Mitutoyo M Plan Apo objectives are famous as being among the best objectives for photography, but Mitutoyo also has other lesser-known objective series, including some that have never been tested anywhere on the internet. These include specialized infinity-corrected objectives, and also finite telecentric toolmakers' objectives similar to Nikon's MM series. I'll deal with the Mitutoyo telecentric toolmakers' objectives later; here I want to present the first internet test of a Mitutoyo G Plan Apo 50x objective.
For my work these odd objectives are very interesting. One of my main tasks is to photograph microstructures of orchid flowers under water or oil. These thick media can cause aberrations for normal lenses and objectives. I've written a few posts here about my solution for high resolution photography at low to moderate magnifications. However, when high magnification is needed, aberrations are much more severe, and a better solution is required.
This is where Mitutoyo's G Plan Apo objective series, and Nikon's little-known LCD Plan series, could be useful. These objectives are made to shoot through thick glass. Mitutoyo's G Plan Apo objectives are corrected for 3.5mm of glass, while Nikon's LCD Plan objectives are usually corrected for thinner glass. Either of these objective series should also be able to image through an appropriate amount of water, so I have been on the lookout for them on eBay for a long time. The G Plan Apo objectives are especially attractive because they have long working distances and are apo corrected. They are prohibitively expensive new (Edmund Scientific sells the 50x G Plan Apo 0.50 for $4380) but after a long wait one of them came up on eBay and I negotiated a price of $600 from a seller with not very good feedback (96.5%). The objective seems to be in perfect condition. I added a black filter ring to take a 3.5mm glass filter, so it can also be used in air.
I am using it with a full frame camera and a favorite FF tube lens, the Nikkor-Q 135mm f/3.5, which pushes the objective down to 34x. There is no vignetting at all on the FF sensor. A 90mm tube lens did cause very minor vignetting. In the picture above, I have removed a diffuser made of a cylinder of tracing paper. I am sure better diffusion would improve the image.
I think the optical path through 3.5mm of glass is equal to the optical path through 4mm depth of water (4mm = 3.5mm * 1.51/1.33, with the conversion factor being the ratio of the indices of refraction of the media). So I shot a wafer through exactly 4mm of water. To measure the depth of water precisely, I first focused on the surface of the wafer with no water, then moved the objective 4mm above that, and added water drop by drop until the dirt on the surface of the water was in focus. I then refocused on the wafer; this required moving the objective 2.961mm down towards the wafer. I then made a normal stack. The stack log showed that the objective is telecentric as an added bonus.
Here is the image of the wafer under 4mm of water, with no sharpening and no contrast adjustment, though I adjusted the brightness:
There is very slight pincushion distortion, easily corrected.
Here is a 100% crop from the edge; not too great, but reasonable considering the magnification. It looks decent at 50%. There is color fringing on blown highlights, not present in the center of the main image, but not horrible and probably avoidable with more diffused lighting:
Later I will try it with oil instead of water; that should give better results, since its depth will be almost exactly equal to the 3.5mm of glass that the objective expects. But overall I am happy with this; it is a breakthrough for my work. Better results can be obtained by water-dipping objectives, but these have much smaller working distances and are far from telecentric, so they can't be stitched and they often hit some part of the flower I am photographing.
