telecentric cicada

[mtuell]

I was reading one of the posts here that mentioned that some objectives were telecentric. That got me to thinking about what a regular telecentric lens might do as a macro lens, which of course is what they are for...

I borrowed an Edmund 0.25x lens from work that has a c-mount, so screwed a 10 MP USB camera on there. It has an iris, which I closed much of the way and a built-in focus ring, so I didn't even deal with putting it on a rail for this initial test. I used a cicada as a test specimen, set up 9" (225 mm) from the front of the lens, which is a longer WD than the specs claim.

http://www.edmundoptics.com/imaging-len ... ses/55349/



This is a quick stack of four images, cropped and 50% size reduction. I'm sure I can do better with lighting and other factors, but for a quick test, I think it looks pretty good. Overall, it seems like a reasonable route for small sensor long working distance work at moderate magnification.

Mike

[mtuell]

I was curious how well this lens performed in reality. The datasheet says that it is f/6 wide open and at f/10 it could get an MTF of 55% at 40 lp/mm. I've actually got an older version of this that doesn't have the "gold" collar - it may be the same unit though.

Looking at a resolution target (thanks, Steve!) of chrome on glass USAF 1951, there are many resolutions represented.



With the aperture wide open, we get a crisp image with small depth-of-field. There is white paper behind it for illumination.



Closing the aperture much of the way (probably similar to how the cicada was imaged, but I'm not sure...), we get the following, with an obvious increase in DOF (which is why I stopped it down for the cicada).



Zooming in on each of the central regions, and expanding by 300%, we can see that the 10 MP camera isn't the limiting factor, but it is close...





So, with the iris open, we can resolve group 5, element 5 or so, which is 50.8 line pairs / mm, or 101.6 lines per mm - right about 10 micron features.

However, with the iris stopped down, we can only get up to maybe group 4, element 3, or 20.1 lp/mm - which corresponds to about 25 micron features.

These are the minimum resolvable features - not what it would be easy to image, but still, not too bad from 250 mm away, with a cheap OMAX USB 10 MP camera! Looking at this target with a microscope blows this pattern out of the water - the smallest features of group 7 are resolvable even at 10x. It is useful for 2x-4x maybe.

I hope this was informative!

Mike