What is Depth of Field?
Depth of field! Absolutely my favorite concept in photography. Mastering depth of field, in my opinion, is what separates the amateur from the professional photographer.
Focus and definition of depth of field.
Before talking about Depth of Field we have to talk about Focus. Focus is the point at a specific distance from the lens where the image is at its sharpest.
Usually, the focus point is where we want the subject of our picture to be. We control the focus manually with the focusing ring in the lens or via the autofocusing system of our camera.
Now, let’s give Depth of Field a definition: Depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image. In other words, in every picture, there's a distance that is in focus but also an area further away from the camera and one closer to the camera where objects or people are still fairly sharp.
Now the interesting and important part is coming up so hear me out:
Depth of field and Aperture. How do you control depth of field?
Depth of field has a direct correlation to Aperture.
Large Aperture (therefore low F-numbers) gives us a small, shallow depth of field.
Small Aperture (therefore high F-numbers) gives us a large, deep depth of field.
In this picture, the focus is on the letter D, on the left side of the image. This picture was taken with the aperture, with the diaphragm fully open. The setting was F1.4. As you can see the letter D is in focus but as we move further away from the camera the other letters and numbers quickly become blurred. So here, in this case, we have shallow Depth of Field.
As we stop down the lens the area beyond and in front of the focus point becomes wider and objects in that area become sharper. In this picture, shot at F4, the focus is still on the letter D. The cubes immediately beyond the first stack are now sharp.
Eventually, if we stop down the lens completely, the depth of field, which is the area of acceptable sharpness, ends up covering the whole scene. This picture was shot with a very small aperture setting. F 22. You notice how now almost everything in the frame is in focus. From the original focus point, the letter D on the left, as far as the last stack of cubes, on the right side of the frame.
Deep and shallow depth of field
Ok, now we know that Depth of Field is a byproduct of Aperture.
Large aperture translates into Shallow Depth of Field.
Small aperture translates into Deep Depth of Field, into what it's called deep focus.
Depth of Field on camera
What else affects Depth of Field? Focal length affects Depth of Field.
All settings being equal, long focal length lenses, or tele lenses, have a shallower Depth of Field. Short focal length lenses, or wide-angle lenses, have a deeper Depth of Field. This is one of the reasons why we shoot landscapes with wide-angle lenses and portraits with tele-lenses. In landscapes we want to include more, show more, because we are describing an overall view while in portraits we want to focus on the subject and show less of the background.
Distance also affects Depth of Field.
The closer the focus point is to the camera the shallower the Depth of Field becomes. So be aware. If you shoot details or close-range portraits, you will have a smaller area beyond and in front of your focus point that is still sharp. In these cases super-accurate focusing and subject tracking are essential.
There is also an interesting ratio involving Depth of Field.
The Depth of Field beyond the focus point is always greater than the Depth of Field in front of the focus point. The ratio is mostly 2 to 1. The Depth of Field beyond the subject is usually twice the size of the one in front of the subject. When you shoot very close to the subject this ratio changes and decreases. For very very close up images the ratio becomes almost equal. 1 to 1.
Maximum depth of field
Max depth of field is reached when the lens is completely stopped down (i.e. when you are using the highest F number your lens allows you.)
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