Possible reasons for objectives being non-parfocal?

[Planapo]

I am using Olympus objectives made for the BH-2 on a BH-2. All else seems fine, I am having nice clear views through them.
However, they seem not to be parfocal. I've tried with the SPlanFL 2x, SPlanApo 4x and SPlanApo 10x and always have to focus back or forth a good portion. I don't dare to try to swing in the 10x, 20x or 40x with their short w.d. without lowering the stage before, and then approaching the cover slip carefully from a distance which starts becoming a bit of a time consuming pita.

Hmm,... what could the reason for the non-parfocality be?
(Yes, I seem to have screwed the objectives all the way in.)

--Betty

[Pau]

I am using Olympus objectives made for the BH-2 on a BH-2.

I assume that all of them are designed for 160mm TL
With most systems I used some refocusing is usually needed, but in a good system IMO you must see an image, even not well focussed, when you change to the next magnification objective, with not problems at all when inserting high magnification/low working distance ones.

I don't dare to try to swing in the 10x, 20x or 40x with their short w.d. without lowering the stage before, and then approaching the cover slip carefully from a distance which starts becoming a bit of a time consuming pita.

Do they touch the slide if you change directly from a focused image with low power?
If the lack of parfocality is important it may be due to out of specification microscope tube lenght. Check the position of the eyepieces and, if both are focusable check its focusing mecanism to set them towards the midle of its range. If you use an intermediate piece or magnification changer, test if its optics are at its right position. (if I remove the telan lenses in the intermediate piece the tube lengh increases from 160 to 200mm an I lost parfocality, and if I switch its position as I accidentally did the result is really crazy)

[Charles Krebs]

I've tried with the SPlanFL 2x, SPlanApo 4x and SPlanApo 10x and always have to focus back or forth a good portion

Let's define "a good portion". Don't expect them to be "right on", but the range I have seen mentioned as "normal" (or at least nothing to get excited about) is up to 1/2 rotation of the fine focus knob.

I would not use the 2X as the base-line since its great DOF gives quite a range where your eye can accommodate and perceive correct focus. Less so with the 4X. But you should be able to switch between 10/20/40 with little "error". (But the 40/0.95 Apo can actually vary significantly with the correction collar setting).

[DanGuy48]

If you have 2 focusing eyepieces, do this.

1. Set both eyepieces to roughly midpoint
2. Go to top mag hi-dry objective.
3. Focus image and make sure illumination is proper
4. Do not touch focus again
5. Switch to low mag objective
6. Focus each eyepiece independently to give the sharpest image for each eye
7. Your microscope optics are now parfocal for image viewing

If you have one focusing and one fixed eyepiece, the procedure is almost the same except that you must use the fixed eyepiece only to do your hi mag, focusing, then adjust the focusing eyepiece at low mag to give the best image.

[DanGuy48]

Oops...didn't notice how old thread was....

[Planapo]

No need for any oopses, DanGuy48!
Your entry is most welcome from my side, not only that it's very helful, it also has reminded me of this thread that I had totally forgotten about. (Haven't gotten around to using my scope for months. )

So oopses from me, my apologies for not answering in due time and a belated:
Thank you guys for all your entries and help!

I will come back to this, when soon my time allows for checking it out and thus doing my homework properly.

--Betty

[Pau]

Betty,
When you resume microscopy, if the former ideas don't work, you can use parfocal rings like:
http://www.ebay.com/itm/Microscope-Obje ... 1272396517?

[Planapo]

Thanks for this link, Pau!
Good to know that such rings are readily available off-the-shelf from there, and wouldn't have to be custom made.

--Betty