Part 4 - The relationship between aperture, shutter speed, and ISO

 Making the right exposure in auto modes

Nairn Falls.  Left +1  Middle 0  Right -1  Bracketing 3 frames by one stop.

When light falls onto a digital light sensor, such as a CMOS chip in a digital camera, the tiny wells making up the individual pixels generate small amounts of current.  That current is converted into pixels (numbers that possess position and colour)  and then processed through the camera's processors.  The data is stored on memory cards after this.  Jpegs are formed in this manner; raw files receive less processing and are stored as proprietary files.

It is important that just the right amount of light fall on the sensor making up the image.  Too much light causes overexposure (top image, left) while too little light causes underexposure (top image, right).  What is desired is just the right amount of light to produce a proper exposure.

The problem with over and under-exposures is that details are lost in the blown-out highlights and black shadows.  They are permanently gone for jpegs but may be retrieved from raw files depending upon numerous factors.  Although a modest amount of under and over-exposure can still produce a reasonable image, it is always best to use the correct exposure so that fewer corrections are required and light and dark values are preserved.

If you are going to err on an exposure, it is usually best to underexpose rather than overexpose.  This is especially true using raw files, where dark areas may still contain significant data while blown-out areas may have little or no useable data.

Your camera can create an exposure using various modes.  As recommended in an earlier section, aperture priority exposure mode (A) tends to be the best as it gives good flexibility over setting aperture and shutter speed settings while not limiting exposure combinations.  Other auto modes including shutter priority (S or Tv), program (P), and even Auto will also allow you to capture images without spending too much time worrying about particular settings.  Remember that you can alter ISO to tweak the resulting aperture and shutter speed values according to needs or desires.

Exposure compensation (+/-), Exposure lock (*), and manual mode (M) are ways to alter the exposure from what the camera recommends.  Even though modern cameras and metering systems are capable of making food predictions on what the proper exposure should be most of the time, they are not always right.  You can use these methods to alter the camera's given exposure to compensate for the fact that backgrounds or shade predominance may cause an improper exposure.  Have a look at some examples below.

We will look into this more on another section.

Thanks for reading.

Eric Svendsen     www.ericspix.com