so we set a low number. This means that we let more light into the camera.

Remember back to the suntan, if the sun is brighter we need to either:

* Spend less time in the sun
* Decrease our sensitivity to sunlight by applying suntan lotion

What about the opposite, we what lots of things in focus, so we pick a large number, which means that less light enters the camera.

Back to the suntan, we now need to either spend more time in the sun or increase our sensitivity by applying less lotion.

So how do we spend more or less time in the sun with our camera?

This is controlled by the shutterspeed. This determines how long our sensor (our skin) is exposed to the sun.

The consequence in terms of cameras of spending more time in the sun is that things can move while the shutter is open. If they more while the shutter is open then they will blur.

So lets say I don't want blur in my image, I need to set a fast shutterspeed (a high number on the camera).

A fast shutterspeed means I spend less time in the sun. What do I need to do in order to get the perfect tan?

* Increase the amount of light (set a lower aperture)
* Increase how sensitive my skin is to the sun

Of course the reverse holds true too. If I want blur within my image, set a low shutterspeed and spend more time in the sun so I need to:

* Decrease the amount of light
* Decrease how sensitive my skin is to light

You can now probably guess what ISO represents in getting our perfect tan.

ISO determines how sensitive our skin is to light. The higher the ISO number the more sensitive to light our camera becomes.

The trade of of making the camera more sensitive to light is that the noise within the image also increases. Again, there is a technical reason for this, but is is really not important to our image making.

So lets put this into practice. Here I have a lovely scene (camera with 50mm f1.4 setup pointing at a table containing different objects at different distances to the camera. One item contains a motor and causes movement)

Q. What do I need to do to get everything in focus?

A. I need to set a big aperture number, as this gives a big depth of field

Q. But what does this mean in terms of light?

A. Less light hitting the sensor so I need to either decrease the shutterspeed (spend more time in the sun).

Q. What happens to the same scene if we introduce movement?

A. The slow shutterspeed means we get a blur.

Q. What can we change if we want to keep he large depth of field and a high shutterspeed to stop movement?

A. ISO, we can make the camera more sensitive to light

Q. What is the consequence of increasing the ISO?

A. We end up with more noise in the image.

Q. Lets say we want a shallow depth of field, ie a small aperture number. Does this increase or decrease the amount of light?

A. Increase the amount of light.

Q. If we increase he amount of light what do we need to do?

A. Remember back to the suntan, we need to either spend less time in the sun or we need to make our skin less sensitive. So set a high shutterspeed or a low ISO number

As you can see you need to decide on the aesthetics that you want within your image first.

Do I want everything in focus or do I want to isolate something with a shallow depth of field?

Do I want to freeze movement or do I want to give the impression of movement?

When this has been decided you can use the camera controls in order to turn that into reality by adjusting aperture to control depth of field, shutterspeed to freeze or allow motion. ISO to allow you to achieve the correct balance between aperture and shutterspeed, potentially at the expensive of noise.

In my opinion the technical details behind why changing the aperture results in different depth of focus can remain a mystery, same with why a fast shutter freezes motion - the details don't matter, only the effect on the image. If you don't like the effect you see on the back of the camera then just reshoot changing one of the variables.