Useless eyepiece?

[Dale]

I have a set of 10X, 15X, and 20X oculars. They pair with 4X, 10X, 20X, 40X, and 100X objectives.
The 10X and 20X oculars are Bausch & Lomb wide field , the 15X is unmarked. The 15X does not give enough of
an increase to bother inserting, and the field of view is markedly reduced.
Is there any point in keeping the 15X?
I have not oiled up the 100X yet, but I do not expect I would get a good image with the 20X.
Given the reputation of the 20X oculars, I think a lot of you would have me get rid of that one!
Dale

[75RR]

There is a formula (well a rule of thumb really) to calculate this: Range of Useful Magnification (500-1000 x NA of Objective)

In which the 500 x NA (Numerical Aperture) is the minimum necessary for the detail present in an image to be resolved,
and anything over 1000 x NA gives empty magnification.

as an example:
For a Plan 40/0,65NA Objective
the range would be between
500 x 0,65 = 325 and
1000 x 0,65 = 650 so
below 325x total magnification, detail will not be resolved
above 650x total magnification, no additional detail will be resolved

best Useful Magnification Range for this objective is between 325x and 650x

A quick test is to multiply the NA of an objective by 1000,
then multiply the magnification of the objective by the magnification of the eyepiece,
if the second figure is higher then using that objective with that eyepiece will result in empty magnification.

Another useful test is to divide the max magnification by the objective's magnification, this gives you the max eyepiece magnification for that objective.
In this case 650/40 = 16. Therefore a 16x eyepiece would be ok (just) but a 20x eyepiece would give empty (too much) magnification with this objective.

It is good to keep an eye on this, especially if you have a multiplier like an Optovar or its equivalent which can increase total magnification by 1.25x, 1.6x and 2x.

Here is a previous post on the subject: viewtopic.php?t=1182

and a link to practical magnification, i.e. what we actually need: http://www.microbehunter.com/how-much-m ... do-i-need/

[gekko]

For what it is worth, while what 75RR said above is true, it assumes one has normal vision. For a person with sufficiently poor vision who may not be able to see sufficient detail using optics obeying the above rule of thumb, higher eyepiece magnification may be useful.

[Charles]

Also, those 15X and 20X are not useless...they can be used on your stereo microscopes.

[billbillt]

I use 20x oculars on my Leitz all the time... Does a good job for me.. I am happy with them... But, I am not a perfectionist... 95% perfection suits me...

BillT

[apochronaut]

Also, those 15X and 20X are not useless...they can be used on your stereo microscopes.

.... where the same N.A. to mag. ratio applies, so as many people have found out 15 and especially 20X eyepieces can give a blurry image on a stereo as well, especially if used on the wrong microscope.
Stereo microscope eyepieces are specifically plan compensating, unless they are huygens or ramsden types, which have no claims or specific capacity to produce a flat field. Flat fields on older stereos, were attained by careful engineering of the objectives to meet the demands of Huygens or Ramsden eyepieces. Later generation stereos designed proprietary objective/eyepiece combinations. Sometimes swapping eyepieces around will yield good results but more often ,not. As an example, if we take the Bausch & Lomb Stereozoom 7 for instance, the 15X eyepieces corrected for that instrument are cat. # 31-05-68. Most B & L 15X you will encounter are either #31-05-63 , used for the stereozoom 1,2,3,4 and 5 only., or #31-15-62, which was for the Balplan diascopic microscope. They are easy to swap around and often can be found mis-appropriated but they do not give good results. Eyepiece/objective matching, is a critical component of manufacturing an excellent microscope , otherwise companies wouldn't invest tens of thousands of dollars doing eyepiece/objective matching and making clearcut catalogue recommendations.

Your, narrow field 15X eyepieces are almost for sure huygens type, for which narrow fields are one of their hallmarks. Despite their narrow field, they are a universal type of eyepiece, so are well corrected for many older achromats. One thing I have done is increase the field stop which sits inside the eyepiece and creates the field of view. Huygens have curvature of field anyway, so increasing the stop from it's original 12mm f.o.v. to about 16, didn't give a better peripheral image but gives the impression of a more expansive field and seems nicer to use. The field stop should be in sharp focus; if it isn't it has moved. A lot of eyepieces have a field stop insert, which is held in place with a pressure fit. They can move. The other thing that can happen, is with some eyepieces the thread for the bottom lens is the same as the thread for the top lens and these can accidentallly be reversed in the process of cleaning them. This will place the field stop , closer to the eyelens and give a ridiculously restricted field of view but in this case the edge of the field stop will not be focused.

