Written by David Xu, MD
Ophthalmic lenses are often a lifetime investment, but the multitude of choices can be daunting for the beginning ophthalmologist.
Which Ophthalmic Lens to Choose?
The Volk Classic series lenses were designed around the 1970’s. Slit lamp lenses had powers of 60D, 78D or 90D, while indirect lenses ranged from 15D to 40D (most commonly used are the 20D and 28D). The Super series came about in the mid 90’s. They used new optical glass and had a better optimized aspheric shape to improve field of view, magnification, and small-pupil capability. The Digital series had further improvements in the lens shape and used newer optical glass with a higher index of refraction.
Basic Lens Theory
Slit lamp lenses are high plus power condensing lenses which create in an inverted view of the fundus. These lenses are differentiated by their dioptric power and their aspheric shape, resulting in different fields of view, magnification, working distance, stereopsis, and small-pupil capability.
There is a lot to consider when choosing lenses. For example, a higher magnification (actually lower dioptric power) lens yields a smaller field of view. Though this small field of view can be widened by increasing the physical size of the lens, the larger lens can be more difficult to hold. The higher power (lower magnification) lenses are generally better at converging light through a smaller pupil, but have the disadvantage of worsened stereo effect. In this article, I will explain the advantages and tradeoffs of each lens in order to make recommendations for the beginning ophthalmologist.
Slit Lamp Lenses
Digital Wide Field and 90D
I’ve used the Digital Wide Field and classic 90D lenses for a long time. The 90D is the classic all-around slit lamp lens with good field of view, comfortable magnification and excellent ability to examine through small or undilated pupils. In comparison, the Digital Wide Field has a wider field of view, the magnification is similar to the 90D, and it still has excellent small pupil capability. I like this lens because you can perform a complete exam of the posterior pole with the patient looking in primary. The lens can also be used to examine almost the entire retinal periphery except the far anterior retina. It is very capable through an undilated pupil, comparable to the 90D, and allows greater field of view. Glare and reflections, most pronounced with an undilated pupil, do take a little time to get used to especially when coming from a 90D or similar lens. Some people worry about the Digital series lenses causing optical aberrations, but I think the view of the fundus is sharp and aberration free – no need to worry about this. The working distance needs to be shorter to get the wider field of view, which leads to more lens cleaning to remove oils from patients’ eyelashes. All said, I think the strengths of a Digital Wide Field substantially outweigh the drawbacks and strongly recommend this as the primary workhorse lens for slit lamp examination. There is no need to buy a separate 90D.
The Digital Wide Field is on the left, 90D in the middle, and 78D lens on the right.
78D, Super 66, and Digital 1.0x
These higher magnification slit lamp lenses are, in my opinion, quite essential for any ophthalmologist. The traditional 78D has been in use for decades and is an excellent general purpose fundus or macula lens with better stereopsis than the 90D. The Super 66 and Digital 1.0x, both 66D, provide 1.0x magnification for optic nerve measurements without applying a scaling factor. The physical size of the lenses are all about the same and they are all easy and comfortable to use. They all need pupil dilation for a good stereo exam, but you could probably get away an undilated view of the optic nerve. The field of view is also very good, comparable if not better than the 90D, which make them great general purpose lenses as well. They all have a proven track record, so it’s up your individual preference for which to get. I think choosing a Super 66 would be an excellent all around choice.
The 78D lens is on the left, Super 66 in the middle, and the Digital 1.0x is on the right.
Digital High Mag
The Digital High Mag yields a very magnified view for macula or optic nerve exams to replace the classic 60D. It is a 55D lens with effective magnification of 1.3x. Compare that against a 90D lens which has 0.76x magnification or a 66D (1.0x magnification) lens. Stereopsis is excellent and allows better visualization of macular fluid, optic nerve elevation or simply getting a higher mag view when findings with a 90D are unclear. It does require a well dilated pupil, and the field of view is more limited than the other slit lamp lenses. Compared to a 78D or 66D, the Digital High Mag has noticeably greater magnification and smaller field of view. This makes the lens more suitable if you know a priori that you want to examine the macula or optic nerve and less suitable as a general purpose scanning lens.
