I think without doubt that the majority of microscope users naturally tend to think that the objective is the most important part of the image making process. This is entirely understandable since we might think that the image produced by the objective cannot be improved upon when it enters the eyepiece?

The fact is that the eyepiece does have an important bearing on how the final image is perceived by the eye.

Before I dwell on this I must make it clear that this article is essentially about achromatic systems. Apochromatic and fluorite objectives need compensating eyepieces, whose properties are essential to the making of the final image, such that these objectives cannot produce a fully corrected image by themselves. In the achromatic system it is generally understood that the objective is designed to produce a corrected image, which is then magnified by the (non compensating) eyepiece . Usually, but not always, these eyepieces are marked with a 'K' or similar to denote the compensating property.

Whilst all eyepieces will produce an image from a given achromatic objective, most manufacturers design their objectives and eyepieces to complement each other, and therefore it should be expected that the best imaging comes from such pairings. In reality however, many used microscopes have 'adopted' objectives and eyepieces from other makers, so the perfect match should not be expected on all used 'scopes.

Eyepiece Designs

There are a handful of eyepiece types which have evolved over time, each trying to address the various problems of amplification of the prime image from the objective. Standard types have been assigned the names of the men who first designed them, amongst them are names of scientists/astronomers.......Huygens, Kepler, Kellner, Tolles, Ramsden etc.. In actuality most microscope eyepieces are based on the Huygenian designs and Kellners'. Other designs such as the Steinheil monocentric, Orthoscopic and Erfle are applied to other optical instruments in military use and binoculars, telescopes etc.

Of the many parameters that eyepiece design has to deal with, SIX of them are of importance to us :-

1) Magnification ( usually an amplification of the primary image of x5 up to x15 or x20 ).

2) Field width ( Diameter or angular separation between edges of view)

3) Flatness of field ( Ability to show the whole field in same plane of focus, 'flat field' or 'Plan')

4) Colour correction ( The 'perfect' eyepiece does not add any colour artefacts to the objective's image )

5) Eye relief ( The larger the eye relief the further away from the eye lens the eyes can be, and sometimes referred to as " high eyepoint ". Spectacle wearers need specially designed high eyepoint oculars, which tend to be expensive new, or in used condition ).

6) Cost.

It may not come as a surprise that all six are interrelated, and thus the eyepiece designer has to juggle with all of them to produce the "perfect" eyepiece. Of course there is no such thing, so the designer has priorities and therefore deals with these in preference to the others, and thus the result may be an eyepiece with excellent colour correction, field width and eye relief, but there may well be a lack of flatness of field. It's simply 'horses for courses', and almost certainly we cannot expect an eyepiece which has all six parameters suited to our liking. Generally speaking, the wider and flatter the field, the more complex the eyepiece's optics need to be to sustain image quality.

Huygenian Eyepiece

This is probably the most commonly produced eyepiece design. It is relatively straightforward and possesses the basic elements of design common to a larger number of related eyepieces. 

The eye lens does most if not all the magnification, and the field lens focusses the image in front of the eye lens in a manner that allows both lenses to nullify any chromatic aberrations etc. The field diaphgram limits the field diameter to the central area of best quality to be viewed, which in this design is at its lowest compared with more complex designs. A field of view of about 30-35 degrees is about the maximum that can be expected from the Huygenian eyepiece. Both lenses are usually plano-convex in section, and the flat sides face the observer.

However, the quality of imagery produced by this design when well made and using coated optics can be first class, though the eye relief is limited, but not uncomfortably so.

Kellner

This is a variation on the Huygens eyepiece, and has a two piece achromatised eye lens. In fact this design has many variations, some having both the field and eye lenses in achromatic forms, making for more perfect corrections than the Huygenian, with wider fields and better eye relief, but this of course comes at a price. This arrangement can be found in all types of optical instruments, and when well constructed can provide excellent high contrast imagery with moderately wide fields.

Some thoughts

Have the manufacturers, in their quest to provide us with wider fields, greater eye relief and magnification etc., lulled us into accepting this 'enlarged view' of the microscopic image as the norm?

