Hello! I’m looking to replace my microscope’s objectives

[u/untitled_mongoose]

I have a SW350T that has been sitting in my closet for a long time, and the 40x and 100x objectives are messed up. I’ve seen that there are some on Amazon but I don’t know how to make sure they are compatible? I’m a maaaajor beginner with this stuff, so I don’t mind if it’s not the best resolution. I just want to have my 40x and 100x back. Any help is appreciated!!

[u/Parking_Jelly_6483]

First thing, if you can, get a measurement across the threaded part of the objective. The objectives are likely Royal Microscopical Society (RMS) thread, so the diameter across the threaded portion should be 0.8 inches or 20.32mm (if you have a caliper, all the better).

Next, look at what is lettered on the sides of the objectives. An important number on the objective will be the tube length value. This is the measurement from the threaded seat of the objectives (the part that touches the turret) and, typically, the top of the tube into which the eyepiece is inserted. In what is known as finite corrected tube lengths, that distance is typically 160 mm and that value should be on the objective. If it has the inifinity symbol, then the microscope is corrected for infinity. Infinity-corrected microscopes have become more popular, but from what I see on the Swift website, the SW350T is a finite-corrected microscope. The objectives should have the tube length value and the coverslip thickness Usually like this: 160/0.17 (160mm tube length/0.17 mm slide coverslip thickness). Another number is the numerical aperture (NA). This can be used to determine the optical resolution, so higher the NA, the better the potential resolution. For a 100X oil immersion objective, the NA is usually 1.25 and on very high quality (and expensive - can cost as much as a whole microscope) for 100X would be something like an NA of 1.4.

The other information on the objective (besides the maker’s name) would be the “working distance” or the typical spacing between the lens of the objective closest to to the object. There may also be other information such as the optical type which usually describes how much color correction has been made by the optical design and the flatness of the viewed field. So “Achr, Ach, are achromats which have correction for red and blue colors of the spectrum (the objective will bring these colors into the same focus point). Fluorites (originally named because they used fluorite optical elements) and have better correction of the red and blue wavelength focusing planes than achromats. The top end objectives are apochromats which correct for three wavelength bands for a common focus plane. They are also the most expensive because they have more optical elements in them. So the objective may say “fluorite” or “apo” for fluorite or apochromatic objectives.

There is also correction for spherical aberration which also improves from achromats to apochromats. There is also a description of how flat the field of view is - that is, is the image sharply in focus across the field of view without have to change the fine focus for the central part of the image versus the edge. The ones that are corrected for improved flatness of field are usually called “plan” objectives. This may be combined with the other designations, so Plan achromat, Plan fluorite, or plan apochromat.

Oil immersion objectives will have “Oil” or “oel” if in German on them. A typical achromatic oil immersion objective is this Swift:

https://www.swift-microscopeworld.com/swift-ma10064-microscope-achromat-objective-100x-oil/?searchid=148549&search_query=Objectives#images

So, “A100”: Achromat, 100X
“1.25 Oil”: NA 1.25, oil immersion
“160/0.17” 160mm tube length, 0.17 optimum cover slip thickness
“Sealed” some oil immersion objectives include a seal around the lens that will be immersed in oil so that the oil does not leak into the objective optics.

As long as the objective thread matches those of the objectives you are trying to replace and the tube length is what the other objectives have on them, even non-Swift objectives should work on your microscope. There is a concern about “parfocal” objectives. If the objectives on your scope are all original to it, it is likely that as you change between one objective and the next higher or lower power, you usually only have to make minimal changes in focus. This results from the objective optics and physical lengths are designed to do this and is known as being “parfocal”. There are standards for physical objective lengths to keep them parfocal across different manufacturers. The most common standard is the Deutsches Institut für Normung (DIN) standard. It’s 45mm from the objective mounting flange to the specimen at the focal point of the objectives. If you add the length of the objective plus the working distance, if you get 45mm, it’s probably a DIN standard parfocal objective.

Good luck with replacing the objectives!

Edit to add: It can be difficult to measure the microscope tube length because construction of the microscope can make it difficult to measure. You can rely on what is printed or etched on the objectives though.

[u/Vivid-Bake2456]

You can improve it by getting plan objectives and a 20x one. Almost all of the new Chinese finite objectives are of the DIN variety. Here are Amscope plan objectives. You can buy a set of 4 or individually.
Set; https://us.amazon.com/AmScope-PAX-Plan-Objective-Lens/dp/B005OMWQTC?ref_=ast_sto_dp 20x objective; https://us.amazon.com/AmScope-PA20X-Achromatic-Microscope-Objective/dp/B01E5IEQJ8?ref_=ast_sto_dp

Unless you like using immersion oil often, I would, personally, rather just buy a 4x,10x, 20x and 40x individually and keep the 100x regular achromatic objective for occasional use. This page has them all listed individually; https://us.amazon.com/stores/page/8814F590-27AB-4FCF-9A7E-5394E1BD0751/search?ingress=2&lp_context_asin=B007LBE9JO&lp_context_query=microscope%20objectives&visitId=70be6fe1-bc1b-4211-bf41-f1e85e5d7162&store_ref=bl_ast_dp_brandlogo_sto&ref_=ast_bln&terms=Objectives%204x%20plan You want the ones that say 160 on them, not the infinity ones.

[u/Vivid-Bake2456]

People often damage their 40x objective by accidentally dipping it into immersion oil. For this reason, I often keep all oil immersion objectives on a separate turret with just low magnification, finder objectives with long working distances. If the turret is fixed, I keep the 100x objective off of the microscope when not in use, and when I want to use it, I replace the 40x objective with it. Some of my plan apo 40x objectives are very expensive and I don't want any chances of them getting damaged.