About the effect of a barlow lens on binocular microscope

[ygi]

I'm puzzled about something I just noticed on my binocular microscope and was wondering how come no one ever talks about it.
So I'm running the classic setup with an Eakins binocular µscope with 10x/20 eyepieces and a 0.5x Barlow lens in front of the objective. Per specification, without Barlow lens, my µscope has a working distance of 100mm. With the additional lens, viewing field widen and working distance increases to 165mm, awesome. That's how I've using it from day1.
My eyesight's been going down lately so I thought the eye strain and feeling like something's off looking through the lenses - like I can't ever find comfortable viewing angle from both eyes - was a "me" problem. As a result I was always concentrating on one eye even though both were independently in focus. Then I noticed that approaching an object from the right side, I would see it entering the left eyepiece field of view before I could see it through the right eyepiece which made no sens at all. As I know close to nothing about optics and microscopy I went looking for an answer and found out some optical microscope types give and upside down or mirrored image... Still, I was no satisfied with that dead end answer and thought maybe some sloppy worker at the factory misaligned something. I took apart the damn thing comparing how mine's built with a video of a very similar Amscope and... everything was in order.
Then it hit me. That's a binocular microscope, it has 2 objectives, right? Those objectives are at an angle aiming at the same focal point, converging at bottom tip of a "V" shape. Remember how I said Barlow lens increase working distance? Yeah, but here's the catch: the microscope becomes cross eyed! I could confirm that shining lights through the eyepieces in the dark.
Like I said, I know close to nothing about optical stuff but my theory the "V" shape becomes an "X" shape as we go past the focal point because the angle of the objectives isn't adjustable (on those Amscope clones at least). How come this combination of Chinese trinocular + 0.5x Barlow be so popular and all over youtube with that major flaw? What am I missing there?

Please do share your input,

Thanks for reading!

[ebastler]

It took me a while to understand the problem you are describing, since the "object approaching from the right" seems to be just one special case of the effect. You mean that the two fields of view are no longer fully overlapping, right? Which you should be able to observe directly, by looking at e.g. a PCB and observing which part of it you can see through the left and right eyepiece, respectively?

I don't think your assumption is correct that a Barlow lens will systematically cause this effect. When you insert a Barlow lens, the central optical axes of the two objectives are tilted vs. the Barlow's optical axis. So the two beam paths will each experience a refraction effect, which makes them converge (cross) at a distance further away from the lens. So your "V"-shaped beam path does not become an "X", but rather a "V" with kinks on each of the two sides, and a longer more pointed tip.

This should make the fields of view still match, at least "to a first order approximation". The match might not be perfect, e.g. due to effects from the finite thickness of the lense elements. (I think it should be perfect in the thin-lens approximation.) Maybe other Amscope or Eakins users can report what they see?

[ygi]

You're probably be right about my assumption being incorrect. Looks like what I thought for sure was because of the Barlow lens is actually caused by zoom level. The extra lens just makes it more noticeable. Basically, as I zoom in the 2 overlapping fields of view start diverging and the observed object gets "duplicated" like I'm drunk and crosseyed. Since the brain naturally tries to compensate, it's hard to notice where it starts but at max zoom if I observe a needle centered on the right eyepiece, it's 1/3 of the view field more to the left on the left eyepiece.
So that's some kind of misalignment but I don't really know if it's a prism thing or a lens thing. I guess I'll have to figure it out but I wish I had some pointers.

[jfiresto]

I have three questions. Can you make or still make the microscope parfocal? As in, as you zoom in and out, does the object you are looking at stay in focus in both eyepieces – or if just one, which eyepiece stays in focus?

Second, does the object in one of the eyepieces stay centered as you zoom in and out, and if so, which one?

Third, can you return the microscope under warranty as this one appears to be out of alignment? You may very well need a factory jig to align (or realign) the microscope.

