Prosilica - High Resolution - How high is too high?

High Resolution: How High is Too High?

Achieving high resolution performance is more than simply buying a camera with millions of pixels...

In order to achieve a high resolution image, both the camera and the optics must be capable of achieving the desired resoluion. Not only that, but the focus and aperture settings must also be optimized for maximum resolution.

There are a number of issues that need to be addressed in order to achieve high resolution performance from a digital imaging system.

Resolving Power of the Lens

Most machine vision lenses are not capable of achieving higher than megapixel resoluion. The resolving power of a lens is usually quoted in line pairs per millimeter (lp/mm) if it is quoted at all. Most inexpensive machine vision lenses offer an optical resolution of around 50 lp/mm which corresponds to a horizontal pixel resolution of about 700 pixels. Such a lens is adequate for use with a VGA resolution camera, but will not acheive megapixel resolution.

Some exceptionally good machine vision lenses have a resolving power of 100 lp/mm which corresponds to a horizontal pixel resolution of 1280 pixels (for 1/2" format). In this case, the lens will achieve megapixel resolution, but not more. If resolution is important to you, you will need to ensure that your lens resolution is capable of achieving at least the resolution of the camera that you are using.

Focus Setting of the Lens

Even when a lens is capable of high resolution performance, there are other issues that need to be addressed in order to acheive full resolution performance. For example, a slight mis-adjustment of the focus setting on your imaging system will greatly reduce the effective resolution of your imaging system. It is no use having a megapixel camera connected to a lens that, because of poor focus setting, can only achieve the equivalent of a 100 pixel resolution. Because of this, it is generally recommended to use lenses with screw-lock mechanism on the focus adjustment and to take special care in setting the focus of the camera.

Aperature Setting of the Lens

There are a number of issues surrounding the aperture setting of your lens that affect system resolution. Generally speaking, the wider the aperture, the greater the resolving power -- but the smaller the depth of field. Conversely, the smaller the aperture setting, the larger the depth of field, but the lower the resolving power of the lens. But of course, life is not quite so simple. When the lens aperature is opened fully, there are usually lens aberations that negatively affect resolution. Generally the best resolution performance of a lens is near the full-open setting, but not fully open.

Other Issues

Other issues to consider when designing a high resolution system includes the data bandwidth requirements, and the processing limitations of, the of the host computer. The higher the resolution, the higher the required data bandwidth to achieve a given frame rate. Also, the higher the resolution, the more computing power required to analyze the data.