[Dale]

That was a lot to digest, especially apo's post. I will keep them all.
I have to sell an extra stereozoom, but it will go without eyepieces now!
thanks,
Dale

[einman]

Interestingly, though B&L designated different codes for the 15X 31-05-62 for the Stereozoom 3-5 and 31-05-68 for the SZ7 B&L indicated the 20X 31-15-64 could be used for all the SZ's 3-7. I often pondered why this was the case. I have several pairs of B&L 15X eyepieces that came with my SZ7's neither of which are labeled as to part number. One set is black while another is aluminum without paint. I have used both 15X eyepieces in my SZ7 and don't recall observing a difference. They could be the same part number though given the difference in physical appearance I have my doubts. Though my eyesight is not to be used as a baseline under any circumstances.

Does anyone have a picture of the 15X 31-05-68 with the part number on it?

[Dale]

Mine are black painted metal, and are marked W15X-10MM. Aaaargh, I
just unscrewed the cap and the inside looks like dried white toothpaste!

[einman]

I personally use the 10X, interestingly no 10X eyepiece is specified in the manual for the SZ7. I also like to use the 15x on occasion. Rarely have I ever used the 20X eyepieces as the DOF is so shallow as to not be very useful.

[Dale]

I just spent 4 hours with mine, and found all 3 useful, but I could surely live with just the 10X.
What was really frustrating was with the 2X aux lens it was awkward manipulating many subjects.
Plus the led illuminator kept falling off because the aux lens is tapered.
So many projects!
Dale

[apochronaut]

Interestingly, though B&L designated different codes for the 15X 31-05-62 for the Stereozoom 3-5 and 31-05-68 for the SZ7 B&L indicated the 20X 31-15-64 could be used for all the SZ's 3-7. I often pondered why this was the case. I have several pairs of B&L 15X eyepieces that came with my SZ7's neither of which are labeled as to part number. One set is black while another is aluminum without paint. I have used both 15X eyepieces in my SZ7 and don't recall observing a difference. They could be the same part number though given the difference in physical appearance I have my doubts. Though my eyesight is not to be used as a baseline under any circumstances.

Does anyone have a picture of the 15X 31-05-68 with the part number on it?

The brushed aluminum 15X with a little B&L inside a triangle and a 15X W.F. on them are 31-05-62. They have a black ring around the top. At some point, likely around the mid-70's, they went to black anodizing on the eyepieces, similar to AO . The B&L logo should still be on them. Your two sets of eyepieces could be the same but there are also Chinese clones of B&L eyepieces out there, unmarked as to which model they are clones of and they are black.
Then , later the catalogue #'s were stamped on them. I have seen SZ 7s with brushed aluminum eyepieces and black ones. Eyepieces are the Rodney Dangerfield of optics. ....."oh , uh, here's a pair of 10X , put those in", No one respects their uniqueness, like they do objectives, so they get swapped around and quite often end up mis-matched, with the rest of the system. Another effect of eyepieces, I didn't mention is W.D. Two different oculars might work o.k. and give a well corrected flat image but they can each cause a different W.D. Sometimes a certain pair of eyepieces are recommended in order to give a correct or desired W.D.

[einman]

yes I noted the change in WD when I swapped the eyepieces from the SMZ-U to the SMZ-2T. The image was fantastic but the WD changed dramatically.

[Dale]

I found the tiny 'Japan', but I can't figure out what the junk is on the lower lens. It's
only around the periphery, and on the inside.

[apochronaut]

aluminum oxide?

[billbillt]

Hi,
Why do you suspect aluminum oxide?.. I know it is used for polishing lens glass, but that would have been caught upon final inspection... Obviously this objective has been in use for a time...

[Dale]

It is an eyepiece, probably older, but its partner is pristine.

[75RR]

Galvanic corrosion?

[apochronaut]

I am assuming it is an aluminum barreled eyepiece, maybe it isn't. I had one microscope , that had it everywhere, that the anodizing or other coatings, like enamel or grease, didn't cover. Threads, edges of tubes, where holes were bored. I don't know where it had been. but since Dale has since pointed out, subsequently, that the other one is pristine, then that doesn't make sense, assuming they have been a pair for a long time.

[Dale]

The stuff is on the barrel, the retaining ring, and a small amount around the glass. Looks like someone cleaned just the center
7/8, but did a good job as the glass is unscratched. I will attempt to clean it, veeery carefully!