How parfocal should things be? (Nikon finite phase)

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TomFid
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How parfocal should things be? (Nikon finite phase)

#1 Post by TomFid » Sun May 08, 2022 2:01 am

I've been gradually picking up phase contrast bits for my Optiphot. I bought the CFN 10x 0.3 Ph1 objective, and by an accident of ebay, also wound up with a 40x 0.55 Ph3 LWD. As I understand it, the LWD versions are typical on inverted scopes, and there isn't a LWD version of the 10x (not needed), so these are commonly found together. However, I'm finding that they're not quite parfocal. If the 10 is in focus, really nothing is visible in the 40. I think the difference is fairly small - if I unscrew the 10 by about 2/3 of a turn (thickness of a sheet of paper maybe), they match OK. Is this common? Should I just shim one?

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zzffnn
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Re: How parfocal should things be? (Nikon finite phase)

#2 Post by zzffnn » Sun May 08, 2022 2:27 am

What else does your 40x 0.55 PH3 LWD say on its shell? Is there a number near 1.0, 0.8-1.2 or something close? If so, it is for inverted scopes and ~1.0mm cover slips.

Even with the same objective series, some objectives won’t be perfectly parfocal.

abednego1995
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Re: How parfocal should things be? (Nikon finite phase)

#3 Post by abednego1995 » Sun May 08, 2022 4:39 am

That 40x LWD is equipped with a coverglass correction collar that allows for a 0-2mm adjustment. Changing that correction effectively changes the focal length of the objective slightly. So, it will be parfocal at a certain correction value, but not at every value with this objective and others.

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Phill Brown
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Re: How parfocal should things be? (Nikon finite phase)

#4 Post by Phill Brown » Sun May 08, 2022 7:13 am

LWD /1.2mm corrected is for viewing through a 1.2mm slide from underneath as much as anything?
LWD condenser with phase might be worth a look into,might not be a direct fit but that doesn't mean it can't be made to.
Or get an inverted phot type thing.

TomFid
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Re: How parfocal should things be? (Nikon finite phase)

#5 Post by TomFid » Sun May 08, 2022 1:43 pm

@zzffnn, abednego is right - it's the version with a 0-2mm correction collar. I had noticed that the correction affected focus a little, but I didn't put the pieces together. I was assuming it would be parfocal with the 10x at 0.17, but it's probably at 1.2. I'll experiment.

TomFid
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Re: How parfocal should things be? (Nikon finite phase)

#6 Post by TomFid » Sun May 08, 2022 3:59 pm

It seems they split the difference (assuming my objectives are within spec). The 40 is parfocal with the 10 with the coverslip adjustment at roughly .65mm. It's not exactly convenient, but I'll probably just live with it for a while, because it was so cheap (after a shipping mixup), but eventually swap it out for a non-LWD 20x.

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Re: How parfocal should things be? (Nikon finite phase)

#7 Post by zzffnn » Sun May 08, 2022 7:32 pm

Tom,

At NA 0.55, assuming you are using 0.17mm cover slips, you may want to set it at 0.17 and use parfocal shim rings (or a rubber band) to shin either the 40x or 10x and other objectives. Setting the 40x to 0.65mm will compromise image quality by adding some haze / fog (spherical aberration), which is much more visible in darkfield.

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