A key factor for selecting loupes-magnification

A Key Factor for Selecting Loupes: Magnification Power or Resolution?

/in ,
Print Friendly, PDF & Email
ErgoPractice News – September 2020
By Jin Chang PhD
jchang@surgitel.com

Marketing Slogans

Long ago, some loupe manufacturers sold their loupes as higher magnification power loupes than actual magnification power as a selling gimmick, for example 2.5x loupes as 2.8x. And, still today many companies are selling their Galilean loupes as higher magnification powers. Some of you may have those loupes. Therefore, Designs for Vision started to use “True Magnification” as their main marketing slogan which implies that only their loupes offer true magnification power. Designs for Vision is still using this slogan today.
Then Orascoptic has created its own selling tactic, “Resolution is more important than Magnification,” claiming that their 2.5x loupes offer the highest resolution among loupes tested by a third-party lab. This third-party lab has not been disclosed to the public, making peer-review impossible. Orascoptic has been claiming that “image clarity is the product of resolution and light transmission” which sounds fancy but means very little. Most clinicians not only cannot verify that these claims are true but also may not understand these claims. But the mystique of the (unknown) third-party lab lends an immediate feeling of authenticity. Since Orascoptic’s claims may be designed to create confusion, we would like to review these claims by telling you the whole story.

Recognizing that improperly designed loupes were the main cause for chronic neck pain, in the early 1990s, SurgiTel started to develop ergonomic loupes and coined its slogan of “Vision and Ergonomics at Work.” SurgiTel has continued to develop many world-first products with patented concepts, including Ergonomic FLM (front-lens-mounted) loupes, Ergonomic TTL (through-the-lens) loupes, Portable Headlights with Small Halogen Light Bulbs, LED Headlight with Touch-free Switching, True Color LED Headlights, Loupe-mounted Digital Cameras, Loupes with Interchangeable Working Distance Caps, and Custom Nose Pad Systems. Today SurgiTel offers a full range of ergonomic loupes (2.5x to 8x) in both FLM style and TTL style.

Before answering questions about Orascoptic claims, we will review the functions of the human eyes as an adaptive optical system (Figure 1). This is important because loupes work with the eyes to create what the user perceives. Loupes are just a vision aid device to help eyes see more details.

Eyes as An Adaptive Optical System

The eye is a sophisticated imaging system that can control incoming light to maximize the quality of images and to protect the retina by adjusting pupil diameter and the focal distance of eye lenses.3 The pupil diameter can quickly vary from less than 1mm to 3mm when illumination levels change. The eye’s resolution capability increases as the pupil diameter increases because there is more surface area of the lens used. The increase of resolution capacity usually stops around 2 mm to 3mm because human eye lenses are not a perfect lens. Eyes (on average) can perceive details larger than 75 micron-size details located at about 16 inches (Figure 2). Using 2.5x loupes set to that working distance, we can see about 30 micron-size details. It is noted that high-resolution 2.5x oculars (25 LP/mm) can resolve about 20 micron-size details. Even poor resolution 2.5x loupes (20 LP/mm) can resolve about 25 micron-size details which are above the resolution capacity of eyes!

Without any vision-aid devices, we can see greater details by simply getting closer to objects because the size of the image projected onto the retina is increased as the distance decreases. But getting closer is not a good solution for clinicians. Not only would you be unable to use your tools, but your posture would be unsustainable.

To compensate for the limitations of our eyes and to avoid poor working postures, clinicians are required to use ergonomically designed loupes. Not only do they bring the object’s image “closer” through magnification, but well-designed loupes also support safe working postures not to have neck or back pain, or neck or back injury.

Resolution of Oculars (Loupe Optics) and Resolution of Loupes

Two core designs of telescope systems were invented by Galileo Galilei in 1609 and Johannes Kepler in 1611. These core designs are still used for the design of loupes. Gallian loupes consist of an objective lens and an eye lens, while Keplarian/Prism loupes consist of an objective lens, prism, and an eye lens.

Lenses used in loupe optics, even though they are now designed with advanced computer programs, function similarly and are not of proprietary designs. The imaging quality and resolution capability of major brand oculars are very similar if the size of objective lenses is similar. The resolution of oculars with smaller objective lenses will be lower. Comparing objective lens sizes, we can compare the resolution capability of oculars. To take high-resolution photos with a camera, we open the iris of the camera to expose more of the objective lens (small F-number). Casually looking at photos taken with small F-number (large lens area) and photos taken with large F-number (small lens area), one cannot tell the difference, but we can see the difference if the images are magnified.

The resolution of loupes as a binocular optical system depends on the oculars’ resolution and the alignment of two oculars to each other. Since oculars’ resolution is higher than eyes’ resolution capability, the alignment of two oculars to each other will make a greater impact on the resolution of the loupe image. The final resolution of loupes is determined by loupe alignment technologies used to assemble loupes.

