Tuesday, April 17, 2012

Evaluating lenses – the most important tip!

I’ve just finished upgrading an inspection system that I installed a year ago. Besides some software improvements the most important thing I did was upgrade the lens.

The issue, as is so often the case, was that once detection of flaws of a certain size has been achieved, the quality people asked if the system could do better. This led me to evaluate a number of lenses, but also highlighted an essential aspect of lens evaluation: you can’t do it by eye!

All too often I see people who should know better trying to decide if lens A is better than lens B simply by looking at the images on a monitor.

What’s wrong with this picture?

Well how do you know the monitor is accurately displaying what your camera and lens combo see?

If the camera is just VGA resolution then the monitor probably presents a pretty accurate facsimile of what has actually been captured by the sensor. This is because most monitors have beeter than 640 x 480 pixel resolution. But if you’re playing with a 5Mp camera with a 2,500 x 2,000 pixel sensor some compression will be involved to display the image to your eyes.

And let’s not even start talking about how subjective and uncalibrated human vision is anyway.

So what do you do to assess performance?

You use your vision tools, especially edge detection, to produce numbers that indicate the relative ability of each lens to pass photons to the sensor. That way you’re being objective, and that can’t be bad, can it?

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