Objectives...

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DocDodgers
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Joined: Fri May 31, 2019 7:35 am

Objectives...

#1 Post by DocDodgers » Mon Jun 03, 2019 2:47 pm

Hello, what is the difference between these two objectives, apart from the price and its appearance ... should I buy the cheapest or the other?
https://www.amazon.com/AmScope-Plan-Ach ... =8-2-spell

https://www.amazon.com/AmScope-PA40X-Ac ... =8-8-spell

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

Re: Objectives...

#2 Post by apochronaut » Mon Jun 03, 2019 3:02 pm

the same in terms of the optical design. China has been making the same 160mm designs for decades, just changing the barrel to make them look more like a clone of a name brand, they have chosen. There might be some variation in the coatings used, so it's theoretically possible there are slight differences in contrast but it would be a very small difference. There may be differences in parfocality or working distance but that would be slight with a 40X too.

Just make sure you are looking for a 45mm parfocal objective (D.I.N.)

Probably a better option https://www.ebay.com/itm/Nikon-E-Plan-4 ... SwxS9cdsqd

PeteM
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Re: Objectives...

#3 Post by PeteM » Mon Jun 03, 2019 3:52 pm

I've had a dozen or so 40x 160mm tube objectives pass through. Can't directly tell you if the two objectives are equal, but can make some comments on what has been seen. I have used one of the AmScope versions in a 20x and 40x and they were pretty good.

1) The parfocal differences as suggested above.

2) Some have been pretty lousy - low contrast, not a plan view even if marked plan or pl.

3) Some have been pretty good - not quite as well built, parfocaled, or parcentered as their Olympus DPlan or Nikon E Plan equivalents but of similar image quality. That was my experience with the AmScopes which are built in a way that looks like the cheaper (not DPlan) Olympus objectives.

4) Someone inquired about an AmScope trinocular the other day. It had regular achromats. The Chinese plan achromats I've seen are both significantly better and significantly more expensive.

5) The fluorite versions a bit disappointing image-wise compared to their Olympus, Nikon, Leica equivalents (in infinity versions). Still, better than the plan achro versions.

5) Note that the main makers typically made around five or six levels of objectives such as cheaper achromats, educational level semi-plan achromats, middle of the road plan achromats (e.g. D Plan etc.), better plan achromats with wider field and often a slightly higher numerical aperture (e.g. S Plan), plan and sometimes not-plan fluorite objectives with higher NA, and plan apochromats.

6) There are the issues of objectives not being fully color corrected and requiring a matching tube lens, set of eyepieces, etc.

If you buy from a source that allows returns, you might consider buying both, compare images, and report back?? Otherwise I'd be inclined to try to find the AmScope objective for a bit less.

apochronaut
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Re: Objectives...

#4 Post by apochronaut » Mon Jun 03, 2019 5:00 pm

The problem with precision optics that are made in cut rate production is a general lack of consistency. You cannot test one objective and think the next will be the same. An objective is an optical design, and even though that design looks to be such and such on paper; the Q.C. level or lack of, can allow for far greater tolerances from the specs. than would be the case with optics made to tight tolerances by a company whose reputation relies on quality, not just sales volume. The lenses produced by the microscope companies of high repute, were and are ground to 1/2 lambda or better, some even reputedly 1/4 lambda. When you are kicking out thousands upon thousands of lenses to be utilized in generic designs for bargain basement products, you can bet that 1/2 lambda ends up being 3/4 lambda a lot of the time....or worse.
In literature, a quality brand will give a guarantee of a 2 micron parcentering variance. With cut rate production optics, two objectives next to each other in a box from the factory that look identical might be 5 microns, and the other 150 microns. You don't know, because materials have impurities and the machining is substandard. Threads are cut at breakneck speed. Some of the burrs on them you could shred your skin on.
Is any specific objective .65 N.A., if that's what it says on the barrel? Did it get tested , to ensure that it conforms to that spec.? Does it's resolution match it's N.A.? Are the coatings an even thickness? Is it parcentered or parfocal ?
Testing a cut rate objective model would need, probably 20 examples minimum, in order to get an average performance level. It's a lot easier to figure that they are what they are , more or less, and realize that for the same cost, one can almost for sure, find a good used objective of known performance.
Modern budget 160mm optics are just cheap copies of some of those from the 9 or 10 quality manufacturers that made microscope optics over the past 30 years. With those you know that the precision level required to meet a certain standard was maintained.

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