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Monday, December 30, 2013

Introduction to Photography Exposure Terms – Part 4: Exposure

Written by Marc B

Last week we introduced beginners to ISO, and in previous week’s we introduced aperture and shutter speed. This week is our fourth and final part in our Introduction to Photography Exposure Terms blogs; we’re introducing exposure ie. how to bring it all together. 
The correct exposure for an image is a combination of shutter speed, aperture value, ISO setting and scene brightness, with each of these values having an effect on the exposure. 
If any of the values of these settings change, it is necessary to change one or more of the other values to make up for it. 
For example, where an exposure of 1/125th at f8 and 200 ISO is the correct exposure for the scene brightness, and you choose to change the aperture to f5.6, more light will pass through to the sensor so the image will now be over exposed.  To accommodate this change you will need to adjust another setting. 
Assuming you couldn’t change the light levels in the scene, options would be:

·           A faster shutter speed (in this case 1/250th).  This would halve the amount of light that the shutter would let in and therefore bring the exposure back to the original value, making up for the fact that the aperture is letting in double the light as it was originally.
·           Lower the ISO setting to 100 ISO.  This would lower the sensitivity of the sensor by half and would also make up for the original opening of the aperture being opened up.
·           A combination of both.
Below are some examples of photos taken at the correct exposure, then under exposed by two stops and over exposed by two stops.
This is the correct exposure according to the camera - a good overall exposure with some loss of detail in the brighter and darker areas.


This photo has been under exposed by two stop of light, resulting in a much darker image as compared to the image above. There is significant loss of image information in the darker and mid-toned areas.


The below photo has been over exposed by two stops, resulting in a much brighter image as compared to the first image. There is significant loss of detail in the light areas.
 
 
This diagram shows the relationship between all the values that need to be considered to get the exposure right on a camera.


The most important thing of course, is to experiment.  Get out of Auto mode; see what happens when you change the camera settings.  It costs nothing and you have everything to gain.  If you have any questions about any of the information you’ve learnt over the course of this Introduction to Photography Exposure Terms blog posts, feel free to contact us or ask in the comments section below.

We are always happy to talk cameras!

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