Depth of Field TutorialBy John Honovich, Published Oct 19, 2012, 12:00am EDT
Ignore depth of field concerns for 95%+ of surveillance applications, despite what you hear. While this is a common consideration in digital photography, its goals and logistics differ. In this note, we explain the drivers, why it is generally not a surveillance concern and the exceptional application where it is plus what can be done.
Photographers Often Prefer Shallow Fields of View
Photographers often want to emphasize a particular subject in the field of view while essentially blurring out the background. To do this, they reduce the depth of field. Take a look at the image below. A photographer would most likely prefer the one of the right if they were showcasing the figurines:
However, in surveillance, it is the opposite. One almost always want a very wide depth of field so that you do not lose details that might prove important for later investigations.
Drivers of Wide Depth of Field
The closer a subject is to the front of the FoV the shallower the depth of field will be. This can happen either because the subject is physically quite close to the camera or a long lens used. Here's an example from a youtube tutorial when the subject is very close:
Notice at 3 foot, depth of field is poor but at 9 foot, it is much better.
Now, here's an example where the subject is far away but the lens is long and the subject is the front of the FoV:
In Surveillance, Not a Common Issue
95%+ of real world surveillance consists of:
- Wide angle shots (e.g., 3 - 8mm)
- With subjects 5 or more feet away from the camera
Because of this, shallow depth of field is simply not a common field problem. Trying to deepen the FoV in these conditions will do little to no good but can cause big problems in low light.
Calculate For Yourself
Depth of Field size can be calculated mathematically. Here is a nice Depth of Field calculator we recommend.
Below is a short 3 minute video showing you the drivers and the calculator in action:
The Surveillance Exception
The most common, though still infrequent exception, is when a surveillance camera uses a long lens to observe far from the camera. Below is an example looking down a 200 foot hallway.
To eliminate the shallow depth of field above, increase the F-stop to reduce the iris opening.
However, two important conditions need to be met:
- Use a manual iris lens to close down the iris and keep it fixed there. An auto iris lens may automatically adjust the iris opening creating depth of field problems.
- Ensure that you have lots of light 24/7 or whenever you want images. Closing down the iris will create terrible image quality with even moderate low light.
All in all, do not worry about depth of field but if you have a rare case where it is an issue, think carefully about the tradeoffs and issues involved.
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