What you need to know

  1. It works with Shutter Speed and ISO to determine the exposure of a picture.
  2. It affects how much of your picture will be in focus.

What it is

It’s how you know how wide the opening in your lens is.

Controlling Exposure

So what happens to the pupil when more light tries to come into the eye? It get smaller. What effect does this have on the amount of light getting into the eye? When the diameter of the pupil gets smaller less light comes into the eye.


8 sec video shows how light affects the eye

Shooting tip: In portrait photography if the subject has more light on their eye their iris get bigger making their eyes more colorful.

Understand: Diameter getting smaller means less light gets in.

New Term

Talk about the diameter of the lens and you’ll be talking about f/stop. It goes the opposite way you would think it should at first. Choosing a smaller f/stop number means the diameter is getting bigger. (More light is coming in.) (Those with experience with electrical stuff can think it’s like when choosing a wire, the American Wire Gage measurement of wires.)

The rest is in italics because it’s some background info you may not be interested in.

It’s called f/stop because it’s just not convenient to keep talking about the size of the diameter of the hole inside the lens. It’s better to talk in a number that is more closely tied to it’s affect on exposure. So they chose a system that allows you to compare diameters of opening in a way that compares it in stops of light. It makes comparing f/stop just comparing stops of light. It’s actually just a ratio of diameter to focal length. f/stop=focal length ÷ diameter. This is why f/stop is often written with the “/”  in it. This is also why why f/stop goes down as diameter gets larger.

  • Ex.1 at f/4.0 on a 200mm lens the diameter would be 50mm.
  • Ex.2  at f/8.0 on a 200mm lens the diameter would be 25mm.
  • Ex. 3 at f/5.6 on a 800mm lens the diameter would be 143mm. (Thats almost 6 inches! Thats is why super telephoto’s can weigh 10lbs and costs $13,000!)

Remember: You make the diameter in the lens smaller by selecting a larger f/stop number.

Creative Effects

Changing the aperture effects the Depth of Field.

Depth of Field

In a picture only a certain part of it is in focus.

f/2.8 can cause the DOF to only be a few inches

f/2.8 can cause the DOF to only be a few inches

To find out exactly why Aperture affects focus is a bit academic. The purely academically stuff is all in italics.

Why does Aperture affect focus? It’s based on the angle the light converges at the film plane. As the Aperture shrink the light hits the film plane (where the film or sensor is)  at a more parallel path. This means it’s easy for the correct light rays (photons) to hit the right part of the sensor. So a small diameter aperture means the light travels almost parallel. For more reading on what makes something in focus check out this article. If you don’t know what circle of confusion is this article is worth it to those who really want to know the nuts and bolts about whats going on. The only practical use this knowledge has though I can see is to understand some intricacies about what effect sensor size has on focus.

Choosing a high Aperture, like f / 14 here, makes the DOF much larger.

Choosing a high Aperture, like f / 14 here, makes the DOF much larger.

Remember: To make more of the scene in focus choose a smaller Aperture by selecting a higher f/stop number.

New Term

This portion of the subject that will reproduce clearly on the final product is known as the Depth of Focus of DOF. (Do not confuse this with Focal Length.) It’s measured in length.

Remember: To make more of the scene in focus is to increase the DOF.

Wrap up

Choosing Aperture effects the DOF and the amount of light in. You are only three concepts away from having all the fundamentals of photography (and videography).

Here is a video I did not make that explains these concepts as well.

Once you know Aperture, Shutter Speed and ISO move on to Exposure.

This article discusses how you can use this knowledge to get your pictures in focus.

If this article helped you out then it probably will help someone you know so share is below or with email.

Ask questions below, in our community page, our Google+ community or send me an email at mark@technologyformedia.com

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