When I was starting out in photography, the relationship between aperture shutter speed and ISO was mind boggling to me, no mater how many tutorials I watched or blogs I read I was just confused when I picked up the camera. So I thought I would take some time to explain in the most basic of terms how these 3 concepts work and why it is important to understand their relationship if you are just starting out and you want to move off of auto settings.
The Exposure Triangle
Aperture, shutter speed, and ISO make up the three sides of the exposure triangle. They balance together in order to produce a photo that is properly exposed. If one element changes, then one of the others must also change to keep the correct or same exposure.
What is Shutter Speed?
Shutter speed is the length of time light is allowed to hit the sensor. It is measured in seconds. Shutter speed is probably the easiest of the exposure triangle sides to understand. To increase the amount of light, you need to increase the length of the exposure.
Below I have taken 3 photos of a duck in a shaded area of a stream. The first image was taken at a speed of 1/200 of a second and as you can see the shutter was too fast so there was not enough light traveling through the shutter to expose the image. The second Image was taken at 1/25 of a second which added much more light and worked really well as the duck was relatively still. The last image I went right down to 1/2 a second shutter speed the picture is much lighter and if you look at the water you can see the movement in it, which is great for water but if the duck had moved it would also have that blur. It really depends on what you are shooting and how much you need it to be frozen in time. In the most basic of terms in order to capture a clear image you should remember the faster something moves ideally the faster your shutter speed would need to be.
What is Aperture?
Aperture is probably the one that I get most confused over because the numbers are seemingly opposite in description so f/22 is a small aperture and f/2.5 is a larger aperture, I will try and explain. Aperture refers to the size of the circular hole in the lens that lets the light in. The higher the f-number the smaller the hole which means less light can get in. The lower the f-number the larger the opening and the more light that can get in which increases the exposure.
Aperture also influences the depth of field and creates the blurred back ground effect or can be used to enhance the bokeh effect. The higher the aperture number the greater the depth of field is and so more of the image will remain in focus. The lower the aperture number the shallower the depth of field is and there for the greater the background blur.
Confused? Yes me too! So the way I remember it is; the low numbers means the hole is bigger and and so depth of field is bigger which makes it blurry. the higher number means the the hole is smaller and more focused so the whole image is much clearer. I find the aperture priority settings easier on my fixed lens because the focal length is already fixed. Zoom lenses that have a variable aperture will show the maximum aperture range. For example, when “f/3.5 – f/6.3” is written on the lens barrel. The 3.5 and the 6.3, are referring to the maximum aperture the lens can achieve for each end of the zoom range.
What is ISO?
The final variable in the exposure triangle is ISO which is basically the sensitivity of the image sensor. Higher values of ISO mean that the sensor is more sensitive to light and so does not need to collect as much light which allows you to use your camera in darker situations. Low ISO values mean that the sensor is less sensitive and will need to gather more light to make the exposure. In the images below I shot at night time with just my regular dining room light on. I was shooting at a shutter speed of 1/200 and aperture of f/1.8, I only changed the ISO for each picture. You can see when the ISO is low the camera has not absorbed enough light and when the ISO is high you can see the grain or noise which takes the sharpness out of the image.
An example of a situation you might want to choose a higher ISO would be photographing an indoor sporting event where the light is low and your subject is moving fast. By choosing a higher ISO you can use a faster shutter speed to freeze the movement.
As you can see, there are lots of combinations of ‘shutter speed,’ ‘aperture’ and ‘ISO’ that create the perfect exposure. You also have to take in to consideration the capability of your equipment, the camera and the lenses also play a key roll in how light is processed in order to capture your image. I’ve read blog after blog that just over complicates the technical side of photography you can find detailed descriptions of each of the elements of the exposure triangle. My advice is to just practice and see which combinations give you the desired effect for your style of photography. I personally don’t prefer certain combinations more than others as it really depends on what I’m shooting and what my end goal for the image is. What is important is the understanding that if you increase or decrease one variable in the exposure triangle, you must make up for that by decreasing or increasing one or both of the others.