What is chromatic aberration for an optical system? Well, according to Wiki, it is the failure of a lens to focus all colors to the same convergence point. You usually see this effect when you have high contrast edges in your image. Well, it happens not just with microscope objectives, you will also see this effect with normal photographic lenses, for example along the edges of a mountain and the sky. But I will focus on this for microscope objectives as it is rather important.
Mitutoyo QV 2.5x 0.14, a 5x 0.15 “APO”, and a generic 4x PLAN objective.
When choosing a microscope objective, very often you would encounter three common types of objective for their ability of correcting chromatic aberration: achromat, fluorite (semi-apochromat), and apochromat. The achromat objectives can correct 2 colors chromatic aberration, the fluorite (semi-apochromat) can correct 2-3 colors chromatic aberration, and finally the apochromat type of objective can correct 4-5 colors chromatic aberration. Without going to too much details, here is a good read on this subject. Knowing all of these, our intuitive instinct might be telling us that the apochromatic objectives are probably most expensive one, the fluorite type comes next, and finally the cheapest (relative speaking) is the achromat. Yeah, trust your instinct and then some. The apochromatic (often marked as APO) are many time more expensive than the a simple achromat doublet, some times even tens of times more expensive.
How do we check if an objective is what we paid for in terms of chromatic corrections? Let me just share one of my personal experience. I was once offered an exciting deal, an 5x NA of 0.15 APO M Plan objective for a “mere” 160 USD, it has whopping long working distance of 45mm, too! I was enticed and jumped on the deal. But after some stacking sesseions, it does provide a working distance of 45mm, but it does not seem to have an NA of 0.15, worse yet, it does not seem to be an APO lens at all. Why, because the images shot with this objective has a lot of chromatic aberrations!
Well, at the time, I do not have a genuine APO objective to compare with. Now I do have a beat-up used Mitutoyo QV series objective, 2.5x and NA of 0.14. So I set out to do a comparison test. The objectives are shown in the picture at the beginning of this blog and they are Mitutoyo QV 2.5x 0.14, the “APO” (or “Mitutoyo M Plan clone”), and a generic 4x Plan. Here the tube lens used is the Zhongyi 135mm and because the QV objective requires 100mm tube lens, its magnification is same as the “clone”, both are at 3.375x. The 4x PLAN objective is at 3x magnification. The subject is a piece of white paper with some patterns printed on it in black. Here is the setup:
The following is the center part. It looks the “APO” objective is holding up against the generic 4X PLAN objective in the center. It shows just a little bit of chromatic aberration as arrows are pointing to. In general, the right hand side of high contrast area shows some yellowish color while the QV does not. The 4X generic PLAN shows more chromatic aberration as the color is more or less pinkish.
However, things change quickly when comparing left hand side (or corner). The good deal “APO” objective shows severe chromatic aberration, even worse than the 4x generic PLAN objective. The Mitutoyo QV objective holds up well.
Conclusion – it is very easy to set it up to check chromatic aberration for your objectives, to create a high contrast scenario in the test subject and take some pictures. The “good deal” is really too good to be true. However, given its shiny look and its 45mm long working distance, I kept the objective as a prop in photo shoot as it looks cool. Then again, only looks cool.