Written By onci on Sunday, March 28, 2010 | 7:01 PM
So, what are the effects of changing the SS or F values? Obviously changing the SS will affect how you can “stop” motion in your images. If you are shooting sports where fast action is taking place you will most likely want a fast SS. Basketball, football, swimming, lacrosse etc will need about 1/800 to 1/1000. In the image of the Osprey landing with a stick for his nest a fast SS was required to “stop” the action. If you want to make an image of a waterfall with the water showing a lot of motion blur creating that silky effect in the flow you will need a SS of 1/20 or longer.
Changing the F value will change what is known as depth of field (DOF). The DOF is defined as the portion of the scene from near to far that appears acceptably sharp in an image. While there are many things that affect DOF you can control it with any given lens by changing the F stop. The smaller the F number the shallower the DOF and the larger the F number the wider the DOF.
If you are making a landscape image where the foreground as well as the distant parts of the scene need to be in focus you will want a large F number. If you are trying to isolate a small bird by blurring the background you will want a small F number. This is generally referred to as stopping down or opening up respectively.
SETTING YOUR CAMERA TO UTILIZE THESE PRINCIPLES
Most people set their camera on “P” or “Green Square” to use their camera’s totally automatic functions. Most serious amateurs and pros use either Tv, Av or manual (M) modes. Green Square does not allow you to change F or SS; it selects what it thinks is best. P allows you to change the values but offers less flexibility in adjusting the EV to suit the conditions. Some cameras have picture icons that attempt to select parameters based on the “scene” you are shooting. This does not work well for serious photographers and should be avoided.
Manual allows you to use the meter to read the light and you set the SS/F stop combination best suited for the conditions. The choice of which mode to use will depend on your subject matter and conditions. If you are shooting fast sports where stopping action is the priority, set your camera to Tv and select the appropriate SS. If choosing DOF is the priority, set your camera to Av and select the desired F value.
Manual is more complicated but allows you to maintain the proper exposure on a subject regardless of background brightness. For those old enough to remember when built in exposure meters first came out in 35 mm cameras using manual mode is similar to the old “match needle” adjustments we used to do. If a bird is flying against bright blue sky and you choose an EV to properly expose the bird using Tv or Av the exposure will change if the bird flies in front of a dark forest. When this occurs the bird will be over exposed as the camera will try to lighten up the forest. Manual allows you to maintain the correct bird exposure regardless of the BG.
Sometimes you will encounter situations when you can’t achieve the combination of DOF and SS your image requires. This often occurs when a fast SS is required and your camera just can’t go to a wider aperture because it isn’t available on your system. This is when you can change the ISO to a larger number. In the “old days” this meant changing film to a faster (higher) ASA number. Modern digital cameras allow you to select various ISO values (this is what the old ASA is now called). Selecting a higher ISO is really like turning up the volume of the amplifier used to send your image data to your memory card. It does not come without compromise. Turning up the ISO introduces noise in the image. ISOs up to about 800 are very usable on most cameras. However, only high end pro level cameras can produce noise free images above 1000.
I hope you have learned something from this article. In another section to be published later I will discuss modifying the EV. This process is called Exposure Compensation and is also vital to obtaining professional quality images.
Posted by onci at 7:01 PM