You
probably know that MTF charts are the way to assess the performance
of a lens. What you may not know is that those charts are not very
helpful. Writing in the July/August 2013 edition of Vision Systems
Design magazine Andy Wilson summed it up this way:
“Although
MTF charts may allow similar lenses from a single manufacturer to be
compared, the different testing methods used by individual
manufacturer makes it difficult to compare lenses from different
vendors. Finally, since other factors such as filters, imaging
sensors and camera readout electronics will also affect image
quality, MTF charts should only be used as a starting point when
comparing lenses.”
So
if MTF charts are the start point, what comes next?
Actually,
it’s really hard. I have two techniques, both of which rely on
having the lenses in my hands. First, I set up a simple line pair
test and measure the contrast. I think it’s important to do this
out at the periphery of the field-of-view rather than at the center.
Every lens manufacturer makes sure their products work well on the
optical axis. And of course, make sure the field-of-view and
resolution are the same for both lenses.
Second,
I weigh the lenses. It’s a crude measure I know, but glass is heavy
so I conclude that the heavier lens contains more of it. More glass
implies, to me at least, more optical elements for optimizing the
image quality, thus a heavier lens is better. Usually, a heavier lens
is more expensive, and while price doesn’t always relate to
quality, (ask any wine taster,) I do think there’s a closer
correlation than say the price of a pinot noir to how much you’ll
enjoy it.
1 comment:
http://ocularcomponentmv.blogspot.sg/2014/04/machine-vision-lens-and-camera.html
detail explanation on how lens and extension tube is selected.
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