Machine vision lenses from various manufacturers may look similar, have identical focal lengths, but perform different… but why?
The images above were taken with the same 5MP CCD GigE camera, identical iris and focus setting BUT with two different $250 class “Megapixel” C-mount lenses. What lens would you choose?
The correct selection would be the lens that resolves the sensor pixel size and provides you with crisp images. Too many times we have seen lenses paired incorrectly providing blurred images as seen on the right image even if they are classified as “Megapixel” lenses.
This can be avoided by understanding the lens performance in terms of the modulation transfer function also known as MTF which gives you the performance of light through a medium. It compares the intensity of the light before the optics vs. the intensity of the light after it goes through the optics. This is not a single number, but rather it varies as light hits the lens on or off axis, and is also dependent upon wavelength of the light. MTF is normally given in line pairs/mm or lp/mm vs. % transmission. Essentially, it tells you how well the lens can resolve a certain size spot. If you draw lines that get closer and closer together, at some point the optics system is going to see the 2 lines as a single blurred line. This is basically where the lens breaks down, and this is just past the limit of its resolving power.
In the diagram below you can see as the lines get closer together the intensity fades. (picture courtesy of Schneider Optics)
Note: Some lens manufacturers give MTF as only lp/mm and not vs. % transmission. E.g 60
lp/mm. This does not mean that you cannot see objects smaller than this MTF, it is just that the intensity of the image is lower than 100% at this rating. As the intensity drops at some point your eye or the processing SW can not distinguish between line pairs.
Ideally, the total MTF is derived from a multiplying all the MTFs of the system. This would include the MTF of the lens, the filter, the camera, the electronics, etc.
So if you have a megapixel sensor with a high MTF, but put a low cost lens in front, you have degraded the MTF of the system. Garbage in, garbage out!
The bottom line is to know the pixel size of the given sensor in which you can then derive the lens resolving power in terms of lp/mm. In some cases, curves are available to plot lp/mm versus contrast providing the MTF of the lens. You are now in a position to select a lens matched to your sensor!
For a comprehensive understanding on “How to Choose a Lens”, download our whitepaper HERE.