Photography SLR basics: Shutter Speed, Aperture, ISO

Photography SLR basics

So you got a new SLR for Christmas or upgraded from your previous camera and wondering how exactly do I get the most out of this new camera?

When it comes to taking an image or exposure, knowing how the aperture, shutter speed and ISO interrelate will put you in a position to take better photos.

You can use these there elements to control the exposure of whatever you are shooting. How you do it with the dials and buttons will be specific to the camera you are using, but the principles remain the same regardless of which camera you use.

A camera works by recording light onto a light sensitive medium, either film or a digital sensor. Shutter speed, aperture and ISO all play a part in how this light is captured. We can use an analogy of filling up a glass full of water from the tap to explain each of the three functions.

Shutter speed

Inside your camera is a shutter, which remains close until you press the button to take an image. The shutter speed refers to how long the shutter is open for to let in the light and is expressed in fractions of a second. Using our glass and tap analogy, if the tap was on for only a fraction of a second, it will provide less water than if it remained on for a full minute. A long shutter speed, say ¼ of a second will allow more light than a short shutter speed of 1/400 of a second. With a long shutter speed, moving objects will be blurred, but a shorter shutter speed will be able to ‘freeze’ the action.

Slow Shutter: 1/30

Fast Shutter: 1/320

Aperture

Aperture refers to the size of the hole in the lens through which the light passes. The size of the hole can be large to let more light in or small to let less light in. The aperture is measured in ‘f-stops’ which indicate how wide the aperture is. The confusing thing about f-stops are that the smaller number, the wider the aperture is and therefore the more light is let it. A f-stop of f/2.8 will let in more light than one of f/8. This is like how much you open up the tap to fill the glass of water. If you open the tap fully, the glass will fill much more quickly than if it is only partially open.

Aperture also affects the ‘depth of field’. The depth of field refers to the portion of the image that appears sharp or in focus. A smaller f-stop (wider aperture) such as f/1.8 will have only a tiny portion of the image in focus, whilst a larger f-stop (narrower aperture) of f/16 should keep more of the image sharper or in focus.

Wider Aperture: f/1.8

Narrower Aperture: f/9

ISO

ISO refers to the sensitivity of the film or sensor. A more sensitive ISO or higher value will be able to capture more light than a less sensitive ISO value. A higher ISO would be similar to using a smaller glass, as less water is required to fill the glass.

ISO is measured as a number such as 100 being on the low end and 3200 being on the high end. The higher the ISO, the nosier the image will be with more grain, but it will allow you to capture images in low light or shoot with a faster shutter speed.

Putting it all together

All three, shutter speed, aperture and ISO numbers are measured in ‘stops’. A stop is simply double or half the amount of light you have previously. For example a shutter speed of 1/100 lets in double the light as a shutter speed of 1/200 but half as much as a shutter speed of 1/50. ISO 400 is twice as sensitive as ISO 200. An aperture of f/2.8 lets in twice as much light as f/4 but half as much as f/2.

For a given exposure, if you increase the shutter speed by one stop (double the shutter speed), you will need to increase the aperture by one stop. (1/100s @ f/2.8 is the same as 1/50s @ f/4).

On SLR cameras you can control all of these values to get the right amount of light and the correct exposure or the camera can do it for you. In ‘shutter priority mode’ you select the shutter speed and the camera will automatically work out the aperture required. In ‘aperture priority mode’ you select the aperture and the camera will work out the shutter speed required to take the correct exposure. In ‘manual’ mode you select both the shutter speed and aperture and this gives you the most control of your image.

Hope this article was useful and feel free to add comments or ask questions!

2 Comments

  1. by Catherine on August 3, 2011  2:06 pm Reply

    Congratulations! This is about the most succinct and clear explanation I have ever read on shutter speed, aperture and ISO. When are you launching your book? You really should - you'll kick all the other beginners photography books out of the market! Well done!

    • by Anthony on August 4, 2011  9:17 am Reply

      Catherine, thanks for your comment - not so sure about a book, that would be a lot of work! :)

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