Holding a Lens for Testing

OK, I have to do a test on a lens I have. I have this gadget called a lens holder which looks like a large open clamp, with an adjustment screw, which when turned tightens the rubber covered clamp tines around the edge of a lens.  It kind of reminds me of a clamp I used in chemistry lab to hold some glassware equipment? Is this thing the right way to hold my lens? Do I need a more sophisticated holder?

Well, we older folks have all been there at some time.  The true answer is, it really depends on what you are trying to do or measure regarding that lens. If you intend to make a quick and dirty measurement of the focal length of the lens and you are going to use a wooden desk ruler and a 3″ x 5″ index card to see the “image” etc., well then that clamp they gave you, which might be designed to hold an Erlenmeyer flask in the chemistry lab, could be good enough. You might have a budget of zero. We would understand.

But, if you are trying to do a sophisticated test or measurement, you will benefit from having a more sophisticated holder. There are so many what-ifs to answer.

Do you need to hold a bare glass lens or a lens in a barrel or housing? Do you need to tilt it around some axes in order to align its optical axis with something? Are you going to measure its image in air with some kind of measuring microscope? Are you going to try to “focus” that lens’s image to a fixed focal plane in space or on a detector you have? Or, (good grief) are you going to have to do interferometry using that lens? The Erlenmeyer flask clamp may not work at all in these cases. This post will get horribly long if I try to consider all of these possibilities in detail. So, to keep this one short, let’s say:

There are many kinds of opto-mechanical lens holders available. If you expect to make very small angular adjustments or very small linear movements with that lens assembly in order to make really accurate (or repeatable) focusing, or (good grief) interferometric measurements, then you are going to need a good, sturdy, stable, adjustable, lens holder, which may cost much more than that lens you have to work with! Even when the boss tells you that your measurement budget is “zero,” the fact is that your time spent in the lab is costing way more than zero. Time wasted on making a “serious” measurement or a “go/no-go test” on a lens will be worse than wasted, if your results of the test are erroneous because they gave you a cheap clamp to use.  Do good work!