04 Apr understanding depth of field
Welcome to the depth of field master class! The general understanding of depth of field is simple enough. More in focus is a wide depth of field, less in focus is a shallow depth of field. While this is the most basic understanding, there’s is quite a bit more to talk about. To begin we must understand how light travels through the lens and onto the image sensor to create depth of field. Light is reflected off of a subject, redirected through the camera lens, and then projected onto the image sensor. Its the angle of light, changing inside the lens that is of concern. To start, we’ll need to focus our camera on a single point.
In the diagram, the point of focus is indicated by the intersecting lines. This is the most in focus point. As the lines spread from that point, light begins to scatter and that part of the image is gradually less in focus but our eyes are unable to tell. There is a point at which the part of the image is considered to be out of focus and that’s where the end of the field is. The area filled in with blue is representative of the total area in focus, or the total depth of field. This volume is fixed. Now that we have the depth of field defined, how do we change the depth of field? There are three ways. By manipulating the distance to your subject, the aperture, and the focal length.
Distance to the subject
I’m sure a few of you have noticed that when shooting, you can make a subject stand out more in your image by moving closer to your subject. How does that work? Lets take the coin example again. By changing only the distance from the coins we are able to change the angle in which the light is reflected into the camera. Farther away, makes the angle narrow, and will squeeze the blue stuff farther out, thus the depth is greater. Move closer and the angle widens, resulting in a narrow depth of field.
This is maybe the most common method of affecting change. Inside the camera lens there is an iris. Its measured with aperture value. You’ll see it written a couple different ways, either as f/2.8 or f2.8. The slash actually serves a purpose however, its the mathematical equation part. Where f= focal length of the lens, divided by the aperture, equals the diameter of the opening in the lens in millimeters. For example, a 50mm lens @ f/1.8 = 27.7mm opening. Which brings me to another point, why do people call a large aperture large if the f number is small. Because they’re really talking about two different things. I suppose its easier to say large aperture vs large iris diameter. Back to our coin example. Like the first example we had with distance, the blue is indicative of the total depth of field and is a fixed volume. In the middle of the lens there is now a representative aperture. From the top illustration, f/4 will yield a wider angle, and a more narrow depth of field. Stop it down to f/16 and nearly everything is in focus.
First, what is focal length, and how is it measured? Lenses are designated in millimeters. 10mm, 28mm etc. This is the actual distance between the image sensor and where the light converges inside the lens. The smaller the number, the wider the lens, and the larger the number the more narrow. Similarly to changing the distance to the subject, we are changing the distance again in a different way. The effect is reversed however. The larger the distance between the point of convergence, as indicated by the cyan lens below, the larger the angle and more narrow depth of field.
Lets bring all this together now!
If you got this far, great job! If you learned something new, please let me know in the comments below!