Reading optics diagrams

Photo: Rainer Ebert used under CC license

This post is an extra resource for the Photography 101 series I am writing for the Digital Phototgraphy School blog.

Explaining some of the concepts of optics and physics covered in the lessons with words alone, would make understanding very difficult. To help visualise optics and light, simple diagrams can be used that show the optical components, and path of light through them. I have a confession to make: I haven’t drawn these diagrams formally since my high school physics classes, so I’m probably not following the standard convention, but hopefully they’ll help nonetheless.

Diagram components:

• Lens: a pale blue spindle shape of varying thicknesses.
• Tree: a generic object used as an example subject.
• Lines: generally these represent the path of the light. They will have arrow heads on them if the direction is not obvious. If it is important to distinguish different lines, I’ll show them in different colours, this is for illustration purposes only.
• Dashed line: this line is drawn vertically through the focal point and shows the image plane – the surface where the image is focused, whether it is a screen or film or the sensor chip. This is helpful for drawing diagrams, but is often omitted from the final diagram, especially when the subject’s symbol is used to represent the projected image.
• Labels: whenever a new element is used, it will be labelled. Some concepts will be described in the diagrams with labels.
• Direction: unless otherwise indicated, the path of light is from the left to the right.

Here’s a typical diagram with some of these features labeled:

Here’s one helpful point to keep in mind when trying to work out where to draw an image as focused by a lens. If you know the focal length of the lens, draw the lens and the subject (for example our tree symbol). Draw a dashed line vertically through the focal point to represent the image plane. Now take a ruler and draw a straight line from the top of the tree (or any other point), through the centre of the lens, to the image plane. We can do this because we know that any light that passes through the center of the lens does not get bent from its path.

Now that we know where light from that part of the tree is focused on the image plane, we can draw as many additional lines as we need to illustrate our point. For example, draw a line from the top of the tree to the top of the lens, then from that point to where the first line intersects the image plane. The same can be done for the bottom, or any other part of the lens. We can do this because we know that all light will be focused at the same point.

Keeping it up to date

As the lessons progress, I may need to update this reference for a variety of reasons: to correct any errors, to expand further on future digrams used in the lessons or to provide further information or resources on optics diagrams. I will endeavour to make this reference as accurate as possible, but cannot guarantee that it will be. If you have any corrections or suggestions for improvements, please leave a comment below. If you would like to link to this reference from your own web page or blog, please feel free to do so. You may also be interested to read the lessons to which this resource refers, you can find links to them in the first paragraph of this page.