Proper combination of objective and eyepiece

Everything relating to microscopy hardware: Objectives, eyepieces, lamps and more.
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AntoniScott
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Proper combination of objective and eyepiece

#1 Post by AntoniScott » Mon Apr 26, 2021 9:50 pm

The 5x obective with a 20x eyepiece yields 100x
The 20x objective with the 5x eyepiece yields 100x
The 10x objective with the 10x eyepiece yield 100x,
The same can be said for similar combinations of objectives and eyepieces.


Is there a formula for the correct combination to give best image results ?
I was led to believe that the best results were obtained by using the highest magnification objective although in some uses the lower magnification objective gives greater depth of field and working distance.

apochronaut
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Re: Proper combination of objective and eyepiece

#2 Post by apochronaut » Mon Apr 26, 2021 11:09 pm

The ideal combination can vary based on the user's criteria but there are some rules that must be followed, so the results are as ideal as possible and you don't stray into empty magnification or exceed the resolution limit of your eyes.

The total magnification of any combination should always be between 500 and 1000X the N.A. of the objective. For fluorite and apochromat objectives, the upper limit stated above can often be exceeded by about 10%, sometimes a bit more.


5X eyepieces have an apparent field of view much narrower than a 10X, so using one gives the impression of looking down a tube. Their f.o.v. is restricted by the diameter of the eyepiece tube. That is why many mfg. made a 6 or 6.3X, which give a much wider field than a 5X. 5X tend to be used for very special purposes and are not a very good choice for general purpose viewing.

For many apochromat systems with a high power objective of 1.3 or 1.4 N.A. , a W.F. 15X eyepiece was the standard default. The objective would be made to in the vicinity of 90X, so the total magnification did not exceed the practical limit.

Low power objectives below about 6X often see more area than many condensers can illuminate and will need some kind of modification of the condenser to fill the field with light. Objectives of 10X and up seldom need any condenser modification.Using a low power objective can be useful if you require working distance but it can be difficult to fill the field with light and in an attempt to gain extra magnification , you can easily enter empty magnification. You will also lose the working distance, referred to as eye relief at the other end. High magnification eyepieces, unless they are of a superior and therefore usually expensive design always have shorter eye relief and are often stressfull to use, so a combination of low power objective and a high power eyepiece is usually a difficult and unnecessary choice. Working distance and potential f.o.v. are about the only reasons to opt for such a combination.

BramHuntingNematodes
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Re: Proper combination of objective and eyepiece

#3 Post by BramHuntingNematodes » Tue Apr 27, 2021 2:16 am

Those old B&L eyepieces had relief all over the place, with the 12.5 conpens over 21mm and the 15x huygens at a pretty much unusable 4.2mm.

The 5x are good for afocal photography when using small sensor size cameras. I don't know much other use for them. Maybe if using g a projecting prism, but then they made a separate projection eyepiece for that
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

Adam Long
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Re: Proper combination of objective and eyepiece

#4 Post by Adam Long » Tue Apr 27, 2021 12:28 pm

The pdf linked from this blog: http://bettermicroscopy.blogspot.com/20 ... ts-of.html
is a great primer on this topic.

TL:DR - the resolution available with low mag objectives often exceeds what can be seen with 10x eyepieces, so higher mag eyepieces can be usefully deployed. With high mag objectives the opposite occurs and you may be better using lower mag eyepieces.

apochronaut
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Re: Proper combination of objective and eyepiece

#5 Post by apochronaut » Tue Apr 27, 2021 2:40 pm

Adam Long wrote:
Tue Apr 27, 2021 12:28 pm
the resolution available with low mag objectives often exceeds what can be seen with 10x eyepieces, so higher mag eyepieces can be usefully deployed. With high mag objectives the opposite occurs and you may be better using lower mag eyepieces.
There are some qualifications to that.
The main one is that low magnification objectives are designed for primarily one of two purposes. The cost structure of each varies widely.
Some are for scanning and are sold as an inexpensive lens for mainly that purpose, often having a fair amount of field curvature , distortion and modest N.A. Based on the N.A. alone, they should give a good image with 20X eyepieces at say 100X total magnification but peripheral distortion and aberration in a condenser at it's field limit , the objective and further ; eyepiece limitations, conspire to provide one that is usually less than desirable. Not as useful as a 10X x 10X configuration. At magnifications below about 6X, the technical difficulties of producing a fully plan, highly resolved and distortion free condenser/objective/eyepiece combination increase inversely. That is why really good imaging objectives of low magnification are more expensive than 10X objectives in the same series, especially when one includes the additional condenser requirements, in order to meet it's demands.

So, then. There are low power objectives intended for high quality imaging.

No doubt, one could put 25X W.F. eyepieces coupled to a 2X .06 or higher plan objective and get a pretty decent image at 50X, certainly if the objective was planfluorite or planapo but the additional cost associated with all 3 of the components involved in order for the image to equal the quality of a 10X x 10X configuration and have the same ease of viewing would be substantial.

Seta
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Re: Proper combination of objective and eyepiece

#6 Post by Seta » Tue Apr 27, 2021 5:29 pm

Coming from a bellow set up and used to direct projection I never understood this objective/eyepiece magnification combo; looks like a gimmick to improve sales to me. When taking pictures I normally I just specify magnification on sensor

To me a 5X image is just that, a 5X no matter what camera I use(FF, APS-C or m4/3); eyepiece "cropping" factor would be similar to sensor crop factor. In fact what you see via direct projection to an APS-C camera is very similar to what you see via an 10X eyepiece, maybe a m4/3 camera would be a better match.

BramHuntingNematodes
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Re: Proper combination of objective and eyepiece

#7 Post by BramHuntingNematodes » Tue Apr 27, 2021 5:54 pm

With old finite apos you need the eyepiece to impart the final color corrections
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

apochronaut
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Re: Proper combination of objective and eyepiece

#8 Post by apochronaut » Tue Apr 27, 2021 10:03 pm

Seta wrote:
Tue Apr 27, 2021 5:29 pm
Coming from a bellow set up and used to direct projection I never understood this objective/eyepiece magnification combo; looks like a gimmick to improve sales to me. When taking pictures I normally I just specify magnification on sensor

To me a 5X image is just that, a 5X no matter what camera I use(FF, APS-C or m4/3); eyepiece "cropping" factor would be similar to sensor crop factor. In fact what you see via direct projection to an APS-C camera is very similar to what you see via an 10X eyepiece, maybe a m4/3 camera would be a better match.
Yes. excessively high magnification, low quality eyepieces are used as a gimmick to increase sales. It is important to attend to the N.A./ total magnification ratio, so understanding of the effect the eyepiece plays in that ratio is important.
Electronic imaging, especially without a photo lens is a somewhst different case.

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