Depth of field and aperture relationship goals

Focusing Basics | Aperture and Depth of Field

depth of field and aperture relationship goals

What is depth of field? Understanding depth of field is one of the first big hurdles in photography. Knowing how your aperture, focal length and. My goal with this step-by-step depth of field tutorial is to help you easily overcome Depth of field depends on aperture, focus distance, focal length and circle of to show you the relationship between depth of field and aperture numerically. Understanding depth of field is crucial to creating a good photo. Understanding the relationship between background and foreground is key to where you want to use a shallow DoF since the photographer's goal is to highlight the subject.

The basic factors determining DOF are: There are other technical factors involving the sensor or film in your camera, but we're not going to worry about those, because they are basically a given and out of your control. Compare the two following illustrations to see how changing the focal length and aperture affect the DOF of an image.

Example 1 Example 2 I'm not going to explain the math of why this happens. All you need to understand are the following general rules of thumb: The wider the aperture, the shallower the DOF e. In each case I'm focusing on the red guitar. In fact, the DOF extends to infinity in this image. Even the buildings in the background, on the horizon, are in focus. If you want a fancy term to toss around at cocktail parties, when you focus on an object beyond what is known as the hyperfocal distance, you produce a depth of field that extends to infinity in the background.

If nothing else had changed, moving farther from my subject would increase the DOF, but this factor was overwhelmed by radical changes in aperture and focal length. As you can see in the image, not only has the distant background gone out of focus, but even the second guitar is slightly blurred.

If you do the math, the DOF for this image calculates to just over 1 foot. Everything outside of the 1-foot-deep zone around the red guitar will be out of focus, increasingly blurry with distance from that guitar.

The mm focal length gives us a tight field of view the relatively wide aperture makes for a shallow DOF. However, both of these factors are offset by moving farther away from the subject. Crunching the numbers tells me that the DOF for this image is just over 2 feet, actually slightly deeper than in Image 2. Despite the big increase in focal length, the greater distance and slightly smaller aperture combined to give us a wee bit more DOF than in the previous image but still not enough for anything but the red guitar to be in focus.

I can't keep the entire red guitar in the frame any more. They can do, but the choice of aperture has to be balanced with the shutter speed and ISO in order to maintain a consistent exposure. Check out our guide to the Exposure Triangle for a more detailed explanation, but here's a brief overview.

The relationship between aperture and depth of field

Larger apertures let in more light, so faster shutter speeds can be used to freeze movement. Switch to a smaller aperture, and the amount of light passing through the lens is reduced. Consequently, the shutter speed has to become slower, increasing the risk of camera shake and subject movement. To get round this, you could increase the ISO.

This allows you to use smaller apertures to increase the depth of field and use faster shutter speeds.

Depth of field explained | TechRadar

Okay, so how does the type of camera affect depth of field? It's the size of the imaging sensor inside the camera that makes the difference. The larger the sensor, the shallower the depth of field will be at a given aperture. This is because you'll need to use a longer focal length or be physically closer to a subject in order to achieve the same image size as you get using a camera with a smaller sensor - and remember the effect that focusing closer has on depth of field.

Is it true that longer lenses produce a shallower depth of field? The focal length of the lens does appear to have a significant impact on depth of field, with longer lenses producing much more blur. A mm lens focused at 12ft will have a wafer-thin depth of field compared to a 20mm lens focused at 12ft.

However, if the subject occupies the same proportion of the frame, the depth of field the area that appears sharp is essentially the same whether you're shooting with a wide-angle lens or a telephoto! You would, of course, have to move closer with a wide lens or further away with a telephoto lens to maintain the same subject size.

Tutorial: Understanding Depth-of-Field

So the diameter of the cone is mostly smaller than a pixel. Therefore almost every part of the frame appears to be in focus. Extremely small aperture would let only a ray of light from each point of frame to each point of sensor and everything would appear on focus.

There's a reason macro lenses often usually? As Mark Scott Abeln said, it is simple geometry. Consider a point in the background.

The Relationship Between Aperture and Depth of Field

The light rays entering the camera from that point form a cone. Where that cone of light crosses the subject plane on which the camera is focussed it appears as a disc of light.

depth of field and aperture relationship goals

That is how the point in the background appears in the image: If that blur disc is smaller than a certain critical size, the image appears to be sharp and the background point is said to be within the depth of field. Simple geometry shows you that the further away the background point becomes, the larger will be the diameter of the blur disc where the cone of light crosses the subject plane.

If the background point moves out to infinity, the blur disc diameter will be equal to the lens aperture, so this defines the maximum size of the blur disc. All this ignores diffraction, which comes into play only if the aperture is small enough.

The f-stop is the ratio of the focal length of the lens to the diameter of the virtual entrance pupil the apparent diameter of the opening in the lens when viewed from the front of the lens. OK This affects the picture's depth of field, which is the plane in which objects appear in focus. Technically, a plane has no thickness. So the Depth of Field is the distance from the farthest beyond the plane to the farthest in front of the plane where points are rendered in the image as acceptably sharp.

depth of field and aperture relationship goals

Now, I've been told that the actual focus plane is always a really thin slice Yes. It is a true plane - it has no thickness. And it is called the "subject plane". The focal plane is where the sensor or film lies. Any point on the subject plane will be rendered as a point on the image ignoring the effects of diffraction and lens aberrations.

That is to say, all the light coming from a point on the subject at the focus distance will, no matter where it strikes the lens, be refracted to the same point in the image. All the light from a point in front of the plane will be refracted to a point in front of the image sensor, and then will continue on to fall in a circle on the sensor. If that circle is sufficiently small, our eyes won't able to tell the difference between that circle and a point.

In that case we call the light from the point in the scene "acceptably sharp" and that point in the scene is within the depth of field. Meanwhile all the light from a point beyond the subject plane will be diffracted to a point beyond the focal plane, but before it gets there it will fall on the sensor or film in a circle. Again, if that circle is sufficiently small, that point in the scene is within the depth of field.