This post is about the most important setting people with bridge/DSLR cameras do not know about. I will tell you why in a second, but first, some technical stuff: Aperture is the size of the area through while light passes on its way to the sensor. One important thing to keep in mind is the relationship between aperture and shutter speed.

Suppose you get a perfectly exposed picture at shutter speed 1 sec, but you decide to change the aperture and make the lens opening bigger while keeping the same shutter speed. What will happen?
Your picture will become over-exposed. This is because when the lens opening becomes larger, it allows more light in to the sensor in the same amount of time. Luckily for you, if you have a bridge or a DSLR camera, it has a mode marked with an A. This mode allows you to change the aperture, and lets the camera change other settings to adjust for the picture.

Now comes more tricky part. The different settings which represent   the different size openings of the lens are called f-stop values. The tricky part that if you want the opening of the lens to be big, you have to choose a small f-stop value. The figure below shows you this:

Original Image: http://blogs.sitepointstatic.com/images/tech/2010-06-11-photography-for-the-web-1/figures/chap2_apertures.jpg

As you can see, the number is the f-stop value is actually in the demoninator but in the camera the f-stop is represented by just the number. Decreasing the denominator increases the overall value of f-stop, and increases the opening of the lens. If you actually care, the f here is the focal length and the resultant value is the actual opening of the lens. 🙂

Now that some of the science is done, let me tell you why I think this is the most important feature of modern DSLR.

Consider the following picture:

You might have noticed the shallow depth of field.

Depth of field is the distance between the closest point and the furthest point in an image which seem to be in focus. Smaller this distance, less area is in focus, and the image is set to have a shallow depth of field.

Only certain distance from the camera is in focus, and other things are blurred. This is a property which is often utilized to create distinct portraits as well as creative pictures.

So now we come to the essence of the this post. If you want shallow depth of field, reduce the f-stop value (increase aperture) to make the size of the lens bigger. This is particularly useful for portraits, close-ups and pictures with single focal point.

On the other hand, when taking pictures of landscapes and environments with many details worth capturing, increase the f-stop value (decrease aperture) to make the lens opening smaller. 🙂

The results can be seen below:

Aperture setting: f/4.5

Aperture setting: f/29

Did you notice the blurred background? 🙂