20D and Pan Retinal 2.2
These two lenses are great general purpose binocular indirect lenses. Between the two, the Pan Retinal has greater field of view at about the same magnification (2.68x vs 3.13x). This makes the lens easy to use and allows very efficient scanning of the retinal periphery or posterior pole. The Pan Retinal has become a fairly popular option for many new residents. It is as easy to use as a 20D, although perhaps less ergonomic since the lens is slightly bigger and requires a slightly longer working distance. Nonetheless, I’ve seen ophthalmologists with small or large hands use this lens without any problem. If you were to pick only one of these two lenses, I would recommend the Pan Retinal 2.2.
From left to right, the Pan Retinal 2.2, 20D, Digital Clear Field, and 28D.
Digital Clear Field and 28D
The 28D lens is a classic lens with many strengths. It is comfortable to hold, has great small pupil capability, is suitable for laser, and can be used for neonatal exams. The Digital Clear Field is the new indirect lens in the Digital series. I think of this lens as splitting the difference between the 20D and 28D – when I’m holding the Clear Field it feels like I’m using a (slightly larger) 28D. Whereas with a 20D the view of the fundus is brighter and more magnified, the 28D is more ergonomic and has a wider field of view. The Clear Field is more ergonomic than the 20D and actually has a wider field of view than the 28D. This “hybrid” lens blends the strengths of the 20D and 28D, so it’s possible to replace both with just one lens. Still, it does seem to need more accurate alignment with the pupil and proper working distance to fill the view. Also, when viewing the far anterior periphery, the smaller sized 28D lens is still the easiest to maneuver and align especially in patients with a larger nasal bridge.
|Field of View||Magnification||Working Distance|
|Slit Lamp Lenses|
|Digital Wide Field||124||0.72||5mm|
|Super Field NC 1||116||0.76||7mm|
|Digital High Mag||70||1.3||13mm|
|Pan Retinal 2.2||73||2.68||40mm|
|Digital Clear Field||72||2.79||37mm|
|1The Super Field lens was not included in this review. It is comparable to the digital wide field lens, but with less glare.|
3-Mirror vs 4-Mirror Goniolens
The Goldmann-style 3-mirror lenses allow examination of both the angle and retina using three mirrors of different orientations which visualizes the 1) iridocorneal angle and pars plana, 2) ora serrata to equator, 3) equator to the edge of posterior pole, and finally the central lens is a direct view of the posterior pole. The traditional 3-mirror lens has a large flange which vaults over the cornea, which is why it requires a coupling fluid like Goniosol. One must turn these lenses to examine the angle over 360°, which makes it more time consuming. On the other hand, they are more suited for performing gonio laser along with their single or two-mirror counterparts. Construction is either with glass or plastic – I recommend if you want to get a 3-mirror lens to get the glass variety.
The 4-mirror lenses allow faster exam of the angle without turning the lens. They directly contact the cornea with approximately the same curvature which requires no Goniosol, and allows for indentation gonioscopy to dynamically examine the angle. For most residents, the 4-mirror lens is more suitable since it’s faster to use, allows dynamic gonioscopy, and is the more modern gonio lens choice.
The G-4 is on the left and the G-6 is on the right.
Volk released a relatively new 6-mirror goniolens, the G-6. I tried this lens as part of the review over the course of a few months, and I came to see its advantages over the 4-mirror lens. The mirrors were reshaped to allow better access in a deep orbit, and the contact area over the globe is smaller leading to less squeezing by the patient’s eyelids. As far as having six mirrors, it did help me get a better sense of the 360 degrees of the angle. Keep in mind, there are six mirrors not eight, so you get superior and inferior views along with four diagonal views. Ultimately, I liked the orientation of the four diagonal mirrors in the 6-mirror lens as it made it easier to examine the angle compared to the nasal and temporal views in a 4-mirror lens.
Note: I wanted to review different Volk lenses and approached Volk for help. They were gracious to lend me lenses, but I did not receive benefits of any kind to write this review.