[ygi]

So, the microscope's still parafocal but not all the way. Zoom knob goes from 0.7x to 4.5x, let's say I can go from 0.7 to 2.5x then I have to refocus a bit. Both eye pieces are in sync when it comes to zoom level. If one's clear or unfocused, both are.
Second question describes what was the main issue for sure. I could look at an object in x1 zoom where both fields of view were ~95% overlapping. Then as I zoomed in, both fields of view would converge then keep going and reverse position. At that point, not only were they significantly diverged (including a minor vertical misalignment), left eye piece would look to the right of the object and left eye piece to the right. Thus what I called "the cross eyed microscope". Note that I wrote it "was" the issue. I managed to get it OK-ish by very slightly untightening the right "tube" where it's mounted to its support plate. Then spent 2 hours fiddling with with both objective lenses, overlapping as best I could using a printed calibration target at max zoom. Considering my amateurish approach and general lack of knowledge in that area, the result's better than expected.
Regarding the third one, as you may have guessed, sending the damn thing back to China wasn't in my plan. It's costly and another kind of waste of time dealing with seller, shipping and customs.

[Electro Fan]

I went down much of the same path you are going - figuring out microscope basics, and it's been awhile since I used the scope so I might have something wrong here.  Without a doubt jfiresto knows more about microscopes than I do, so if I get something wrong and he corrects it, go with what he says.

Having said that, here's what I'd recommend.

Put the Barlow lens on that you think you are going to use over a PCB with some discreet components such as ICs with legs.  Set the lighting so you can clearly see a particular IC with legs, maybe with some writing on the top of the component.  Notice the direction of the light, shadows, etc.

With both eyes open and looking into the eye pieces move the PCB so the targeted component sits centered in the field of view. 

Try to focus as best you can up close, zoomed in with the head focus control, so the writing on the center targeted component is highly sharp.  When you get it as sharp as you can with the head focus control, then close one eye and adjust the eye piece for your eye that is open to make it as sharp as possible.  Then close the adjusted eye and try to adjust the other eye piece to make it as sharp as possible.  Open both eyes and confirm that the view is sharp.  If not, redo everything up to this point, and then proceed.

Now zoom out, all the way to fully zoomed out.  Ideally the microscope will retain sharpness on the targeted component all the way zoomed out to the widest view.  If it isn't sharp, redo everything up to this point, and then proceed.

If you are lucky (and the scope is good) you should be able to see the targeted component sharply in focus and most of the rest of the wider viewing area sharply in focus, however it's likely that the overall viewing area edges will not be quite as sharp as the center component you set as the target.  Then when you zoom back in you should go back to your optimum sharpness on the target component.

After doing this procedure a few times you might start to notice that what you thought was well focused on the zoomed in center target could potentially be improved based on where exactly you were focused.  At this point you might start to notice how much depth of field the scope reveals.  For example, when zoomed in maybe the component label is sharp at the top of the component but by the time you look at the legs of the component where they meet the PCB maybe they will be a little less sharp.  Some optics allow more depth than others.

After following this you should have good control over the scope and be able to set it for it's best performance, which should allow it to provide parafocal operation, ie it should retain focus when zoomed from in to out, and then zoomed from out back to in.  It's possible that it will not be 100% parafocal, but ideally it will be substantially if not perfectly parafocal. 

At this point you are ready to enjoy soldering with a good view. 

[ygi]

Thanks for that but all this assumes the microscope's operating properly and just requires user level adjustment. That's not the case, it is out of alignment. I can only guess optics took a hit at some point or the factory worker was in a hurry to go home. Anyway, what I need is a DIY way to realign it that doesn't require specialized factory equipment. Obviously I don't know much so it might be unrealistic.

[thm_w]

I'm sure its doable, just a huge time sink.
Did you ask the seller what their options were? They specifically said you have to ship it back?

https://www.microscopy-uk.org.uk/mag/artfeb04/pjcollim.html

[ygi]

It's been over three years since I bought it on AE, it's out of warranty if there ever was one. As a hobbyist, it's not a tool I rely on daily hence why I never really dug into the issue until  I thought now.

