Another part of the technical aspect of photography is exposure (how bright or dark the image is).  F-Stops and shutter speeds do not only impact focus and motion but exposure.  A large F-Stop (big hole in the lens) allows a lot of light through while a small F-Stop allows less light through.  This obviously impacts how bright or dark the image is going to be.  Similarly, a long shutter speed lets the sensor see a lot of light because it is looking at the light for a long time while a fast shutter speed only lets the sensor see a glimpse of the light.

          A good way to visualize the interaction between these two settings is to think of water.  Let's say we want to fill a glass with water for a glass full of water is a good exposure (not too dark or too bright).  If the glass is not full the image is too dark.  If the glass overflows the image is too bright.  Now there are two ways in which to fill the glass.

1. You can expose the glass to an open fire hydrant for a split nano second
2. You can wait a minute for the glass to fill up while under a drip

          Both methods will fill the glass.  The size of the stream of water (fire hydrant or drip) is controlled by how much you open the faucet (or fire hydrant).  In the same way you control the size (strength) of the light stream entering your camera by the F-Stop.  Yes, the hole in the lens determines the strength of the light stream.  Your shutter speed is how long the glass (sensor) gets water (or light in our case). You can now clearly see that these two settings impact each other.  If you want the same sized glass full of water a bigger F-stop (more light) will require a faster shutter speed to prevent the water from overflowing (too bright), just like we needed to let the glass fill up in front of the fire hydrant for just a split second.  Similarly, a weak small beam of light will need time to fill up the glass, so a small F-stop requires longer shutter speeds to allow enough time for the glass to fill up.

          So what is ISO (ASA)?  Simple, ISO is the size of the glass.  A large vase requires more water to be filled than a small glass.  For our purposes here, the ISO sets your sensor's sensitivity to light.  The more sensitive the less light you need to see well.  The less sensitive the more light you need to see well.  Let's keep it simple.  The lower ISO values result in cleaner and better image quality (I am not going to explain why here).  Therefore, always use the lowest ISO possible for the situation.  There are situations where you need faster shutter speeds to freeze moving objects and you simply do not have enough light to do so.  The only way out is to use a higher ISO which requires less light for proper exposure.

          It is important to understand how these values work:

• F-Stop - Small numbers equate to big holes and large numbers equate to small holes (sorry for the confusion but that is the way it is)
• Shutter Speeds - For shutter speeds longer than a second a bigger number means a longer shutter speed and for shutter speeds shorter than a second a bigger number means a shorter shutter speed because these numbers are then technically a fraction. Remember 1/2 a second is longer than a 1/4 second even though 4 is a bigger number than 2 because we are dealing with a fraction.  A shutter speed of 1/250 is 250th of a second which is twice as fast as 1/125.
• ISO - Smaller numbers require more light while bigger numbers require less light.

          The good news about digital photography is that once you have your gear shooting is free.  So play around and shoot.  Experiment with different settings.  Change one setting and see how your camera changes the other settings to keep the glass full.

Happy learning.