Lens Iris Tutorial

Published Aug 08, 2019 13:46 PM
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Cameras, like humans, have irises, controlling the light which hits the imager and impacting image details. However, cameras have multiple types of iris options - fixed, manual, and auto, and variants of each which can be confusing for new users and over-hyped by manufacturers.

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In this tutorial, we look at:

  • Iris basics: What they do and how they work
  • How iris relates to shutter speed
  • How iris relates to F-stop
  • How opening or closing the iris impacts images
  • Auto vs. Manual iris options
  • DC auto vs. P-Iris vs. iCS lenses
  • Iris Type Rarely A Choice

In this tutorial, we explain the tradeoffs of each, how irises work and how they relate to shutter speed.

Controlling Light: Iris, Shutter, and F-Stop

Cameras control light received by either adjusting the width of the opening (iris) or the length of time the opening is open (shutter). Many confuse iris and shutter, as both are described as opening and closing, but they serve different purposes.

Here's what a lens iris looks like mostly open vs mostly closed:

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Iris Components

The animation below shows an iris moving from fully open to fully closed. The iris moves a series of thin metal components, called blades, using a small servo motor. Irises use different numbers of blades, depending on their size (9 blades shown below).

Iris Effects On Exposure

The iris has significant effect on how well or poorly an image is exposed, which affects how cameras handle bright light which may cause overexposure or how they handle low light which may cause dark images.

The video below shows a demonstration of the effects of iris, from fully open and overexposed to fully closed and dark.

Since light levels change over time, the camera has to be able to adjust how much light enters to match the current conditions. Irises are one way to do this. There are multiple iris control methods, detailed below.

Effects On F Stop

When the width of the opening (iris) is adjusted, its F number changes, reflecting that more (or less) light enters. In some cases, this could also result in the camera's depth of field (DOF) being changed and objects becoming out of focus, though this is rarely a practical problem in surveillance. See our Depth of Field Tutorial for more information and examples.

Auto Iris / P-Iris / I-CS

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Motorized iris lenses come in three variants: Auto (DC or video), P-Iris (precise), and I-CS.

  • DC auto iris lenses control the iris opening via motors built into the lens. DS auto iris lenses are most common, which use a standard cable to move the iris as instructed by the camera.
  • P-Iris lenses use a specific software driver to more precisely control the iris, which is intended to provide increases in image quality over auto iris in varying lighting conditions. However, in our tests, P-Iris lenses offered no drastic advantages.
  • I-CS lenses are relatively new, developed in 2016, developed by Axis and Computar. These lenses are similar to P-Iris, but even more complex, with the lens supplying more detailed information to the camera, with control of focus, zoom, distortion correction, and more, instead of just iris control. I-CS lenses are rare, with very limited camera support, and effectively similar to P-Iris in terms of iris control.

While both auto iris and P-iris pigtails and connectors look similar, they are not fully compatible. A DC auto iris lens may be used on a P-Iris camera (though without advanced precise iris control), but a P-Iris lens connector will not physically fit into a DC auto iris jack.

Fixed / Manual Iris

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Though auto-iris is more common, there are two lens variants which cannot be controlled by the camera: fixed and manual.

Fixed iris, as the name implies cannot be adjusted at all. It is set at the factory, typically all the way open to its lowest F stop. Fixed iris lenses are common on small form factor cameras, such as minidomes and covert cameras, but uncommon in CS and other mount types.

Manual iris lenses allow the iris to be controlled by manually rotating a ring on the lens. Though the iris is controllable from all the way open to all the way closed, manufacturers generally recommend setting manual iris lenses either fully or 90%+ open to maintain low light performance. Manual iris lenses are uncommon but still used in some specialist applications such as LPR or machine vision.

Iris Type Rarely A Choice

In the vast majority of cameras today, iris type is decided for the user, as the most popular camera form factors (domes and bullets) typically do not use interchangeable lenses. Models with motorized or manual varifocal lenses typically use auto-iris (or rarely, P-Iris) while fixed focal length models such as low cost domes and bullets generally use fixed iris.

For those using box cameras, we have not found a major difference in image quality between iris types. Some slight gains may be found in extreme scenes or in how quickly a camera adjusts to changes in light, but overall other factors have more impact. See our P-Iris Lens Performance test for full details.

If your camera supports P-Iris or I-CS, we suggest using it, as there are few, if any, drawbacks and some limited potential benefits. However, we do not recommend rejecting cameras simply because they support only DC auto iris.

The reality is that IP cameras today typically adjust the shutter speed dynamically and automatically, allowing them to handle super strong sun light (fast shutter) to pitch darkness (slow shutter).

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[Note that this tutorial was originally published in 2011, but was updated in 2016 and 2019 with more detailed examples and additional P-Iris and iCS lens information.]

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