Field Curvature and Depth of Field

When we think of depth of field, we talk about a plane of focus. By definition, the plane is expected to be perpendicular to the sensor. However, if you take a deep dive, you’ll come to know that all the lenses exhibit field curvature. Hence, the plane of focus is not flat in all cases. So, when you might be thinking that things are in focus, they may not be in reality. In this video, photographer Ted Forbes talks about field curvature and how you can visualize it using a simple trick:

Each lens has its typical characteristics that affect what and how much of the scene is in focus. To visualize the plane of focus, you can use a simple Photoshop filter. It’s in no way a scientific method but it does give a pretty good idea of the plane of focus.

Simply open an image in Photoshop and go to Filter>Stylize>Find Edges. The filter looks for edges that has significant transition to them. Areas in focus appear darker while other areas appear lighter. If you try this out with images taken with different lenses, you can see that the zone of focus is close to linear in some, and curved in others. So, you need to understand the characteristics of your particular lens and predict its behavior for your job.

In order to determine the depth of field you’ll be getting when using a lens at a particular aperture, learn to use the depth of field scale that you can find in some lenses. Some cameras even show the depth of field in-camera making your work more convenient.

Having an understanding of how your lens behaves, and how your aperture settings affect the depth of field is essential to have better control over how much of the scene is in focus. It’s quite important that you know them well so that you can extract the best results from the equipment you’re using.

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