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Aperture Shutter Speed and ISO: The Ultimate Trade Off

by Jo Ferguson (follow)
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How do shutter speed, aperture and ISO interact to produce correct exposure?
How can you manipulate them for creative effect?
What is the ultimate trade off with your camera settings?

Scarborough Marina Sunset
There was enough daylight to capture this image hand held and still have it sharp from front to back

To achieve a correct exposure both shutter speed and aperture can be adjusted to allow the right amount of light to enter the camera. Different lighting conditions will require different aperture/shutter speed combinations for accurate exposure.

It is a reciprocal relationship between shutter speed and aperture, meaning one affects the other. If you change one to let less light in, you will have to change the other to let more light in to balance things out.

So what happens if for example, you need a small aperture such as f16 to keep everything sharp from front to back.

Remember that the smaller your aperture, the less light getting into your camera. In this instance your shutter speed will have to be slower to allow enough light to illuminate your scene. The darker your surroundings, the slower the shutter speed will need to be.

Scarborough Marina Sunset
This shot needed a tripod for front to back sharpness as the dimmer light meant a slower shutter speed was needed

Keep in mind that the longer you leave your shutter open for, the more movement that will be captured in the shot. This includes vibrations underfoot or shaky hands.

Even with a large depth of field nothing is going to be sharp in your image if the camera picks up hand shake or vibrations. Your images will be soft and slightly blurry all over from the movement.

What if you don't or can't use a tripod?

What happens if you want to freeze the action in your scene, but you want to keep things sharp from front to back?

Are you starting to see where problems might arise?

So how do we get around these problems with aperture and shutter speed?

The answer is that photography is all about compromise.

You need to work out what is most important to your scene and select your settings accordingly. The rest will have to be a trade off.

Scarborough Marina Sunset
There is slight movement in the soccer ball and the boy as I had to make a trade off in the dim light to capture front to back sharpness and try to freeze the action

If freezing the action of a moving subject is of greatest importance to your image, select a fast shutter speed and a wide aperture. Your subject will have a shallow depth of field but this is the compromise you must make.

If your subject is a landscape image where there are distant horizons and you want everything in focus, select a smaller aperture and a slower shutter speed. To stop hand shake or vibrations you will need to use a tripod.

What about lugging a heavy tripod into remote areas?

Scarborough Marina Sunset
I didn't have a tripod for this shot so fortunately there was sufficient light to freeze the hippo and water movement while retaining front to back sharpness

The good news is when you have bright sunny consitions you can often still photograph hand held as their will be enough light around to you to allow you to use both a faster shutter speed and a smaller aperture.

You can also purchase lightweight travel tripods and a strap to strap them to your body. At a pinch you can find something to rest your camera on such a a rock, or even lean your body up against a tree to minimise vibrations.

There is one other trade off you can use if you really can't use a tripod. You may be indoors at a venue that doesn't allow them. For whatever reason you are unable to use or don't have a tripod in tow there is a safety valve, and its called ISO.

A higher ISO allows more light to enter the camera without changing your aperture or shutter speed settings. It will save you every time you need to capture hand held shots in dim light.

Thats amazing. Why don't we just use high ISO all the time?

Well like everything else, ISO is a trade off.

ISO stands for international standards organisation. It has carried over from film photography days where films were given an ISO rating depending on how fast or slow they were. A slower film, allowed you to work in darker conditions but meant you had much coarser film grain. This was often used to creative effect.

With the coming of digital photography ISO is simply a term for amplifying the digital signal. Instead of film grain it actually creates digital noise. This is a byproduct of the signal amplifcation and is not at all attractive like film grain.

Newer models of DSLR claim the use of higher and higher ISO settings without much digital noise. However there are two crucial things you should know if you are going to use a higher ISO.

Firstly, no matter which camera you own you will only benefit from the reduced noise at high ISO if your exposure is spot on. If you underexpose even slightly, the noise will be attrocious and be magnified as soon as you try to ligthen the image when post processing.

ISO Noise Comparison
Comparison of digital noise in a low ISO and high ISO shot

Secondly, using a high ISO on any camera is degrading the quality of your image. Ignore all the marketing hype, every time you step your ISO up a notch the quality of your image disintegrates until eventually fine details are destroyed altogether.

For these reasons I strongly advocate only ever using your camera's lowest ISO (usually 100) and using a tripod. Only ever use high ISO settings when you have absolutely NO other choice.
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