Category: Tutorial

In our previous blog, Demystifying lens specs,  we discussed the Modulation Transfer Function, also known as MTF, which gives you the performance of light through a medium.  The MTF helps us understand lens characteristics, however it is extremely hard to compare using manufacturer’s data sheets.  In essence, looking at a 25mm / f1.4 lens from vendor A to vendor B may look similar with basic information, but they are not!  Not all lenses are created equal and in turn need extensive data for comparison. 

The problem is that most lens manufacturers DO NOT supply MTF information, or do not supply complete MTFs. Lens manufacturers with high quality optics, such as Schneider Optics, are one of the few that provide a complete set of MTFs vs. transmission.  (See an example on Pg 2 on this datasheet.)  Many will provide basic information in terms of line pairs/mm (lp/mm) measured in the center of the lens, however this is still not enough for a true comparison.  MTF data will vary with aperture (f3), light intensity and distance from the center to edges.  In turn, if you are not comparing “apples to apples”, you cannot draw a conclusion on which is the better lens.

The short answer to this is: Not so easily! First off, the MTF of a computer monitor is probably around 30 lp/mm. All the lenses we are discussing in this blog are at least 2x this, if not 3 or 4x it. So the limiting factor is the monitor, and you will not be able to see any differences. If you have a resolution chart, and some software where you can get the actual pixel data and plot it vs. the test pattern, you can get a better idea. However, a fairly rigid test set up with constant lighting, constant exact FOV and other identical parameters is required. This is a very lengthy process and requires special equipment. True optical testing is the correct way to determine and compare MTF.

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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.  

For all your imaging needs, you can visit www.1stvision or contact us! to discuss your application in detail.