Aperture, Shutter Speed and ISO

What is Aperture?

Put most simply - Aperture is ‘the size of the opening in the lens when a picture is taken.’

When you hit the shutter release button of your camera a hole opens up that allows your cameras image sensor to catch a glimpse of the scene you’re wanting to capture. The aperture that you set impacts the size of that hole. The larger the hole the more light that gets in - the smaller the hole the less light.

Aperture is measured in ‘f-stops’. You’ll often see them referred to here as f/number - for example f/2.8, f/4, f/5.6,f/8,f/22 etc. Moving from one f-stop to the next doubles or halves the size of the amount of opening in your lens (and the amount of light getting through). Keep in mind that a change in shutter speed from one stop to the next doubles or halves the amount of light that gets in also - this means if you increase one and decrease the other you let the same amount of light in - very handy to keep in mind).


One thing that causes a lot of new photographers confusion is that large apertures (where lots of light gets through) are given f/stop smaller numbers and smaller apertures (where less light gets through) have larger f-stop numbers. So f/2.8 is in fact a much larger aperture than f/22. It seems the wrong way around when you first hear it but you’ll get the hang of it.

Read more: http://digital-photography-school.com/aperture#ixzz0POCvGCXJ


What is Shutter Speed?


As I’ve written elsewhere, defined most basically - shutter speed is ‘the amount of time that the shutter is open’.

In film photography it was the length of time that the film was exposed to the scene you’re photographing and similarly in digital photography shutter speed is the length of time that your image sensor ’sees’ the scene you’re attempting to capture.

Let me attempt to break down the topic into some bite sized pieces that should help digital camera owners trying to get their head around shutter speed:


* Shutter speed is measured in seconds - or in most cases fractions of seconds. The bigger the denominator the faster the speed (ie 1/1000 is much faster than 1/30).
* In most cases you’ll probably be using shutter speeds of 1/60th of a second or faster. This is because anything slower than this is very difficult to use without getting camera shake. Camera shake is when your camera is moving while the shutter is open and results in blur in your photos.
* If you’re using a slow shutter speed (anything slower than 1/60) you will need to either use a tripod or some some type of image stabilization (more and more cameras are coming with this built in).
* Shutter speeds available to you on your camera will usually double (approximately) with each setting. As a result you’ll usually have the options for the following shutter speeds - 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8 etc. This ‘doubling’ is handy to keep in mind as aperture settings also double the amount of light that is let in - as a result increasing shutter speed by one stop and decreasing aperture by one stop should give you similar exposure levels (but we’ll talk more about this in a future post).
* Some cameras also give you the option for very slow shutter speeds that are not fractions of seconds but are measured in seconds (for example 1 second, 10 seconds, 30 seconds etc). These are used in very low light situations, when you’re going after special effects and/or when you’re trying to capture a lot of movement in a shot). Some cameras also give you the option to shoot in ‘B’ (or ‘Bulb’) mode. Bulb mode lets you keep the shutter open for as long as you hold it down.
* When considering what shutter speed to use in an image you should always ask yourself whether anything in your scene is moving and how you’d like to capture that movement. If there is movement in your scene you have the choice of either freezing the movement (so it looks still) or letting the moving object intentionally blur (giving it a sense of movement).
* To freeze movement in an image (like in the surfing shot above) you’ll want to choose a faster shutter speed and to let the movement blur you’ll want to choose a slower shutter speed. The actual speeds you should choose will vary depending upon the speed of the subject in your shot and how much you want it to be blurred.


* Motion is not always bad - I spoke to one digital camera owner last week who told me that he always used fast shutter speeds and couldn’t understand why anyone would want motion in their images. There are times when motion is good. For example when you’re taking a photo of a waterfall and want to show how fast the water is flowing, or when you’re taking a shot of a racing car and want to give it a feeling of speed, or when you’re taking a shot of a star scape and want to show how the stars move over a longer period of time etc. In all of these instances choosing a longer shutter speed will be the way to go. However in all of these cases you need to use a tripod or you’ll run the risk of ruining the shots by adding camera movement (a different type of blur than motion blur).
* Focal Length and Shutter Speed - another thing to consider when choosing shutter speed is the focal length of the lens you’re using. Longer focal lengths will accentuate the amount of camera shake you have and so you’ll need to choose a faster shutter speed (unless you have image stabilization in your lens or camera). The ‘rule’ of thumb to use with focal length in non image stabilized situations) is to choose a shutter speed with a denominator that is larger than the focal length of the lens. For example if you have a lens that is 50mm 1/60th is probably ok but if you have a 200mm lens you’ll probably want to shoot at around 1/250.

Bringing it Together

Remember that thinking about Shutter Speed in isolation from the other two elements of the Exposure Triangle (aperture and ISO) is not really a good idea. As you change shutter speed you’ll need to change one or both of the other elements to compensate for it.

For example if you speed up your shutter speed one stop (for example from 1/125th to 1/250th) you’re effectively letting half as much light into your camera. To compensate for this you’ll probably need to increase your aperture one stop (for example from f16 to f11). The other alternative would be to choose a faster ISO rating (you might want to move from ISO 100 to ISO 400 for example).

Read more: http://digital-photography-school.com/shutter-speed#ixzz0PODhdNgt

What is ISO?

In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films - 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.

In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography - the lower the number the less sensitive your camera is to light and the finer the grain. Higher ISO settings are generally used in darker situations to get faster shutter speeds (for example an indoor sports event when you want to freeze the action in lower light) - however the cost is noisier shots. I’ll illustrate this below with two elargements of shots that I just took - the one on the left is taken at 100 ISO and the one of the right at 3200 ISO (click to enlarge to see the full effect).

Iso-1



100 ISO is generally accepted as ‘normal’ and will give you lovely crisp shots (little noise/grain).

Most people tend to keep their digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in (it will try to keep it as low as possible) but most cameras also give you the opportunity to select your own ISO also.

When you do override your camera and choose a specific ISO you’ll notice that it impacts the aperture and shutter speed needed for a well exposed shot. For example - if you bumped your ISO up from 100 to 400 you’ll notice that you can shoot at higher shutter speeds and/or smaller apertures.

When choosing the ISO setting I generally ask myself the following four questions:

1. Light - Is the subject well lit?
2. Grain - Do I want a grainy shot or one without noise?
3. Tripod - Am I use a tripod?
4. Moving Subject - Is my subject moving or stationary?

If there is plenty of light, I want little grain, I’m using a tripod and my subject is stationary I will generally use a pretty low ISO rating.

However if it’s dark, I purposely want grain, I don’t have a tripod and/or my subject is moving I might consider increasing the ISO as it will enable me to shoot with a faster shutter speed and still expose the shot well.

Of course the trade off of this increase in ISO will be noisier shots.

Situations where you might need to push ISO to higher settings include:

* Indoor Sports Events - where your subject is moving fast yet you may have limited light available.
* Concerts - also low in light and often ‘no-flash’ zones
* Art Galleries, Churches etc- many galleries have rules against using a flash and of course being indoors are not well lit.
* Birthday Parties - blowing out the candles in a dark room can give you a nice moody shot which would be ruined by a bright flash. Increasing the ISO can help capture the scene.

ISO is an important aspect of digital photography to have an understanding of if you want to gain more control of your digital camera. Experiment with different settings and how they impact your images today.

Read more: http://digital-photography-school.com/iso-settings#ixzz0POE0fGIg