SurgiTel’s proprietary and patented manufacturing technologies allow SurgiTel to offer loupes with maximum resolution and a maximum depth of field. For FLM loupes, stability and adjustability of ocular mounting fixtures are two important factors to set up and maintain ocular alignment. SurgiTel’s patented mounting fixtures are used to create stable, binocular resolution for all magnification powers (2.5x to 8x). For TTL loupes, stable frames and precision gluing technology are two important factors. SurgiTel’s patented frames with custom nose pads can support all magnification oculars and proprietary gluing technology allows us to assemble loupes for maximum resolution and a maximum depth of field.

How to Compare Magnification Powers of Different Brand Loupes

We all try to compare different loupes by looking at a target and trying to compare side-by-side, but this technique can be misleading. In the same way, an object looks much bigger when closer to you and a farther image looks smaller, demonstration loupes built for a short working distance make the image look larger and thus more magnified. Some companies will build demonstration loupes with a purposefully short working distance to make the magnification appear more powerful. When the model is built for you at a realistic working distance, the images will appear smaller due to the working distance effect.

Therefore, if you want to compare the magnification power of different brand loupes, you should make sure that the working distance of loupes should be exactly the same because a slightly different working distance will significantly change the visual acuity. For example, imagine we compare two loupes, one with a 15” working distance and another with a 16” working distance. The difference in the working distance is about 7%. With the pair of loupes with 15” working distance, lines appear 7% longer, areas of 2D objects appear about 15% larger, and volumes of 3D objects appear about 20% bigger than with the loupes set only one inch longer.

Although it may be not easy for most of us to compare magnification powers accurately, we may recognize loupes marked with significantly higher power than actual powers. For example, if Galilean loupes are marked with 3.5x, but its field size is large, the marked magnification power may not be true magnification.

In Conclusion: Buyer beware!

In this issue, we discussed how several marketing slogans were created based on what their marketing departments think will work, not on science. Many loupe marketing slogans today are purposefully misleading and are to counter scientific principles.

When purchasing loupes one must first determine the requirements. At SurgiTel, resolution and light transmission requirements are the minimum. Most reputable companies have already met these requirements for many, many decades. The human eye cannot even take full advantage of the resolution of ocular lenses. This is not new.

But the resolution of individual oculars does not guarantee image quality if two oculars are not aligned precisely. SurgiTel developed advanced manufacturing technologies (proprietary and patented) which can provide the fine alignment required for best binocular vision and best ergonomics.

SurgiTel also provides the greatest number of magnification choices, from 2.5x to 8.0x, offering both FLM style and TTL style, to meet all clinicians’ needs.

And finally, so important it is half of our slogan “Vision and Ergonomics,” the best loupe vision in the world could be hurting you, risking injury, and cutting your career short if loupes do not have the best ergonomic design. See our other articles in this series describing in detail how clinicians switching to SurgiTel loupes have saved their health and careers by not accepting working pain.

For more information about this, or any other loupe/headlight topics, you may contact your local SurgiTel Representative at www.SurgiTel.com/MyRep.

 

Questions and Answers

Question #1: Orascoptic is promoting that their 2.5x loupes offer the highest resolution among loupes tested by a third-party test lab. Is SurgiTel one of the loupes tested? If not, what is the resolution of SurgiTel 2.5x loupes.

Answer: Orascoptic has not disclosed the name of a third-party lab and the names of products tested. We never submitted our loupes for this test. The resolution of SurgiTel 2.5x oculars is about 25LP/mm while the resolution of Orascoptic 2.5x oculars is 22.63 LP/mm. It should be noted that the resolution of all major brand loupe oculars may be similar and higher than the resolution capacity of eyes. High resolution should always be a minimum design requirement, but it cannot be a key factor for selecting loupes. To overcome the resolution limitation of eyes. Eyes need higher magnification to make the details bigger.

Question #2: Does the light transmission of oculars affect image clarity?

Answer: If lenses do not have anti-reflective coatings, there will be multiple reflections between lens surfaces which may reduce the image contrast, but this will not reduce the image clarity noticeably if headlights are used. The Orascoptic third-party lab might have used poor quality competition’s loupes missing, or with damaged, anti-reflective coatings for their tests. It should be noted that all major brand loupes optics should have anti-reflective coatings. This is an industry-standard and not a proprietary technology.

Therefore, the major factor which affects the image clarity is the alignment of two oculars with each other. SurgiTel’s quality optics with multi-layer anti-reflective coatings and advanced assembly technologies (proprietary and patented) allow us to offer loupes with maximum clarity and a maximum depth of field.

Question #3: Is the clarity of images the product of resolution x light transmission?

