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1 - Depth-of-field & Lens Opening

Why do different lens openings change the amount of depth-of-field?

First, you have to understand light discs and circles-of-confusion.

Apple Example

Let's say there's a highlight on a drop of water on an apple (the red circle below).

You focus on the highlight.

The point of light from the highlight spreads out as a cone as it travels to the lens on your camera.

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If you were able to watch this expanding cone of light approach the lens, it would appear as an expanding disc of light.

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This expanding light disc reaches the camera lens.

Upon exiting the lens, it becomes a diminishing light disc.

The light disc becomes a point again when you focus it on the sensor (the black line above).

Let's add an orange to the scene.

Apple & Orange

You place the orange one foot behind the apple.

There's a drop of water on the orange with a highlight.

The focus is still on the apple.

When you look through your camera’s viewfinder, the apple is sharp and the orange isn't.

Why Isn't the Orange In Focus?

The orange isn't sharp because the light disc coming from the orange is still a light disc when it reaches the sensor.

The light disc from the orange has not diminished down to the size of a point.

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If the light disc from the orange could pass through the sensor, it would diminish down to a point at a plane behind the sensor (orange line).

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Again, when the light disc from the orange reaches the sensor—it's still a disc—not a point.

This light disc that reaches the sensor is called a circle-of-confusion.

So, everything that's on the same plane as the highlight on the water drop on the apple, expand from points to become discs, and then diminish to become points once again.

Points look sharp, of course.

Everything on other planes in the scene, such as the highlight on the water drop on the orange, expand from points to become discs, and then diminish.

But they haven't diminished enough to become points again when they reach the sensor.

They're still discs.

Large discs don't look sharp, of course.

However, smaller discs look acceptably sharp.

Acceptably sharp is defined as looking sharp on an 8 x 10 inch print at a viewing distance of one foot.

These acceptably sharp discs create the zone of acceptable focus called depth-of-field.

Now, we can answer the question:

Why do different lens openings change the amount of depth-of-field?

Circles of Confusion & Lens Opening

The size of the circles of confusion is changed when they pass through the lens opening.

Small Lens Openings

Small lens openings, such as f/22, "crop" the discs of light passing through, creating smaller discs.

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These smaller discs become smaller circles of confusion when they reach the sensor.

We see the smaller discs of light, even though they're not points, as points.

They're acceptably sharp.

There's more depth-of-field.

Large Lens Openings

Large lens openings, such as f/4, allow larger discs of light to reach the sensor.

These large discs become large circles of confusion when they reach the sensor.

Because these circles of confusion are large discs, we see them as the large discs, the large circles of confusion, that they actually are.

They look out-of-focus.

There's less depth-of-field.

Summary

> Lens Opening = > Circles of Confusion = <Depth-of-field
< Lens Opening = < Circles of Confusion = > Depth-of-field

An Aside

Why does depth-of-field increase the further away the camera is from the subject?

Let's say you're photographing an orchid.

When the subject is this close to your camera, depth-of-field is critical.

Let's say you're now photographing the orchid grower in her greenhouse.

If she's is ten feet away, depth-of-field is less critical.

If your lens is labeled with focusing distances, you can see how the numbers end, after about 30 feet or so, with the infinity symbol (∞).

At infinity, everything is in focus.

There's maximum depth-of-field at the infinity symbol.

Why doesn't your camera have to be focused at 100 feet, 200 feet, or 300 feet?

So far, this writer hasn't found credible answers to the above questions.

Perhaps, when a camera is further away from the subject, only the central portions of the light discs are reaching the camera.

This is similar to the way small lens openings "crop" the light discs to their central portions, at shorter subject-to-camera distances.

Again, perhaps, the central portions of the light discs, from a distant subject, create tiny circles-of-confusion when they reach the sensor, creating more depth-of-field.