It is my humble opinion that we should not forget the basic 'Huygenian' designs simply because they have just two elements and relatively cheap and abundant. Seeing is believing in this department, and believe me my favourite pair of achromatic eyepieces are Huygenian x8, from surprisingly ............'no name' optics! :- 

They elicit crisp imagery to the very edge of the field, which is commendably flat with all low to medium power achromatic objectives ( onboard a Zeiss Photomic 1* ), with fair eye relief and modest field diameter. The secondary colour spectrum is as low as can be expected from achromatic objectives, but the off axis colour fringing seen with some eyepieces is virtually none existent to the very edge of the field. Since having only four air to glass (coated) surfaces, the light transmission and contrast are superlative. Coupled with this delightful performance is the very welcome property that it is more tolerant of dust on the lens surfaces than others, where the dust can sometimes be glaringly apparent.

As photographic eyepieces they leave a lot to be desired, but for 'live' observation they are in my opinion THE oculars to have as they deliver neat, compact, sharply rendered images showing considerable detail, and at low cost in comparison to the high mag. complex wide field hybrids which require eyeball turning manoeuvres to get from one side of the field to the other! In short, they are the best all round eyepieces I have used to date.

* (Used in other stands, these eyepieces still provide quality imagery, but the field is not quite so flat. The internal optics of the Zeiss Photomic such as the 'Optivar' may have something to do with this.)

Buying secondhand eyepieces

Most observers will during the course of time want to try another eyepiece(s) for various reasons. It is important to be aware of the distinction between compensating and non compensating types, and that the latter will provide strong colour artefacts with low to medium power achromatic objectives.

Be cautious about high power oculars, for whilst they might impress initially, they can be very tiring on the eyes, causing excessive eyesweeping movements across the field, which would not occur with more moderate powers.

Some designs have the annoying problem of perpetual dust 'specks' in the fields of view, caused in the main by having the field lens in partial focus with the eye lens, and therefore highlighting all the minute dust specks along with the image. Other designs are more tolerant to this problem, and cause less fatigue.

Be warned though that the two element Huygenian designs vary considerably, and the only way of finding out if they suit your particular optical system, is of course to try them? Some appear to be capable of being used with the high power objectives which normally perform better with compensating eyepieces. These eyepieces have in effect some 'compensating' properties and this shows with lower power objectives, by indication of colour fringing around the outer sectors of the specimen nearer the field edges, but to a lesser extent than with true compensating oculars.

Dust, and testing for defects on glass surfaces.

Inevitably, dust appears in the field of view over a period of time despite efforts to avoid this, but a simple method of 'looking' for permanent scratches or marks on the glass surfaces of prospective eyepieces, is to use them with the substage iris closed right down using a x 40 objective. Under these conditions any foreign matter or undesirable permanent marks on the surfaces of the eyepieces optics will be glaringly apparent, and therefore this simple exercise is the quickest method of 'testing' for surface defects. Any remaining marks using this method after careful cleaning are probably permanent . This 'test' should be performed if possible when buying secondhand eyepieces.

A simple technique to reduce the appearance of these undesirable marks in a favourite eyepiece(s), is to use diffuse illumination to reduce light intensity with little need to reduce the substage iris, OR by using semi crossed polarising filters to achieve a similar effect (of the two, the diffusion method provides the cleanest results) . The object of these techniques is to provide a comfortable level of illumination intensity, yet allowing the objective to operate at or near full aperture, which helps to masks the presence of these marks. Though it must be emphasised that these methods work best with the low to medium power objectives,

Conclusion

Expensive multi-element eyepieces do not necessarily provide the best quality imagery with the humble achromatic objective. I have found that the simplest of designs is capable of extracting the very best out of the primary image from the objective. However, if wide field is an important issue, then clearly the humble Huygenian eyepiece is not for you. It is a simple fact that no matter how high the quality of the eyepiece is, higher powers above about x10 reveal no more detail or information with achromatic objectives, and the image will if magnified too much, appear less convincing, and will be diluted in contrast and colour. A magnification factor of about x8-10 in the eyepiece is enough to reveal all the information that any achromat objective can yield for those blessed with average to good eyesight.

If like me, you are not so enamoured with 'virtual reality' or 'bin-lid' fields of view, then do some prospecting for older eyepieces, and with luck you might find some which reveal super crisp imagery in a neatly packaged and conveniently scanned field of view with no visible artefacts other than those produced by the objective itself.

Sorry my 'NO NAME' x 8's are not for sale!

Photo's taken with Nikon 800 digicam.