MTF and high resolution sensors

You need to see more detail in your image, so just buy a higher resolution camera, right?

Wrong!

More pixels aren’t going to help you if your vision system is limited by a cheap lens. With some assistance from Geoff Adams of Linos, I’d like to explain why.

If you’ve been following my last few posts you’ll understand that the MTF graph for a lens tells you its ability to resolve fine detail. This is expressed as line pairs per millimeter (lp/mm), which you can think of as the frequency of a series of black and white stripes. So the MTF graph shows the % contrast that a lens can achieve at a given frequency.

The sensor in your camera also has maximum frequency that it can detect, which depends on the size of the individual pixels. Sparing you the physics, the maximum sensor frequency can be calculated using this formula:

Fmax-practical = 1/(4 x pixel pitch)

From this you’ll see that smaller pixels result in a higher maximum frequency. And as manufacturers are squeezing more pixels into a given piece of silicon, pixel sizes are getting smaller. In fact the latest generation of 5Mp sensors from Sony have pixels of 3.45 microns (although the pixel pitch is more like 3.97 microns.)

This means that maximum sensor frequency can now exceed the resolving power of the lens. In other words, all those extra pixels won’t do you any good at all if your lens can’t transfer that level of detail. This is why you need to look at lens MTF graphs rather than just buying what’s claimed to be a ‘high resolution” or “Megapixel” lens.

The bottom line; to get the best performance from your new high resolution camera use a lens with a higher MTF than that of the CCD or CMOS sensor.

Incidentally, Geoff very kindly shared with me the results of some lens benchmarking that Linos carried out last year. There’s not room here to share all the details, but the Linos MeVis-C lens was clearly superior to the unnamed competitors.