This is page 3 of 4. "Aperture" begins here.
Each lens has its own numbers and size correlation. But universally the “f-stop” number and the opening size are inversely proportional.
The largest number correlates with the smallest opening and provides the longest depth of field. The entire picture should be in focus. The smallest number correlates with the largest opening and causes a very short depth of field.
Your DSLR camera has a button you can press to see what will be in focus when you shoot. As you push the button the viewfinder grows dim and everything comes into focus. With a Point & Shoot you monitor the screen.
This again can be compared to your eyes. You know how you have to squint then everything kinda dims but comes into focus? Same thing. The ol’ eyes just ain’t what they used to be are they?
A good way to remember is; the larger the number, the more of your picture will be in focus.
The larger the number, the longer it takes light to travel through the lens while the shutter is open; meaning use of a slower shutter speed. The shutter will have to remain open just that much longer in order to get the right exposure for the shot.
Here is how all this adds up.
We have been talking about the largest number. I use the smallest number whenever possible. That’s the largest opening. It does a couple of things.
Being inversely proportional, the smallest number has the largest opening and allows the most light through the lens in the shortest amount of time.
Hence, first you get the fastest shutter speed. If you have read my "Best Camera Settings" page you already know I normally want a fast shutter speed to avoid motion blur.
Also, the smallest number gives you the shortest depth of field. That is the shortest distance within your picture that is in focus. It gives the shot a more professional look and feel by seemingly concentrating on your subject.
So, the second thing a small number does, having this short depth of field, is blurs out the background while my subject remains in clear focus. This helps my subject stand out. It is in focus while everything in front of and behind my subject is blurred.
Thirdly it helps obscure any dirt and smudges that happen to be on and between the outer most lens and the image sensor inside.
Check it out. The first shot is at f2.8, the second at f34.
Quite a difference in the amount of dirt that shows, huh!? Understand, this is super enlarged to make my point but there is always some dirt inside a camera. Even from the factory. When you use a very large number dirt and smudges are likely to show in your finish photo.
There are plenty of times to use the large numbers. For example in the church above I wanted both the foreground and the church to be in focus. So I used an aperture of f14. That was high enough to keep everything in my lens focused.
You can test to see how clean your camera is by setting your f-stop as high as it will go, set your ISO to 100 and using a tripod, shoot a picture of a blue sky. For best results make sure you manually focus your camera to infinity.