When selecting a dslr lens, what maximum aperture do I need?

For beginner, one important must know before selecting an aperture. You have to know that number of aperture (or aperture value, f) and the size of aperture is inverse. The aperture value normally show as “f/5.6″, or “f-5.6″, or sometimes like “1 : 5.6″.

As smaller aperture number, as wider aperture size.
As larger aperture number, as narrower aperture size.

So, now is the question: What maximum aperture do I need?

1. Depends on the object speed
If an object is moving very fast, I need a very fast shutter speed to take its picture (may be 1/2000 second or 1/4000 second). But, do you realize that, as faster shutter speed, as darker picture. Why? Because faster shutter speed allow less light to in. So, may be I can set the aperture wider to balance this situation, because wider aperture allow more light in.

2. Depends on the lighting condition
If you want to take a picture in a low light environment (such as indoor) without flash, there has two ways to get the light that it needs. First method is adjust your shutter speed (open it for long period, may be take 2 seconds to 5 seconds). Second method is open your aperture as wider as possibble.

3. Depends on the depth of field (DOF)
As some example picture I show on previous post, wider aperture will decrease the depth of field (means make the background become more blurry). So, if you want to focus more on the object you want to capture, and reduce the impact of the busy background, you may need wider aperture.

As the maximum aperture is wider, as the cost of dslr lens more expensive. What lens you choose is still depends on your personal demand. Choose it properly, don’t pay more for the thing that you do not really need.

Below is the sample chart source from http://www.digital-slr-guide.com/

Source: http://www.shortcourses.com/use/using1-9.html

Aperture of lens is exactly like the pupil of eye, it function to control the light amount. The aperture adjusts the size of the opening through which light passes to the image sensor. The aperture can be opened up to let in more light or closed (stopped down) to let in less. In respect to just exposure, smaller apertures let less light strike the image sensor so the image is darker. Larger apertures let in more so it’s lighter.

As the aperture number gets smaller (for example, from f/16 to f/11) the aperture opening gets larger and the image gets lighter. The reason you don’t usually see this effect in your images is because when you or the camera change the aperture, the camera changes the shutter speed to keep the exposure constant.

As with the shutter speed, the aperture also affects the sharpness of your picture, but in a different way. Changing the aperture changes the depth of field, the depth in a scene from foreground to background that will be sharp in a photograph. Smaller apertures increase depth of field while larger ones decrease it. For some pictures—for example, a landscape—you may want a smaller aperture for maximum depth of field so that everything from near foreground to distant background is sharp. But perhaps in a portrait you will want a larger aperture to decrease the depth of field so that your subject is sharp but the background is soft and out of focus.

A small aperture increases depth of field so foreground and background are sharp (top) and a large aperture decreases depth of field so the background is soft (bottom).

Aperture settings are called f-stops and indicate the size of the aperture opening. Each f-stop lets in half as much light as the next larger opening and twice as much light as the next smaller opening. From the largest possible opening to increasingly smaller ones, the f-stops have traditionally been those shown to the left. No lens has the full range of settings; for example, the standard lens on a digital camera will range from about f/2 to about f/16. Notice that as the fstop number gets larger (f/8 to f/11, for example), the aperture size gets smaller. This may be easier to remember if you think of the f-number as a fraction: 1/11 is less than 1/8, just as the size of the f/11 lens opening is smaller that the size of the f/8 opening. Many high-end digital cameras have added one or two stops between each of the traditional ones. In the table to the left one-third and one-half stops are shown in red and blue respectively.

How wide you can open the aperture depends on the len’s maximum aperture— its widest opening. The term “fast lens” usually applies to lenses that can be opened to a wide maximum aperture for the focal length. For example, a lens with a maximum aperture of f/1.8 opens wider, and is faster, than a lens with a maximum aperture of f/2.6. Faster lenses are better when photographing in dim light or photographing fast moving subjects. With most, but not all, zoom lenses the maximum aperture changes as you zoom the lens. It will be larger when zoomed out to a wide angle, and smaller when zoomed in to enlarge a subject.

Tips: To get smaller apertures increase the ISO. To get larger apertures, use a neutral density filter.

Reference: http://www.digital-cameras-help.com/aperture.html

Aperture is a device that controls the quantity of light that passes through the lens. It is an iris type mechanism, which shrinks or grows in order to let in less or more light.

Small numbers represent a large aperture, big numbers – small aperture. Most digital cameras do not have this numbers written on their lenses, but they use aperture as part of their construction. It is also the way for you to select aperture priority shooting mode from your camera to control the depth of field.

Picture Source: http://americanmum.blogspot.com/2008/07/get-your-blog-on-apertures.html

This is the picture sample by different aperture value.

What we can see is, while the aperture value is lower (wider aperture), the background become more blurry. Because the background is exceed the Depth of Field (DOF). When the aperture value is higher (narrower aperture), the background become clearer. So, if you want have a blurry background effect, you need a dslr lens with wide aperture to do this.

For aperture, the size of it and the value of aperture is in reverse. Wider aperture has lower value (example: f = 1.4), while narrower aperture has higher value (example: f = 20). Sometimes, this is very confusing for beginner.