G Plan Apo 20x vs M Plan Apo 20x

Have questions about the equipment used for macro- or micro- photography? Post those questions in this forum.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

Scarodactyl
Posts: 1617
Joined: Sat Apr 14, 2018 10:26 am

G Plan Apo 20x vs M Plan Apo 20x

Post by Scarodactyl »

Sorry for the deluge of tests, I've been given something of an ultimatum re:reducing how much microscope stuff I have lying around, so tests I've needed to run for a while are finally getting done.

I picked up this G Plan Apo 20x thinking it would be good for gem inclusions. It is corrected for a thick 3.5mm cover glass, which would theoretically be really nice for looking at fine details deep inside a gem.

In practice I just didn't end up using it much so I sold it, but I did get a few test shots before it went into the mail.

Anyway, here's a quick head to head of the Mitutoyo G Plan Apo 20x vs the M Plan Apo 20x. G Plan Apo on the left M Plan on the right.
With no cover glass as you'd expect the G plan apo suffers from weak contrast, though not nearly as badly as I'd expected. Its lower NA of 0.28 is also evident vs the 0.42 of the M plan apo.

Image Image
Image Image
Center crop^

Anyway, that's an obvious result. I tried to take controlled comparison shots of the same wafer under a layer of water, and while it worked great visually the water wouldn't settle down so my camera couldn't capture it. Anyway that isn't exactly the question at hand, so let's have a look at mineral inclusions inside a crystal of topaz. These were taken with the crystal just laid on its side so the top crystal surface was not 100% perfectly parallel, and with two fiber optic lights shining from the sides (so diffusion was done by the various inclusions inside the crystal itself, not the most formal approach).
Image
Image
Image
Image
These are stacks with no retouching, though I did look through the m plan apo's individual frames to see if there was any salvaging it (there wasn't that I could tell). Once you're a ways into the topaz there is just no contest between these two. Spherical aberrations crush contrast and eat all the details with the M Plan Apo 20x, while the G plan apo gives a nice crisp image with better detail. There is a little bit of axial CA on the g plan apo.
Overall I think this is a good one to consider if you find yourself looking at subjects deep inside a transparent medium.

Medwar
Posts: 72
Joined: Mon Apr 16, 2012 1:46 am

Re: G Plan Apo 20x vs M Plan Apo 20x

Post by Medwar »

Hi!
Nice to meet you here!
It's me who has bought this lens from you on ebay. Thank you! I hope this lens will be able to help me shoot inclusions in crystals.

Let me share my thoughts:
I think that this lens (G) can't be directly compared with 20x 0.42.
I found the following formula for calculating acceped degradation from spherical aberration(SA) that appears because of the additional coverglass (or other medium) before the object.
Allowable thickness of medium (glass in this case) = (0.07)/[15*(NA^4)]
Sorry, I can't remember where I had copied the formula from. It is possible that I met it one of the topics on this board.
According to it, for NA 0.42 the tolerated glass thickness is only 0.15mm, for NA 0.28 - quite large 0.7mm

If shoot a submerged inclusion through medium - 20x 0.42 has no chance vs G - cause NA 0.42 tolerates only 0,15mm of crystal.
Because of the SA even 'normal' 10x 0.28 will give out better resolution than 20x 0.42 if the inclusion is deep enough inside crystal.
If compare G Plan 20x 0.28 combined with proper 3.5mm coverglass vs 20x 0.42 in air - G 20x 0.28 has no chance because of the lower NA.

So, for a better test G 20x 0.28 can be mounted on a 100mm TL to make it work as 10x and then compared with normal 10x 0.28 APO on a 200mm TL. They are almost equal rivals here.
I am probably going to use this G Plan at 10x, because 20x 0.28 (=effective F/36) is too far into diffraction territory.
I am very curious if G Plan will give a big advantage over 'normal' 10x 0.28 when shooting deeper inclusions.

Lou Jost
Posts: 5943
Joined: Fri Sep 04, 2015 7:03 am
Location: Ecuador
Contact:

Re: G Plan Apo 20x vs M Plan Apo 20x

Post by Lou Jost »

Very interesting test! I have the G Plan Apo 50x for the same purpose, shooting through a transparent medium. I do also use it in air with an Edmund Optics borosilicate window mounted on a filter ring, which screws into another filter ring I epoxied onto the front of the objective. It works well in air with this modification.

Scarodactyl
Posts: 1617
Joined: Sat Apr 14, 2018 10:26 am

Re: G Plan Apo 20x vs M Plan Apo 20x

Post by Scarodactyl »

A comparison to the 10x would definitely have been beneficial and I should have thought of that. I spent some time with my 10x trying to find these particular inclusions again but it is a very big and deep crystal with a lot of stuff inside so I gave up.
From working with the G plan apo image quality definitely declined as I got to shallower inclusions so I am still pretty sure it will perform better than the M plan apo 10x as you get deeper into a stone.

Post Reply Previous topicNext topic