The argument for plan.

Everything relating to microscopy hardware: Objectives, eyepieces, lamps and more.
Post Reply
Message
Author
Greg Howald
Posts: 1185
Joined: Tue Oct 20, 2020 6:44 am

The argument for plan.

#1 Post by Greg Howald » Sun May 09, 2021 9:55 pm

I recently purchased an Amscope T400 scope. It's a well made scope similar to the well favored 490. Today I placed a prepared slide on the stage and as I observed it, I noticed for the first time that the focus on the edge of the field of view just wasn't up to par at 4x, 10x and 40x. 100x seemed fine but for the others, when the center of the slide was in focus this was not true at the outer portion of the field of view. I switched over to the plan objectives I've had just sitting in the drawer for such a long time. What a difference! I left the plan objectives mounted but I can't help wondering.... Why this scope? I've never had the problem with any other scope, especially at lower magnification levels. I'm happy with the result.
Greg

Matador
Posts: 48
Joined: Sun Apr 25, 2021 4:55 pm

Re: The argument for plan.

#2 Post by Matador » Mon May 10, 2021 3:52 am

Interesting. I've read somewhere that non-planar objective should be in focus in 60% of there field of view (FOV) diameter (that is, the center part of the FOV will be in focus), while plan-ojectives should have 100% of their FOV in full focus.
The thing is, objectives have a field number (FN). High end objectives often have a FN of 23 mm and up. Some olympus SPlanApo objectives have a 26.5mm FN.
For exemple, if a microscope has say 22 mm FN occulars and 26.5 mm FN plan-Apo 10X objectives, then you will only have 2.2 mm diameter in your FOV [FOV diameter = FN (smallest FN component in the microscope imaging system) / objective magnification).

One thing to consider is when you change microscope but keep the same objectives.
If you use a 10X Achromat (not Plan) that has 20 mm FN eyepieces, you will have 2.0 mm diameter of the specimen in your FOV (20mm/10X=2.0 mm). Of this, 60% in the center of the FOV (about 1.2 mm diameter) will be in focus, but 40% out of the periphery (a ring of 0.4 mm on each sides) will be blurry. And the bluriness increases exponentially as you go further to the periphery.

However, put that same 10x objective on a microscope that has 18 mm FN eyepieces and you will have the impression that there is much less blurriness. Why ? Because now you only see 1.8 mm diameter of the specimen FOV. So you loose 0.2 mm of the more peripheral vision (0.1 mm on each side) from changing occulars, so now the in focus center still has about 1.2 mm diameter, but the peripheral ring of blurriness only has a thickness of 0.3 mm instead of 0.4mm.

Matador
Last edited by Matador on Mon May 10, 2021 9:43 pm, edited 1 time in total.

BramHuntingNematodes
Posts: 1538
Joined: Tue Jan 21, 2020 1:29 am
Location: Georgia, USA

Re: The argument for plan.

#3 Post by BramHuntingNematodes » Mon May 10, 2021 1:41 pm

What was the slide of? Planar corrections are much more noticeable on planar subjects.
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

apochronaut
Posts: 6269
Joined: Fri May 15, 2015 12:15 am

Re: The argument for plan.

#4 Post by apochronaut » Mon May 10, 2021 1:48 pm

With a 2000 micron field, a standard 10X microscope objective will provide somewhat less than 60% of the field with a flat image, in most cases. In fact field curvature begins to occur quite quickly, once you move away from the optical axis. Only about 35% of the center of the field is truly in focus with perhaps another 7.5% on either side pretty acceptable so a total of about 50% of the center of the field.
This is complicated by the nature of the sample. Only very flat objects visible in one way or another across the entire field, either continuously or in regular intervals, can truly display planarity. A stage micrometer or similar device works as a test subject.
Another complication is the eyepiece. Some eyepieces can create a condition of harmonious curvature of field, which masks the curvature of field of the objective by complementing it somewhat. This allows a greater area of the field to stay in focus albeit with curvature. Truly plan objective eyepiece combinations are flat and in focus, with peripheral distortions corrected for.
If an objective is not plan yet has good control over perpheral distortion, when used to view many samples that are relatively thick and amorphous, it can provide almost an as accurate and pleasing view as will a plan objective. Plan objectives only really come into their own where the sample is thin and the portions of the sample off axis need to be referenced against the axial portions. It is hard not to see that the burgeoning need for w.f. plan optics was not fuelled by the needs of pathologists.

Matador
Posts: 48
Joined: Sun Apr 25, 2021 4:55 pm

Re: The argument for plan.

#5 Post by Matador » Mon May 10, 2021 9:40 pm

BramHuntingNematodes wrote:
Mon May 10, 2021 1:41 pm
What was the slide of? Planar corrections are much more noticeable on planar subjects.
Typical histological slide (4-5 micron tissue thickness).

Matador

Hobbyst46
Posts: 4277
Joined: Mon Aug 21, 2017 9:02 pm

Re: The argument for plan.

#6 Post by Hobbyst46 » Mon May 10, 2021 10:00 pm

Matador wrote:
Mon May 10, 2021 9:40 pm
BramHuntingNematodes wrote:
Mon May 10, 2021 1:41 pm
What was the slide of? Planar corrections are much more noticeable on planar subjects.
Typical histological slide (4-5 micron tissue thickness).

Matador
I think that 4-5 um range of non-flatness is not a good indicator. The thickness of the stage micrometer lines is IMHO much smaller. According to the BEREK formula (saw it on the Olympus site), the depth of focus for a 40x0.65 objective with an 10x eyepiece is ~2. So, plan or non-plan, some parts of the tissue will not be in focus, even at the central fraction (say 35%) of the FOV.
Please correct me if I'm wrong.

User avatar
Rossf
Posts: 363
Joined: Mon Jan 20, 2020 12:39 am
Location: Victoria Australia

Re: The argument for plan.

#7 Post by Rossf » Wed May 12, 2021 12:00 am

I think they all have uses-sometimes a simple achro helps isolate an object in the middle as the crispness falls away around it so less busy visually-for photography of course-I agree with thick samples plan makes not a lot of difference-the main advantage of simple achromats also is longer working distance which I always use when looking with marine samples-those tiny plan front elements and close WD would suck up salt water in a flash if the specimen has a bit of sea water from edge of slide-I’m not a great slide mounter-if I had to choose between good plan achromats and non plan apochromat id choose the later for better resolving power and the colours are definitely much nicer-they can resolve an amoeba quite well without phase-different look to phase of course and I’m talking short barrel vintage “affordable” Apos of course-I got a set of Olympus Apos for about $350 from a Japanese ebay seller last year-they rock!
Regards ross

Post Reply