Macro Lens vs Objective at 5X
The purpose of this comparison is to look at the difference in image quality between the Canon MP-E 65 f/2.8 macro lens designed for the consumer market, and a more specialized lens, the Mag.x LD Plan Apochromat 5x/0.20 objective, designed for industrial electronics inspection.
wider aperture = higher resolution
Microscope objectives have a big advantage at magnifications like 5x and higher. The larger the aperture, the more resolution and sharpness a lens can deliver so a wider aperture = higher resolution. At smaller apertures resolution and sharpness can suffer from diffraction, the softening effect that is caused by light dispersing or "diffracting" when passing through a lens aperture.
f/12 vs f/19
MP-E 65
You can predict the resolution from a certain lens simply by looking at the effective aperture. The Canon MP-E 65, with a maximum aperture of f/2.8, seems like it would be free from any issues with diffraction, but the nominal aperture of f/2.8, turns into an "effective aperture" f/16.8 at 5x magnification.
Thats not all, the effective aperture in this test is actually worse since my MP-E 65 did not deliver optimal sharpness at f/2.8, instead my lens was sharpest at f/3.2. That means that the MP-E images made here were at an effective aperture of f/19.2, which is firmly in diffraction territory. With the 24 MP Sony A6300, the softening effect of diffraction starts to become visible at 100% view around f/8!
When a lens has lot of aberrations, stopping down will reduce resolution and sharpness loss due to aberrations, while at the same time increasing resolution loss due to diffraction. In the case of the MP-E 65, the resolution loss from diffraction at f/3.2 was less than the loss due to aberrations at f/2.8.
Mag.x
The Mag.x has an NA of 0.20, which is a nominal aperture of f/2, and an effective aperture of f/12.5 at 5x. Comparing f/12.5 and f/19.2 is a difference of more than one stop between the MP-E 65 and the Mag.x.
The simple formula for effective aperture is: Aperture = Lens Aperture x ( 1 + Magnification )
THE SET-UP
Camera: Sony α6300, model # ILCE-6300, also known as: A6300
Sensor size: 23.5 × 15.6 mm. APS-C. 28.21 mm diagonal. 3.92 micron sensor pitch
Flash: Godox TT350s wireless flash x 2 with one Godox X1s 2.4G wireless flash transmitter
Vertical stand: Nikon MM-11 with a Nikon focus block
A series of images was shot with each lens in 4 micron steps, the MPE 65 was also shot at each third stop from f/2.8 to f/4. At 100% view in Photoshop the sharpest was chosen for center, edge and corner. All images were shot as a single RAW files and processed in PS CC with all noise reduction and lens correction turned off, all settings were zeroed out (true zero) and the same settings were used for all of the images. All of the images shown here are single files.
Sharpness Test CANON MP-E 65MM MACRO LENS VS MAG.X 5X 0.20 LD PLAN APO OBJECTIVE
Center crop at 100%
100% view center crops, Canon MP-E 65mm f/2.8 Macro lens on the left and Mag.x LD Plan Apochromat 5x / 0.20 Objective on the right
Corner crop at 100%
100% view corner crops, Canon MP-E 65mm f/2.8 Macro lens on the left and Mag.x LD Plan Apochromat 5x / 0.20 Objective on the right
Lateral chromatic Aberration test: CANON MP-E 65MM MACRO LENS VS MAG.X 5X 0.20 LD PLAN APO OBJECTIVE
What are Lateral CAs?
Chromatic aberrations (CA) are one of several aberrations, or imperfections, that degrade image quality. Lateral CAs appear in images as purple and green fringing, mostly seen on near-vertical hard edges on the left and right corners of the image. This results in softening of edges in the corners of an image. CAs occur because the different wavelengths of light are magnified by slightly different amounts by the lens.
For this test I used laser printed 6 point type as a target. The MP-E was shot at f/3.2. All of the crop images were made as a single file.
Corner crop at 100%
100% view chromatic aberration corner crops, Canon MP-E 65mm f/2.8 Macro lens on the left and Mag.x LD Plan Apochromat 5x / 0.20 Objective on the right
The Canon MP-E corner crop on the left shows pretty obvious pink fringing at 100% view. The Mag.x image on the right is about 99% free of CAs.
Center crop at 100%
100% view chromatic aberration center crops, Canon MP-E 65mm f/2.8 Macro lens on the left and Mag.x LD Plan Apochromat 5x / 0.20 Objective on the right
Even in the center the MP-E, on the left, shows pink fringing. The Mag.x is almost perfect.
Lateral chromatic Aberration test: CANON MP-E 65MM MACRO LENS VS MAG.X 5X 0.20 LD PLAN APO OBJECTIVE
Longitudinal chromatic aberrations or LoCAs are seen in the out of focus areas in front of and behind the focus point of an image. LoCAs are common, extremely difficult to correct, especially with fast lenses, so chances almost all of your lenses have at least a trace, you just never noticed. Most consumer market lenses, even lenses labeled APO or Apochromatically corrected, still have some LoCAs, only very specialized, very highly corrected lenses are LoCA free.
To test for LoCAs I simply defocused the center crop image. On a lens like the MP-E the color fringing will show up as either green or pink depending on which direction the image has been defocused.
Center crop at 100%
100% view longitudinal chromatic aberration center crops, Canon MP-E 65mm f/2.8 Macro lens on the left and Mag.x LD Plan Apochromat 5x / 0.20 Objective on the right
The Canon MP-E 65 test crops show worse than the average CAs, especially for a lens that, according to Canon, uses UD glass.
The Mag.x suppresses LoCAs almost completely, performing at the very highest level, unlike other lenses labeled APO, more like a Super Apochromat.
CA test conclusion
The Mag.x crops are some of the cleanest you will see from any objective, CA suppression on this lens is even better than Nikon Plan APOs.
The MP-E 65 was released in 1999, that is almost 18 years ago, but given the resources and lens design experts at Canon's disposal they could have done a better job of suppressing LoCAs on the MP-E. The specs include one UD or Ultra-low glass element but looking at the results you would never have guessed it judging by the amount of neon green bokeh fringing. Its not just an average performance, its one of the worst LoCA producing lenses that I own, and I own a lot of lenses. Thankfully we now have smaller companies like Cosina, Sigma, and even Tamron producing better and better designed everyday and forcing Canon to update the old designs. Hopefully a new MP-E will be produced sometime in the future.