If you're shooting: ISO 100, f1.4 @ 1/1000
second, it is the same as:ISO 200
, f1.4 @ 1/2000
second, orISO 400
, f1.4 @ 1/4000
second, etc.. Because each time you double the ISO value, you need 1/2
the light for a proper exposure. The ISO is the camera sensor (or film) "sensitivity to light". The higher the ISO, the more sensitive it is. That's why in the examples above, the shutter is opened 1/2
as long (or it is twice as fast - whichever you like to look at it). But it doesn't stop there..
That same ISO 100, f1.4 @ 1/1000 second picture is also the same as:
ISO 100, f2.0
ISO 100, f2.8
ISO 100, f4.0
second, etc.. This is because each FULL
f-stop (1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22 and 32
) each allow TWICE
as much light than the previous (higher f-stop number). f1.4 allows 2x
more light than 2.0, which allows 2x
more than 2.8 which allows 2x
more than 4.0, and so on. So, if you get twice the light from one aperture than the previous full f-stop, and the ISO is the same, then the length of time the shutter is open must be reduced by 1/2. Hence, 1/500 is half as long as 1/1000, which is half as long as /12000, etc.
It can be represented like the exposure triangle below:
All this shows is that all three variables control the exposure. If your main objective is to change the Depth of Field (DoF), adjust Aperture and one or more of the others to get a properly exposed picture. Likewise, if you want to suggest or stop motion, you'd adjust shutter speed first - faster to stop the motion or slower to suggest motion by creating blur. ISO introduces grain to the image. The lower the the ISO value, the finer the grain is (may not even be perceptible). The smoothest color gradients come from the lowest ISO values - but they need to most light. A tripod may be needed unless shooting in direct sunlight or other brightly lit subject. ISO is a lifesaver for poorly lit subjects, night time photography, or other indoor shooting without a tripod or speedlight. The ability to shoot good looking pictures at ISO 3200 means that you need only 1/32 of the light needed when shooting at ISO 100. That means that under the right circumstances, you could hand hold the camera at ISO 3200 when the same picture taken at ISO 100 would take 32x longer. Of course, grain comes into the mix here. It may be too grainy for your likes. Experiment to how high you can set your ISO with acceptable results.
Below is a chart of the full shutter speeds, stops and ISO values. Many cameras break these down further into 1/3 steps for even more minute control. Basically, if you change the value of either shutter speed, f-stop or ISO values 1, 2, 3, 6, 9 - or however many steps - you need to adjust one or both of the others an equivalent amount to compensate to get a properly exposed picture.
Lastly, make sure you haven't set exposure compensation to a negative value. Press and hold the the "+/-" button (has a green dot) on the top panel next to the shutter release button. Spin the rear thumb dial so that it is niether plus or minus. Minus makes the picture dark (underexposed) and Plus makes it brighter (overexposed).
I hope this was helpful and good luck! Please rate my reply - thanks!