185 or 195 listed in finite objective specs?

Everything relating to microscopy hardware: Objectives, eyepieces, lamps and more.
Post Reply
Message
Author
User avatar
Crater Eddie
Posts: 1827
Joined: Wed Nov 12, 2014 4:39 pm
Location: Illinois USA

185 or 195 listed in finite objective specs?

#1 Post by Crater Eddie » Sun Jul 28, 2019 3:18 am

This might be a dumb question but...
The recent discussion of cheap Chinese objectives has me browsing the offerings again (my grandma always said that I was a glutton for punishment).
I notice that in addition to the usual information, some of the specs include the number "185" and some "195", while others don't say either. I did some googling, but I'm not really sure what this number is. Can someone enlighten me please? This is for a 160mm tube length finite system.
CE

*Edited a bit for clarity*
Olympus BH-2 / BHTU with Olympus E-P1 MFT camera mounted
LOMO BIOLAM L-2-2
LOMO POLAM L-213 / BIOLAM L-211 hybrid
LOMO Multiscope (Biolam)

MichaelG.
Posts: 1913
Joined: Mon Apr 10, 2017 8:24 am
Location: NorthWest England

Re: Conjugate distance value as related to objectives?

#2 Post by MichaelG. » Sun Jul 28, 2019 4:48 am

Crater Eddie wrote:This might be a dumb question but...
The recent discussion of cheap Chinese objectives has me browsing the offerings again (my grandma always said that I was a glutton for punishment).
I notice that in addition to the usual information, some of the specs include the number "185" and some "195", while others don't say either. From googling I believe that this refers to the conjugate distance of the scope's optical system, but I'm not sure how one figures this value (finite system). Professor Google has so far provided only confusion, and a quick search of the forum archives didn't turn up anything useful.
Can someone explain this is simple terms?
CE
I can try ...

In basic school-level physics we learn about lenses having an object distance and an image distance, and that these are related by a simple formula, such that the sum of the two remains constant when one of them is changed ... this is called the conjugate distance.
At this simple level, we assume that the lens is thin, and therefore the distances are measured from a common point.
In reality, things like microscope objectives and camera lenses are complex assembies of elements, and are not thin ... The relationship still holds, but each distance must be measured from its own nodal point.

So ... the physical distance between object and image probably will not be equal to the theoretical sum of the two.

Hopefully: If you search for my emboldened words, you will find a text or demonstration which is at your comfort-level ... if not: I'm happy to take you through it [to the limit of my own ability!]

MichaelG.

.

Edit: I think you edited your post 'for clarity' whilst I was writing that ^^^
... I thought you had mentioned 'conjugate distance' ... was I dreaming ?
Too many 'projects'

PeteM
Posts: 978
Joined: Tue Dec 06, 2016 6:22 am
Location: N. California

Re: 185 or 195 listed in finite objective specs?

#3 Post by PeteM » Sun Jul 28, 2019 5:04 am

If you're looking for a DIN standard finite objective (45mm parfocal obective + 160mm tube length - 10mm for where the eyepiece picks up the image), I believe you'll want a "195." And the "185" would be a short barrel objective.

MichaelG.
Posts: 1913
Joined: Mon Apr 10, 2017 8:24 am
Location: NorthWest England

Re: 185 or 195 listed in finite objective specs?

#4 Post by MichaelG. » Sun Jul 28, 2019 11:07 am

For those who may not previously have noticed the numbers to which Crater Eddie refers:
http://www.qualitek-instrument.com/cate ... cessories/
Qualitek lists objectives in 185mm, 195mm, and Infinity versions

MichaelG.
Too many 'projects'

User avatar
Crater Eddie
Posts: 1827
Joined: Wed Nov 12, 2014 4:39 pm
Location: Illinois USA

Re: 185 or 195 listed in finite objective specs?

#5 Post by Crater Eddie » Sun Jul 28, 2019 12:54 pm

MichaelG. wrote: Edit: I think you edited your post 'for clarity' whilst I was writing that ^^^
... I thought you had mentioned 'conjugate distance' ... was I dreaming ?
I did, sorry. Originally I rattled on a bit about what I *thought* it meant, then edited that out and changed the topic. Looks like I was on the right track, should have left it alone. You answered my original question quite well, I appreciate your explanation.

