Exact parfocal length of short Nikon and Olympus objectives

zzffnn · Post by **zzffnn** » Thu Jun 22, 2017 3:50 pm

Does anyone know, for sure, what is the exact parfocal length of short Nikon S objectives? Are they 34 mm, or 33.5 mm, per Nikon?

The exact one I want to know about is a rare Nikon S DLL LWD 30x NA 0.55 with correction collar 0.9-1.5mm. Let us assume correction collar setting is at 1mm.

Also, are short Olympus objectives always 36.65mm parfocal (per Alan Wood)? I have a short C20x NA 0.4 1.2mm inverted objective. Basically, I am considering to custom make a parfocal extender to make the Nikon parfocalized wth the Olympus.

I don't have an accurate reference to measure myself. If anyone can confirm that dry LOMO shorties are 33mm, then I can measure myself from there.

Thank you very much.

Ichthyophthirius · Post by **Ichthyophthirius** » Thu Jun 22, 2017 5:55 pm

Hi,

Nikon I don't know.

The others are: Tubelength; objective parfocal length; eyepiece parfocal length

Olympus: 160; 36.65; 16
Lomo: 160; 32.3 or 33.5*; 12.5

*Lomo has different objective parfocal lengths and even objectives of the same type are poorly parfocalised among each other, according to the literature.

The old Nikons are also likely to have an eyepiece parfocal length that is not 10 mm.

The difference between Olympus and Nikon objective parfocal lengths is probably very small. Isn't that too short for a normal RMS adapter? More helpful would be some kind of thread extension.

Maybe just use the thickest shim ring that still works (you said 2 mm was too much?). The 30/0.55 will need a lot of refocussing anyway when you set the correction collar.

Regards, Ichty

zzffnn · Post by **zzffnn** » Thu Jun 22, 2017 6:09 pm

Thank you very much, Ichty.

Edit: The diffetence between short Nikon and short Olympus would be around 3.1mm, if short Nikons are 33.5mm parfocal. I feel that number is about right, but prefer to have an exact number for machining.

I think shim rings can work up to 1.5mm. Longer than that, the objective may not engage anymore. The shortest threaded RMS spacer on eBay is 5mm long. So anything in between needs custom machinist work

My experience with LOMO objectives is actually pretty consistent. Maybe I am lucky.

Almost all my dry LOMO objectives (apo or achromat) have almost the same parfocal distance. Per Raf Camera, the dry ones all have 33mm parfocal distance. I suspect Raf got it from his Russian catalog.

With LOMO immersion objectives, parfocal distance seems to be around 32.7mm and they seemed to design them that way on purpose.

I would have to ask my Russian contact to be sure.

abednego1995 · Post by **abednego1995** » Fri Jun 23, 2017 7:58 am

Hi, the old short barrelled Nikon objectives are TL160, parfocal length of 33.6mm with the exception of the 1.2,2,3 plans which are 60.6mm.
Hope this helps.

zzffnn · Post by **zzffnn** » Fri Jun 23, 2017 8:05 am

abednego1995 wrote:Hi, the old short barrelled Nikon objectives are TL160, parfocal length of 33.6mm with the exception of the 1.2,2,3 plans which are 60.6mm.
Hope this helps.

Thank you very much. So you got that 33.6mm number from Nikon catalog? That is quite close to the 33.5mm that I heard of.

And yes, I was referring to the short Nikons of 4x and over. All are 160TL. I have the "3 Plan" iris one and it does have much longer parfocal distance.

abednego1995 · Post by **abednego1995** » Fri Jun 23, 2017 8:46 am

Yup. The Nikon catalogs mention it as so. Also when looking for other objectives from other companies, the 1978 Handbook of Optics has an unparalleled section on specifications. Excellent when looking for equipment from the era. Though, your 30x 0.55 LWDmight be an very special exception:-)

zzffnn · Post by **zzffnn** » Fri Jun 23, 2017 9:51 am

Thank you very much, abednego1995.

So Olympus 36.65 mm - Nikon 33.6 mm = 3.05mm to shim.

I tried to shim again today, with rubber O rings. My Nikon objective does not have enough threads to accept 3mm shim. It can get close but then it has fallen off twice when I pushed it too far.

So a custom 3mm threaded RMS spacer would be needed, to parfocalize the Nikon and the Olympus.

Ichthyophthirius · Post by **Ichthyophthirius** » Fri Jun 23, 2017 1:25 pm

Is that mechanically possible? The length of the thread on your objective determines the minimum length that the adapter must have (usually about 5 mm). You might have to use an adapter for the Nikon and shim the Olympus as well.

Unless you manage to make some kind of thread extension.

Regards, Ichty

zzffnn · Post by **zzffnn** » Fri Jun 23, 2017 2:17 pm

Ichty,

Last time we extended 4.36mm, with a modified version of this threaded spacer:

http://www.ebay.com/itm/162268593012

We had to cut down the spacer parts, removed unthreaded depth from the spacer and my objective and made the threads connect better. The original spacer is 7mm long for the female part and 10mm for the male part.

Looking at the finished 4.36 mm product, I think we likely can make a 3mm version. But I may have an alternative objective to try, so that we don't need to bother my machinist friend again.

My particular scope only has parfocal space for 37mm objectives (not any longer), so shimming both objectives won't be an option (otherwise, we can shim the Olympus 8.35 mm and shim the Nikon 11.4 mm, which would be easy).

Best,
Fan

Ichthyophthirius · Post by **Ichthyophthirius** » Fri Jun 23, 2017 2:40 pm

zzffnn wrote:removed unthreaded depth from the spacer and my objective

that works of course. A bit more invasive than usual.

zzffnn · Post by **zzffnn** » Fri Jun 23, 2017 2:52 pm

Ichthyophthirius wrote:
zzffnn wrote:removed unthreaded depth from the spacer and my objective
that works of course. A bit more invasive than usual.

We only cut 0.5-1mm or so into the objective top surface. Might not even be necessary, for 4.36 mm extension. But probably needed for the 3mm.

And it was done on an achromat objective. But knowing how good my machinist friend is with his work (he can get close to 10 microns WITHOUT proper tools), I won't hesitate to try that on my LOMO apos, if necessary.