The Iris Adjusts the Lens Aperture, but What Adjusts the Iris?

The eye has an iris. When we say we have blue eyes or brown eyes or green eyes, we are referring to the iris. But the iris is more than the colored circle surrounding the pupil. It acts as its diaphragm, reacting to the intensity of light, to widen or narrow the pupil which admits and focuses light on the retina at the back of the eye. Like the image sensor of a camera, the retina reacts to the light and sends a record of it, via the optic nerve, to the brain which makes sense of the image.  The iris constricts the pupil in bright light and dilates it in darker conditions.

The camera also has an iris which serves the same function. The camera iris controls the lens aperture, opening it so as to admit light to the photosites on the image sensor. Iris control for fixed surveillance cameras is of  three types:

·         Fixed  -  set at a specific circumference and cannot be adjusted.

·         Manual - allows you to adjust  the aperture by hand.

·         Automatic   - adjusts  the iris according to the prevailing light

If the camera is mounted indoors where the lighting remains the same, one can get by with either a fixed or manual iris since there is no reason to adjust the iris. However when using an outdoor camera,   automatic  iris control is needed.

 The iris setting affects image sharpness and depth of field. Depth of field refers to the distance both in front of and behind the focal point where objects share the same degree of sharpness. The deeper the field, the greater the portion of the scene that is visible. This is especially important in surveillance when covering a long corridor or escalator or a parking lot.

However, in waning light, the aperture may not admit sufficient light to pass through. The pixels that correspond to the darker portions of the image may not have enough time to collect sufficient photons, resulting in a shallow depth of field. 

A wide iris opening reduces depth of field while a narrow one increases it. The term, f-number is used to define the size of the lens opening, the higher the number, the smaller the opening. 

 

The following chart shows the effect the size of the aperture opening has  on the depth of field. A higher f-number  increases the depth while a lower f-number decreases it.

A smaller opening will also improve image sharpness.  This is because any lens will produce some sort of image aberration if the whole surface is used, so the smaller the opening, the less of the lens used, and the better the error reduction. 

However, and there’s always seems to be a however, too small an opening can actually blur an image due to what’s called diffraction. Diffraction arises in bright outdoor conditions when a lens closes its shutter too much and the light is diffracted or spread over too many photosites, resulting in loss of detail and a dull light-washed image.   Megapixel cameras compound the problem because not only do they have a large number of photosites but in many cases, the photosites are small and close together.

The following set of images illustrates diffraction at different iris settings on cameras within a range of megapixels.

More precise iris control would go a long way in decreasing diffraction, thus increasing sharpness and depth of field but unfortunately  the DC-iris lens as  mentioned above, only  controls the iris in response to light intensity . It does not allow for any finer adjustments that might result in more accurate photon collection. Axis Communications has developed a solution, jointly with Kowa, for minimizing diffraction. It is called P-Iris.

P-Iris provides automatic precise control of the iris opening. Rather than merely regulating the flow of light to the image sensor, P-iris sets the iris at the optimum  f-number at which the central and best part of the lens will be used.  The P in P-iris stands for precise.  Using this preferred setting as the default  insures better contrast, resolution and depth of field.  

But in some lighting conditions, P-iris  may not be enough. In those cases, electronic processing  is called for -  gain (amplification of the signal level),  or an alteration  in exposure time.  Either or both can  optimize the image quality by  maintaining  the best iris position for as long as possible.

In the rare instance when neither the preferred iris position, nor the electronic processing can correct the exposure,  cameras equipped with P-iris will automatically instruct the camera to change the iris position. Axis holds forth that any network camera equipped with P-iris will adjust itself to produce crisp high definition images with good depth of field, no matter what the lighting conditions may be, and will do it all automatically.

In addition to Axis, other camera manufacturers such as CBC Ganz and Vivotek are using the technology. If you have any questions about P-Iris cameras or any IP camera, visit Kintronics at http://www.kintronics.com/neteye/neteye.htmlor fill out a request for information form.