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Fluorite objectives acquired - it is worth it?

TLDR: fluorite objectives are interesting only with the right tube lens to minimize chromatic aberrations.

The problem of MSPlan objectives

MSPlan objectives are not fully corrected, so if you don’t use the right (photo)eyepieces in combination, the image quality will be degraded.

This present 3 problems:

• You can’t use direct image projection
  Because corrective (photo)eyepieces are required, you can’t use direct image projection. So more lenses in the image path, resulting in an inevitable image degradation.

• Low magnification NFK photoeyepieces are very expensive
  NFK 1.67x are very rare. Using a 3.3x photoeyepiece in combination to the U-PMTVC will result in a 1.1x magnification.

• You need a large sensor
  Photo eyepieces such as the NFK 3.3x were originally used with a film camera. For this reason, you need a 1" sensor so the field of view is not heavily cropped.

In the end, photography on an Olympus BH2 will get expensive.

Overcoming optical shortcomings

Most people seems to ignore that focus stacking and channels alignment in post processing can solve this problem once and for all:

• Focus stacking
  Each channel will focus at a different plane, resulting in heavy chromatic aberrations. Focus stacking solve this issue by keeping only focused details for each channel, resulting in a very sharp image.
• Channel alignment
  Channels may be missaligned or having slightly different magnification.
• Short focal length tube lens
  Using a tube lens with a lower focal length allows a wider field of view on smaller sensors with almost no image degradation.

Modern UIS and UIS2 Objectives

• Chromatic corrections are built into the objective itself.
• There’s no need for corrective eyepieces.
• You can project the image directly onto the sensor.

Can fully corrected fluorite objectives improve image quality using the same setup/pipeline? Are they worth the price for silicon imaging if you already own MSPlan objectives?

Setup and post processing

Hardware:

• 5€ D25.3mm FL103mm achromat tube lens
• Swiftcam SC1803R 18MP Camera
• Narrow band monochromatic recombined RGB illumination
• Olympus MSPlan 5x/0.13
• Olympus LMPlanFL 5x/0.13 BD
• Olympus MPlanFL N 5x/0.15 UIS2

Software:

• 14 bit lossless image pipeline
• Focus stacking
• Channels alignment
• Exposure fusion
• Levels / contrast adjustments

The exact same exposure and post processing settings are used.

Visual comparison

MSPlan 5x/0.13	LMPlanFL 5x/0.13 BD	MPlanFL N 5x/0.15

At a first glance, all die shots are very similar. Only the MPlanFL 5x/0.15 is a little less colorful, because of the slightly higher numerical aperture.

Thanks to it’s slightly higher numerical aperture, some details are only visible with the MPlanFL N objective. Image is also slightly sharper.

Fluorite objectives are supposed to have better corrections.

Fluorite objectives further correct for aberrations using advanced glass types containing fluorspar or other synthetic substitutes. Just like achromatic objectives, fluorite objectives are designed to correct for chromatic aberrations for red and blue wavelengths. However, fluorite objectives are designed to correct for spherical aberration at two or three wavelengths instead of just green, typically have a higher NA, and feature a better resolving power and higher degree of contrast. 

Source: edmundoptics

Let’s compare in the blue wavelengths.

And the red wavelengths:

Using this pipeline, results are very similar:

• MSPlan 5x/0.13 seems to result in sharper images than the LMPlanFL.
• LMPlanFL capture slightly more light than the MSPlan.
• MPlanFL N 5x/0.15 have the sharper results, thanks to the higher NA, but slightly less colorful images.

Comparing chromatic corrections

What about results without using focus stacking?

MPlanFL and LMPlanFL will have green and blue in focus at the same time, but red out of focus.

MSPlan will have red and blue almost in focus at the same time.

Here is a table with the Z focus position (in steps) of each wavelength using the motorized focus.

	Tube Lens		Objective		Red 655		Green 545		Blue 456
	FL103mm		Olympus MPlanFL 5x/0.15		210		0		0
	FL103mm		Olympus MSPlan 5x/0.13		160		0		130
	BH2-MA		Olympus MPlanFL 5x/0.15		120		0		120
	BH2-MA		Olympus MSPlan 5x/0.13		80		0		180
	BH2-MA		Olympus Plan 4x/0.10		140		0		390

Results are definitively intriguing. The MSPlan 5x perform much better than expected, even compared to modern fluorite objectives.

This may be caused by the specific wavelengths I use.

Further tests are needed:

• Using more wavelengths to plot a graph
• Trying different tube lenses
• Comparing results on the periphery of the field of view

Important note

My friend Richi, using fluorite objectives on his BX microscope (with the expected tube lens) doesn’t have this out of focus red channel problem.

The tube lens should have the same contribution to all lenses, but mine might exacerbate some chromatic aberrations.

Conclusion

In this particular setup, and using focus stacking, the MSPlan definitively perform well for silicon imaging.

Visually, no significant difference can be observed between both generations.

The travel range required for focus stacking with the MPlanFL is even worse than the MSPlan, leading to slightly longer capture times.

In conclusion the slight difference in numerical aperture (0.13 VS 0.15) is not worth the money for the expensive fluorite modern objectives in this particular setup.

If you plan to invest in modern FL objectives, also get a trinocular head (like the U-TR30) or get the right tube lens first (like the U-TLU).

Note that a good tube lens cost much more than a basic doublet. The U-TLU is $800 on Edmund and usually can be found around $200 on ebay.