Understanding Exposure (by Calvin)
A fellow G35 owner and photographer did an excellent write up on our forum and I thought to share this with the aspiring photographers, or people that just want to understand a little more about how their camera works (even in auto mode behind the scenes).. Understanding these simple rules will help you to get more consistent shots from your DSLR, or non-auto programmable modes on your point and shoot.. Thanks for this Calvin.. Great job..
Understanding Exposure (G35Driver Edition)
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I've seen quite a few new members in this section, so I figured I'd throw this together to help get people started.
Two Ways to Control Light (ignoring ISO for now)
1) The shutter controls the duration of the exposure. A fast shutter speed (1/250 of a second) allows less light to reach the sensor than a slow shutter speed (1/4 of a second). The decision to use a fast or slow shutter speed depends upon the subject matter (i.e. if you're trying to freeze motion, use a fast shutter speed).
It should be noted that your camera will often display shutter speeds as whole numbers, even though they represent fractions of a second. For example, if you're camera displays 250, it is actually 1/250 of a second.
2) The aperture controls the intensity of light reaching the sensor. A small aperture (f/22) allows less light to reach the sensor than a large aperture (f/2). The decision to use a small or large aperture depends on the available light for exposure and the decision to use a great or small depth of field.
Shutter Speeds (in whole stops)
1 | 2 | 4 | 8 | 16 | 30 | 60 | 125 | 250 | 500 | 1000
slow shutter < more light fast shutter > less light
*Note: Cameras typically show shutter speeds in between full stops, too. For example, my camera has 640 and 800 between 500 and 1000.Aperture Values (in whole stops)
1.4 | 2 | 2.8 | 4 | 5.6 | 8 | 11 | 16 | 22
large aperture > more light
small aperture < less light
*Note: Cameras typically show aperture values in between full stops, too. For example, my camera has f/3.2 and f/3.5 between f/2.8 and f/4.Opening up & Stopping down
A stop is equal to one unit of exposure; it is equal to halving or doubling an exposure.
1/60 of a second allows twice as much light to reach the sensor as 1/125 of a second - this is one stop. Going from 1/125 to 1/60 is opening up one stop.
1/500 of a second allows half as much light to reach the sensor as 1/250 of a second - this is one stop. Going from 1/250to 1/500 is stopping down one stop.
f/5.6 allows twice as much light to enter the camera as f/8 - this is one stop. Going from f/8 to f/5.6 is opening up one stop.
f/16 allows half as much light to enter the camera as f/11 - this is one stop. Going from f/11 to f/16 is stopping down one stop.
Opening up one stop will double the amount of light reaching the sensor.
Stopping down one stop will halve the amount of light reaching the sensor.Law of Reciprocity
Exposure = Intensity * Time
Recall that aperture controls the intensity of light and shutter speed controls the duration of the exposure.Let's say that f/16 * 1/250 gives you a correct exposure. Any change in aperture or shutter speed will give you an incorrect exposure. If you change the aperture to f/11 and keep the shutter at /125, then the resulting image will be overexposed by one stop. The Law of Reciprocity says you need to reciprocate by changing the shutter speed to 1/500 (one stop) to compensate. Remember that going from f/16 to f/11 you are opening up one stop (more light), so you need to go from stop down from 1/250 to 1/500 (one stop). This will maintain the balance and keep the correct exposure.
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So let's say I'm getting a correct exposure at f/4 * 1/125 (remember, the table and your camera will show this as just 125, without the 1/___).
What would my aperture be if I wanted to open up two stops? And what shutter speed would I need to use to compensate for this change in aperture while maintaining a correct exposure?
