Understanding Digital Camera Exposure Settings
A brief explanation of how a camera determines what exposure setting to use when taking a photograph:
The camera has to adjust the amount of light coming into the camera to end up with the exact amount of light to create a perfect exposure.
The amount of light is always the same.
What changes is the ambient light surrounding the camera.
In Auto mode....
On a bright sunny day, there is too much light and the camera must choose a shutter speed and an aperture setting that will limit the amount of light getting into the camera.
The ISO will be set to its lowest setting (lets us say it is ISO-50).
The light meter built into the camera will measure the available light and based on that measurement, the aperture and shutter speeds settings are determined.
And here is what the camera has to choose from on this sunny day:
Shutter speed in fractions of a second ----1/500 - 1/250 - 1/125 - 1/60 - 1/30 - 1/15
Aperture in f-stops-----------------------------------2.8------4------5.8---------8-----11-----16
Any of those settings will give you a perfect exposure.
Whoever wrote the firmware for the camera will determine which setting the camera will use for this shot. Most likely they will choose a setting near the middle.
All digital camera have various mode settings.
If the camera is put in "sports" mode, the camera will choose a setting near the left end of the chart above. Because this offers a faster shutter speed.
If the camera has manual controls, you can select the exact shutter speed you want. I usually start with 1/250 of a second and then check to see if that eliminated any motion blurring in the trial photo.
You will notice that I stopped the aperture setting on the left side at 2.8
That is because that in this example the lens on the camera is rated at f2.8. That is the limit that this camera can handle.
How far the right end of the chart can be extended is dependent upon the darkest f-setting the camera can provide. Most small digital cameras stop at about f8. DSLR cameras can go much further.
The right end of the shutter speed chart can be extended into several seconds (exact amount is determined by your camera specifications).
Now....the sun goes behind a cloud, the light meter in the camera has to adjust the sample chart above.
Image that the shutter speed part of the chart remains the same and the aperture part moves to the right by two notches.
Notice that your maximum possible shutter speed went from 1/500th of a second to 1/125th of a second.
Well shucks....I wanted to catch that race car at 1/250th of a second.
Now imagine I double the ISO setting from 50 to 100.
That will move that f-stop part of the chart to the left by one notch. Now I can shoot at 1/250th of a second.
That is exposure in a nutshell.
Now imagine what happens when it gets dusky dark.
You will end up with a maximum shutter speed of about 1/15th of a second or slower.
You should not hand-hold a camera at a shutter speed slower than 1/60th of a second (1/30th if you have image stabilization).
You have two choices if you want a blur free photo .....
1. raise the ISO setting to 200 or 400.
2. put the camera on a tripod.
I do hope you followed all of that....it explains why a camera needs flash when it gets dark.