Find out why understanding F-Stop and Depth of field is crucial for great Photographs

The aperture in photography is the hole in the lens through which light comes through to create an exposure. A bigger hole means that more light will reach the sensor in the camera. A smaller hole means less light will reach the sensor. One of the biggest ways the Aperture manifests itself is by it's ability to narrow the Depth of Field.

Eyes vs Cameras

Your eyes use an iris in a similar manner. When there's not a lot of light, the iris in your eye gets bigger (a bigger aperture) to allow more light to reach the retina. When it's bright outside, your iris closes and becomes small to allow less light into your eyes. Shutters in a photo camera can change size to approximate this.
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When an iris or a shutter is open wide, light at more varied angles enter the sensor, making the images more blurry in certain places
© Ray Villalobos 2009

Another interesting result of having different size shutters is that a bigger hole creates a larger number of angles by which the light enter your eyes. With a smaller hole, the light rays entering your eyes are more paralell and will appear sharper. This is why sometimes when we want to see something more clearly we squint our eyes. We're trying to get the light rays entering our eyes to be less spread out and appear sharper.

Consequently, a smaller hole has the effect of making the image appear to be sharper. A bigger hole on the other hand will appear sharp only around the area that we are focusing on. This effect in photography is called bokeh or depth of field. Think of the typical photo of the football player where only the player is in sharp focus, but the background is blurry. In that case we say that the image has a narrow depth of field because only some things are in focus.

In photography, the aperture is measured by something called an F-Stop. An F-stop is more complicated than the size of the hole, it is a ratio between the focal length of the lens and the diameter of the aperture.

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A lot of times, you want to narrow the Depth of Field to help you emphasize some subjects, but on a photo like this, it's better to make everything in the photo sharp. Say you Visit the Eiffel Tower, you'd hardly want to make the background so blurry, no-one would be able to tell you were there.
© Ray Villalobos 2009

Lenses & Focal Length

So before we can totally understand the F-Stop we have to think about defining focal length. The focal length of a lens is the distance between the center of a lens and it's focal point. The focal point is the point at which an image becomes focused. Remember when you were a kid playing with Magnifying glasses. There is a certain distance at which the objects in front of a magnifying glass come into's part of the reason why you can start a fire with one of those, the rays of light become more focused at the focal point.

In cameras, the focal length is the distance (generally in millimiters) between the optical center of the lens and the imaging sensor. I say the optical center because some cameras use multiple pieces of glass to magnify a subject (and make the lenses physically smaller). We'll talk about lenses in another class, so let's go back to measuring apertures by F-Stops.

Working With F-Stops

The F-Stops is what we care about because it is how we control aperture in camera. The F-Stop is again, a ratio between the diameter of the aperture and the focal length. So an F2 setting on a 50mm lens means the size of the hole is 25mm wide (50mm/2).

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In order to achieve a narrow Depth of FIeld, you want to shoot at your lower F-Stop possible. This effect can also be achieved by shooting in "Macro Mode", where your subject is very close to the camera. At any rate subject to lens and subject to background distance is what makes this effect possible.
© Ray Villalobos 2009

The F-Stops have a couple of characteristics that you should know about. The first is that in zoom lenses, because the focal length of the lenses change, the f-stop is generally variable as the lens gets bigger or smaller. The second one is that the numbers usually used for f-stops are designed to double or half the amount of light just like the shutter speed numbers.

Using F-Stops

There are a couple of uses for F-Stops, the most obvious one is to control the amount of light coming into the sensor. Because it's a ratio, the lower the F-Stop number, the bigger the hole and therefore the more light that comes into the sensor. The other main use for F-stops is to control the depth of field of a photograph. A bigger hole means a narrower depth of field and more bokeh.

Both F-Stops and Shutter speeds will allow you to control the quantity of light coming into the sensor, it's just a matter of working with the different lighting conditions.

Outside, it's tough to get a shot with good bokeh because there is so much light coming into the sensor, you have to limit the sunlight by using a high shutter speed. Sometimes even your maximum shutter speed will still let too much light. Therefor you have to make your aperture smaller by increasing the F-Stop. On ocassion, you can use a Neutral Density filter to allow you to shoot in those instances.

Inside, the opposite is usually the case, you have to shoot at the largest Aperture (smallest F-Stop) and a slow shutter speed, so it's tough to get shots where everything is in focus. You can make up for this by using flash photography, but the light is not always what you want.

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