Essential basics 1 - Exposures

Before going into exposure, it's worth while taking a step back and understanding how your camera works.

A camera is essentially a tool to capture light, and light is manipulated via a set of glass in the lens to create photographic effects such as magnification, etc.

Observe the diagram below (forgive my crap drawing, hehe),


Light reflected off a subject travels through the aperture and a set of glass that acts as lenses, ending up on the sensor where the light information in then picked off and converted to digital images.

For this article, we will discuss about exposures. You should strive to get correctly exposed photographs. A picture can either be underexposed, correctly exposed, or overexposed. And underexposed picture appears dark while and overexposed picture appears overly bright. Too much light entering the sensor leads to overexposed images while too little leads to underexposure.

Exposure is determined by three variables, known by some as the triangle of exposure.

The variables are:

1. Aperture
2. Shutter speed
   
3. ISO sensitivity
   

• Aperture is essentially the gate to the lenses, the larger the aperture opens, the more light enters. (I.e, the larger a gate opens, more people can pass through them at any one time).
• Shutter speed is essentially how long the gate stays open. So logically the longer the shutter speed, the more light is able to enter and reach the sensor.
• Finally, ISO sensitivity is just how sensitive the sensor is to light.

Behind 'Auto'

Exposure Compensation
In the 'auto' setting, the camera does all three automatically. It 'meters' the subject (i.e, calculates the exposure) and then adjusts the variables to achieve the correct exposure.

However, there are some circumstances when the camera fails to 'meter' correctly. In general, two situations will almost always cause the camera to fail:

1. Subject is too white. When the subject has too much white in it, the camera gets confused and tries to compensate the whiteness by decreasing the exposure. It will result in an underexposed picture. A good example is taking pictures while skiing, with snow in the background, or during weddings, where everyone dresses white.
   
2. Subject is too dark. When there's too much black, such as taking a picture against a dark wall, taking pictures when everyone dresses in suits. The camera thinks the scene is darker than it actually is and tries to increase the exposure.
   

To correct this, you'll need to override your camera's automatic metering by doing 'exposure compensation'. It's like trying to tell the camera "dude, you're wrong".

In most cameras, the exposure compensation setting should look like this. The button is usually the +/- button seen to the left of the picture. Just move the cursor left (more -ve) to the "Zero" or right (more +ve) to undercompensate or overcompensate.

In general:

• In white places overcompensate.
• In black places undercompensate.

The decision on how much to set the exposure compensation is partly guided by the histogram, and partly guided by what you wanted the picture to end up as.

The histogram looks like this:
It usually is a bell curve with the apex of the curve more towards the left or right depending on how bright or dark the picture is.

More importantly is the ends of the curve. You'll want to adjust the exposure compensation so that the ends of the curve touches the end of the graph.

In this example, the picture is underexposed. You should increase the exposure compensation in the camera. This would shift the whole curve to the right and finally corrects the exposure.

On the other hand, an overexposed picture will have nothing on the left side and you simple need to decrease exposure compensation.

Here's an explanation I shamelessly copied from another site. hehe

The great thing about a digital camera in this aspect is that you can do this live while preparing your shot. The preview screen can usually overlay this information.

For the rest of us however, the only way to get it right is through trial and error.

You can, if you wish do this during post processing using any programs in the market, including free programs like Picasa or more expensive and powerful ones like Photoshop CS4. The good side is you don't have to worry too much about exposure while shooting, the bad side is that doing this usually increases the noise levels of the pictures and you might loose some details.

It is safer to underexpose that to overexpose. When overexposed, the information of a picture is lost as 'highlights'. The part of the picture looks purely white and there is no way to recover that data through post-processing.

Beyond 'auto'

This portion of the article may not be too relevant to digital camera users. However, a knowledge of some of this may help in your photography.

Most prosumers and dSLRs come with manual and semi manual functions. Using these functions gives you more control of your scene and allows you to be more creative than simply point and shoot. Although some may find it difficult to understand, here's some simple explaination on how it all works.


As you can see above, Nikon labels the manual function as M for manual, A for aperture priority and S for shutter priority. Canon's a little less intuitive with M, AV and TV respectively. In manual --> you can set both the shutter speed and aperture manually

• In A/AV --> you set the aperture manually and the camera sets the shutter speed automatically (you can use exposure compensation here since the camera is still doing automatic metering)
• In S/TV -->you set the shutter speed and the camera automatically sets the aperture.
  

In all three cases (and auto), you can decide on which ISO sensitivity to use.

The next question is, what settings should I use?
Look at the triangle of exposure again. In general, increasing one variable increases the overrall exposure, therefore another variable needs to be decreased. Which variable you increase depends largly on what you wanted to achive and what tradeoffs you want to sacrifice.

Aperture is measured as f-stops. The larger the number, the smaller the aperture.

picture taken from http://jackarcestudio.com/?p=291

• A wider aperture (f2.8, f4.0) gives you more light, but less depth of field.
  
• A narrow aperture gives you less light, but more depth of field.
  

picture taken from http://tomasellodesign.wordpress.com/2009/04/20/little-tip/

A smaller depth of field allows you to focus on the subject and blurs the background. This allows the viewer to concentrate on what you really want them to see and creates a nice creamy background as a backdrop. This effect is called 'bokeh'.

This picture of the bee for example shows a clear bee and blurred backdrop.

Shutter speed:
Shutter speed is measured in seconds and fraction of seconds.

• A faster shutter speed allows less light (less exposure) but freezes movement. This allows sharper images to be taken and also allow you to creatively freeze objects in midair, freeze sports, etc.
  
• A slower shutter speed allows more light (more exposure) but it leads to blur and camera shake if no tripod is used. It is more suitable in night photography, night scenery, fireworks, etc.
  

ISO sensitivity:

• ISO sensitivity is how sensitive the sensor is for light. The more sensitive it is, the more light you can get and it allows for a faster shutter speed and narrower aperture to be used. It sounds all good but it also translate directly to more noise in the camera.
• Noise in some aspect is good, it gives a nice texture to black and white photographs.
• Typically use 200 ISO for normal light, about 400 ISO for indoors with adequate lighting and in dark areas, you can push the ISO as high as you like within limits of your camera and the noise produced.
  

In conclusion, before you shoot, decide what effect you want (freeze, scenery, less/more noise) and select the appropriate settings.

Feel free to drop by any questions in the comments area. Thanks for reading!