F stop or Aperture, Just what the Heck is that?

Last week I wrote about shutter speed, that mechanical opening in the camera that for a specific time will stay open to allow light to come in to make your exposure or image. Fast shutter speeds like 1/250 of a second or higher will freeze action while lower shutter speeds of 1/4 or slower will let more light in or allow the image to show blur for that artsy look.

There is a second part of exposure for the control of light that does add an artistic look and that is the F stop or aperture in the lens. It looks like an iris of an eye and by making the hole bigger or smaller allows a set amount of light through the lens before it gets to the shutter. Some liken it to a window blind that depending on how much it is open or closed controls the amount of light in a room.

The reason it is important is since the shutter is opened for a set piece of time you do not want too much or too little light coming through since that would make the image too light or too dark. The balance of F stop and shutter speed contribute to the correct exposure. There is one other part of a good exposure which is the ISO or sensitivity of the chip/film that I will also blog about later. But I don’t want your head to explode with too much info for now.

So how do you balance the shutter speed and F stop? Well the good news is that with many of the newer cameras they do most of the heavy lifting for you. If you are in the creative or programed modes this is done by the camera saying to it self “Ok, the shooter wants us to be more on a fast shutter mode so we are going to need more light. Let’s open up the F stop to make a bigger hole to let more light in.” Of course this is all done in a split second and with fancy camera speak but you get the idea.

However there is also a creative aspect to F stop. All those crazy numbers of F 4, F 5.6, F 11 and such can be used to give different looks to your images. For example a low number like F 2.8 or F 4 will give a shallow depth of field (DOF). Depth of field is the part of the image that is in focus. A shallow DOF means just part of the image is sharp, like a person or flower, and the background is all blurry. A good long lens or being zoomed out to the high numbers will help in giving this effect.

At the other end numbers like F 8, F 11, or F 22 will give a huge DOF of sharpness. These are the F stops that most landscape photographers will use so all is in focus. Those flowers in the front, that tree about 50 yards out and the mountains in the background.

Here is the trade off, the low number F stops of F 4 and smaller make the iris a big hole so a lot of light is coming in. A faster shutter speed would be needed to hold back the light. The bigger numbers of F 8 and higher is a much smaller hole so the shutter speed has to be much slower to let enough light in for a good exposure. A slower shutter speed could also mean camera shake which is also why many landscape photographers also use a tripod or some kind of steady surface for the camera to rest on.

So there are two parts of the three parts of good exposure. Shutter speed that can either freeze or allow motion blur into an image and F stop which controls the light and the depth of field. Next time I will talk about the third part ISO and how all three can be used both creatively and to get those must have shots.


Bill G.

The Digital Fieldguild to Photography and Photoshop

10+ hours
if (isMyPost) { }