PeteM wrote:If you're looking for a DIN standard finite objective (45mm parfocal obective + 160mm tube length - 10mm for where the eyepiece picks up the image), I believe you'll want a "195." And the "185" would be a short barrel objective.
Excellent, that sums it up very nicely. Thanks very much.

CE
Olympus BH-2 / BHTU with Olympus E-P1 MFT camera mounted
LOMO BIOLAM L-2-2
LOMO POLAM L-213 / BIOLAM L-211 hybrid
LOMO Multiscope (Biolam)

MichaelG.
Posts: 1913
Joined: Mon Apr 10, 2017 8:24 am
Location: NorthWest England

Re: 185 or 195 listed in finite objective specs?

#6 Post by MichaelG. » Sun Jul 28, 2019 3:00 pm

Crater Eddie wrote:... I did, sorry. ...
It's no problem at all, CE
All's well that ends well

MichaelG.
Too many 'projects'

User avatar
75RR
Posts: 6808
Joined: Sun Oct 12, 2014 2:34 am
Location: Estepona, Spain

Re: 185 or 195 listed in finite objective specs?

#7 Post by 75RR » Sun Jul 28, 2019 4:53 pm

MichaelG. wrote:For those who may not previously have noticed the numbers to which Crater Eddie refers:
http://www.qualitek-instrument.com/cate ... cessories/
Qualitek lists objectives in 185mm, 195mm, and Infinity versions

MichaelG.
It is a curious number. Are they adding Optical Tube Length plus parfocal distance or what?
Zeiss Standard WL (somewhat fashion challenged) & Wild M8
Olympus E-P2 (Micro Four Thirds Camera)

PeteM
Posts: 978
Joined: Tue Dec 06, 2016 6:22 am
Location: N. California

Re: 185 or 195 listed in finite objective specs?

#8 Post by PeteM » Sun Jul 28, 2019 5:16 pm

As noted above, I believe it's parfocal distance + tube length - MINUS the distance (10mm for DIN) where the eyepiece picks up the image. Seems to be the only way the math works out. And I've bought "195" objectives which are DIN finite compliant and seen what must have been "185" objectives which have around a 35mm parfocal distance (33 to 37mm what what major makers of older pre-DIN objectives seemed to use). These shorter parfocal objectives are apparently still sold with some cheap microscopes.

User avatar
75RR
Posts: 6808
Joined: Sun Oct 12, 2014 2:34 am
Location: Estepona, Spain

Re: 185 or 195 listed in finite objective specs?

#9 Post by 75RR » Sun Jul 28, 2019 5:36 pm

PeteM wrote:As noted above, I believe it's parfocal distance + tube length - MINUS the distance (10mm for DIN) where the eyepiece picks up the image. Seems to be the only way the math works out. And I've bought "195" objectives which are DIN finite compliant and seen what must have been "185" objectives which have around a 35mm parfocal distance (33 to 37mm what what major makers of older pre-DIN objectives seemed to use). These shorter parfocal objectives are apparently still sold with some cheap microscopes.
Thanks, had missed the minus 10. Adds up nicely now.
Zeiss Standard WL (somewhat fashion challenged) & Wild M8
Olympus E-P2 (Micro Four Thirds Camera)

apochronaut
Posts: 3108
Joined: Fri May 15, 2015 12:15 am

Re: 185 or 195 listed in finite objective specs?

#10 Post by apochronaut » Sun Jul 28, 2019 8:39 pm

The numbers relate to the conjugate distance of the microscope optical tube and follow the DIN 58887 standard of 150mm.

Prior to China becoming a major budget line OEM for the established companies, they had developed an in house industry , likely based on copying the Nikon system, which used a 35mm parfocal objective design coupled to a 160mm tube length. The conjugate distance is 150 mm, the point where the objective intermediary image is collected by the eyepiece, 10mm down the tube. China still manufactures student microscopes to that standard, using 185mm conjugate distance objectives.
The bulk of their internationally known and OEM production however , is based on an additional DIN standard relating to the microscope objective parfocal length of 45mm. The DIN 58887 is still adhered to, so the extra 10mm of objective length , calls for a 195mm conjugate distance.

Post Reply