Thanks for the resources. I don't think I can apply the same trick seen on the video because of the very different design of the optics but the article seems doable.

[spostma]

I would suggest to buy a Mantis Elite viewer with a wiewing window rather than oculars,
because you can keep looking through your glasses.

This is especially useful if your glasses compensate for astigmatism or prism (central axis alignment) errors in your eyesight.

I bought a second-hand Mantis Elite, which works great, but I found the exit pupil to be limiting,
meaning that you have to align your eyes to within a 25mm diameter circle of the viewing window to see a stereo image.

I have doubled the exit pupil by installing 62mm achromatic lenses and placing larger first-wave mirrors in my Mantis Viewer.
This works so wonderfully well that may colleagues do not need to adjust the pupil distance any more.
Maybe I will post the details of my DIY Mantis Elite viewer upgrade on eevblog if there is interest.

Meanwhile, if you want to come to my office in the middle of the Netherlands to try my Mantis Viewer,
send me a PM to make an appointment. If you decide to buy an (used) Mantis, I can help you to upgrade it.

[Psi]

Couple of points, and I apologies if some of these are insulting in terms of being obvious. I don't really know how knowledgeable you are about microscopes.

- Are you aware the eyepieces can be moved together/apart to account for the distance between your eyes.
Had someone once using a microscope for many months without knowing this, they simply accepted that it looked a bit funny

- If your microscope has that thing on the side that you pull in/out to switch the camera port on/off then it can feel a bit weird if that thing is not fully pushed in (camera port off).

- If your microscope is quite far out parfocal wise it may feel a bit strange to work with, in more ways than just the focus changing between zoom levels. That's because you have the focus adjust on the 2 objectives lenses forcing focus one way and then the main focus knob forcing it back again.  Set both objectives lenses focus to center position, then adjust main focus knob until one eye is in focus, then adjust the other objective a tiny amount so it's in focus for both eyes.

That should fix the parfocal issue assuming the microscope was aligned correctly at factory.  (parfocal exists at a point where the objective focus and the main focus are not trying to correct each other)

but.. if it still feels 'weird' you can try doing the same procedure but setting both objective to near their max focus. And if that still feels weird try again near their min focus. This will mess up parfocal but you may find one of these more comfortable if the optics of your specific microscope are a bit messed up.

- Does your microscope have the rubber eye piece things on it, without them it's annoying to hold your head at the right distance and gets quite tiring.


I have found sometimes microscope just have shit alignment, maybe dropped during shipping or just made wrong, I dunno, but sometimes you can have multiple microscopes from the same manufacturer and the same brand and one can feel good and the other weird. 

[lugaw]

Another option he have is adding a camera and a monitor.

[ebastler]

Another option he have is adding a camera and a monitor.

How would that help with stereo vision? He might as well continue to use the oculars and squint or wear an eye patch. 

[ygi]

@spostma Granted I ended up with a lemon but if it were aligned properly (or not messed up in transport), I'd be happy with my 350$ scope for the occasional repair/tinkering project. High end equipment's just overkill for my needs. I appreciate the kind offer though.

@Psi No offense taken at all, I'm here to learn. While I don't know much about how they work internally or how to fix them, I do know how to use one (well, mine at least). So, yes, I know about the papillary distance adjustment as well as the diopter one. My scope doesn't have a manual port selector, left side prism's permanently shared with the camera port. The microscope has simple eye protectors usable with glasses.
About the parfocal thing now. Initially the scope had a nasty collimation issue (yeah, I learned a new word) which I mostly corrected by adjusting both objective lenses. I believe doing so made the parfocal issue worse. Still, I take the minor annoyance any day over the crossed fields of view from before. Anyway, unless I do things in a logical step by step, it feels like I'm fixing something by breaking something else. So I figure gathering knowledge is better than fiddling with every adjustment screws hopping to win the lottery.

@lugaw That's the workaround I've been using to avoid stepping into the cross eyed hell