As these objectives are often used for industrial inspection (e.g. silicon wafers), I suspect that this lens was used to inspect samples of some rectangular item, and repeatedly bumped into the samples.
However, I wasn't terribly worried about the chips. As Charlie Krebs and others have written, a few chips constitute only a tiny percentage of the image forming area, and have little effect on image quality. This makes sense to me, though I've never before had a chance to test the point for myself.
More alarming to me was the way the vendor packaged the objective. Mitutoyo warns us not to "fall or drop" these lenses. And on the second-hand market, a moderate percentage (about 10 percent, in my experience) of used objectives are optically unacceptable. I suspect, but don't know, that a chief culprit in these cases is that the bad lenses have been bumped, jarred, or dropped.
Here is how the objective was packaged by the vendor. Who would ship a lens that costs $2700 new this way?
Apparently, eBay vendor "goodnamestaken" would. I know of three Mitutoyo objectives he has shipped exactly so. You might think he is more used to shipping hockey pucks, but from his history, he has sold quite a few microscope objectives. He has nothing listed on eBay at present, but if you buy from him in the future, you might want to request better packing.
One test subject I find useful at this sort of magnification is a single, dark scale from sunset moth (Urania ripheus). These dark scales are physically and visually quite different from the well-known, metallic-looking scales found elsewhere on the same moth. I find them helpful because they have holes that one can see through, that are close in size to the resolution limits of these optics. For reference, here is a shot of the complete frame, through the lens being tested:
For all images of wing scales in this thread, the following technical information applies: The camera was a Nikon D7100, which places 24 megapixels in an APS-C sized sensor; converging lens was my Bratcam Mitutoyo MT-1 tube lens assembly. Illumination was halogen continuous light through fiber optic light guides, with various modifiers. Shutter speed was eight seconds, after a 3-second delay to permit mirror slap to dissipate. Room was dark. Camera recorded jpegs. All are stacks assembled in Zerene stacker using PMax method, with "Preserve full DR" turned off and all alignment options turned off. Zerene Stacker outputs were saved as TIFF, then post-processed in Photoshop.
An image presented as above, at the 1024-pixel-wide resolution we share here at PMN--doesn't show much about the objective, other than that it isn't a total disaster. To learn more, we need to zoom in and compare with an identical image from a known objective. Here is a very tight crop, overlaid with one from my specimen of this lens. The half-micron scale bar coincides with the half-micron resolution claimed for this objective by Mitutoyo:
What does this comparison tell us? Here are my thoughts:
- 1) My lens is better than the test lens; the holes where we can see all the way through the wing scale are more clearly rendered with my lens, as are the dark structural boundaries between them.
2) However, the test lens isn't all that bad. Would it be useful or fair to describe it as: My lens is resolving about one-third micron, while the test lens is resolving about two-thirds micron?
3) I don't know what causes this difference. Is it the chips in the glass (doubtful, in my estimation), the provenance of the lens (possible, in my estimation), or manufacturing variation (probable, in my estimation)? I know from experience that multiple specimens of a single model of microscope objective usually vary in their rendering, when minutely observed. I'd be interested in occluding the chips with dark ink and re-testing, but I'll leave this for the lens' owner, and doubt, meanwhile, that it would make a noticeable difference.
4) One should ignore any sense that the wing scale appears to shift between lenses. Notice that the see-through holes (bluish white), mostly do not seem to move. The wing scale did not move, and the images are in very tight register to one another. To the degree I understand the elements that create an appearance of movement, they can safely be regarded as "noise" for the purposes of this test.
Here's a crop of the above, layered with a crop from my lens. Source is the darker orange band to the right of the left-most flake of dust. I can easily see differences between my lens and the tested lens, and prefer mine. But these differences appear much less noticeable to me than in the earlier comparison with the dark wing scale. And I suspect that many forum members will find no meaningful differences at all (perception differs greatly from one person to another).
My conclusion (though I welcome other viewpoints
): If one's goal is to post high-quality, uncropped images on the Internet, either lens will likely work as well as the other. This is probably also true for producing prints, unless they are very large and scrutinized up close. However, if one's goal is to record details at the very limit of these optics' resolution, my specimen of the lens is preferable to the tested lens. Since I enjoy supporting research, and routinely work on elucidating details at the resolution limits of my optics, I prefer my own lens over the tested lens.After reviewing my test shots within the return window, the buyer of this lens contacted the vendor about the unexpected chips in the front element. While these chips may not greatly impact image quality, they surely impact resale value. The vendor said he hadn't noticed the chips, and offered an appropriate partial refund that the buyer accepted. From my point of view, the transaction ended up as a fair deal for both, and a happy ending.
I also received and tested a 10x Mitutoyo objective from this vendor, which turned out to be an optical disaster. When contacted, the vendor returned the buyer's payment promptly and in full. So my standard advice remains: Don't be afraid to buy second-hand objectives--you can save a lot of money--but do so only with a return privilege, and test the objective against a known benchmark before accepting. If you lack the capacity to test the objective yourself, ask a forum member who has this capability to help you. Here at PMN, we help one another--so don't be afraid to ask.
Cheers,
--Chris
*The proof of the pudding is in the eating.
--edited for a few typos
) The ink is not actually black, but purple--closely resembling the most troublesome form of axial chromatic aberration.