Answer: This equation is not a scientific formula. This may be an artificial simplification that was created to distract customers from considering truly important factors for selecting loupes such as right magnification power, ergonomic declination angle to support safe neck postures, features of frames to allow necessary declination angles, and maintenance issues. Misalignments of two oculars will affect the clarity of binocular images through loupes. As magnification power increases, the accuracy of alignments is increasingly critical.

Questions #4: Orascoptic’s student buying guide indicates that aged eyes need loupes with a greater light transmission than young eyes. Is it true?

Answer: If headlights or overhead lights are not used, this statement may be true. If adjustable headlights are used, this statement is not true. Often too strong illumination can significantly reduce the clarity of images. This is because strong light causes our pupil size to decrease, making the usable surface area of our eye lens-less – as described in the article above and create glare that washes out details. Brighter is not always better. For the best clarity, the brightness of illumination should be optimized by slowly increasing the brightness of the headlight. Your eyes will naturally tell your brain something is “too bright.” When you get this signal, reduce the brightness slowly until your eyes feel comfortable. Battery packs with full analog control best optimize the brightness of illumination. If you are considering or using an LED headlight, please check spectral quality. Many of LED headlights have too strong blue of a spectral component which not only distorts color but also damages the retina.4

Question #5: I am a dental student and plan to purchase my first pair of loupes. How can I select the right magnification power for my first pair of loupes?

Answer: There are two ways to determine the magnification power for your first pair of loupes. The first method is to use historical data. When 2.5x loupes were recommended as the first pair of loupes, the average working distance of dental professionals was about 15” or shorter. If your working distance is 15” or less, you can choose 2.5x for your 1st pair of loupes. But if your working distance (WD) is longer than 15,” the desired magnification power (MP) will be determined by MP = (2.5 x WD)/15. If your working distance is 18,” desired magnification power should be 3.0x or higher. You may purchase 3.0x or 3.5x, but 3.5x loupes may be a better choice. The second way is to use curves shown in Figure 2 above. Let us use the middle curve with 1.5 pupil diameter. If your working distance is 18” (about 45cm), your naked eyes can see 175 micron-size details of objects located at 18” away. If you want to see 50 micron-size details, a required magnification power will be 3.5x (175/50 =3.5).

Question #6: I am a practicing dentist and have used my current loupes for about 5 years. Because I feel neck pain at the end of the day, I am considering an ergonomic pair of loupes which may help me to avoid my poor neck posture. Can I purchase loupes with the same magnification of my current loupes?

Answer: In general, your working distance will be increase by about 3” if you change from tilting your neck forward to having an upright position. If your current working distance is 15,’ your new working distance with an ergonomic pair of loupes will be about 18.” The increase in your working distance will be 20%. Therefore, you should consider loupes with at least 20% more magnification power. See Figure 3 for an example.

 

Summary:

Magnification power has long been used as the key factor for selecting loupes for enhancing vision,1,2 but Orascoptic has been saying that the resolution of loupe optics (called oculars) should be the key factor. Based on the basic optical functions of eyes as an adaptive optical system, which we will review below, we understand that the resolution capability of human eyes is relatively poor compared to loupe optics. The human eye cannot perceive the full level of detail that loupe optics transmit. Thus magnification, the function of loupes which makes details larger so our eyes to perceive them, should be the key factor for selecting loupes for enhancing vision.

Optical principles and eye functions relevant to this article:

  1. The resolution of imaging optical systems with larger front, objective lenses is higher than that of imaging optical systems with small objective lenses. Comparing surface areas of objective lenses, you can recognize which oculars can offer higher resolution.
  2. The resolution of loupes as a binocular optical system depends on both the resolution of oculars and the alignment of two oculars. The resolution of oculars is significantly better than that of eyes, but the resolution of loupes can underperform the eyes’ if the two oculars are not aligned precisely.
  3. Eye lenses are small and thus the resolution capability of eyes is significantly lower than the resolution of oculars with larger lenses. Therefore, eyes cannot see all details of images formed by oculars.
  4. Resolution capacity will decrease as the working distance increases. This is because the size of the target image on the retina becomes smaller as the distance from the object increases. Higher magnification power is required to keep the same detail as the working distance increases.
  5. Eyes naturally control incoming light to maximize image quality by adjusting the pupil diameter from less than 1mm to 3mm. This means human eyes can control incoming light by up to 10 times.

 

References:
  1. Leknius C, Geissberger M, The Effect of Magnification on the Performance of Fixed Prosthodontic Procedures, J Calif Dental Assoc. 23 (12): 66, 1995
  2. B. J., Chang, Ergonomic Benefits of Surgical Telescope Systems: Selection Guide, J Calif Dental Assoc. 30 (2): 161-169, 2002
  3. Optical Society of America, The Eyes and Vision, Handbook of Optics, Section 12, McGraw-Hill, 1978
  4. B. J., Chang, ErgoPractice News, Aug. 2020, https://surgitel.com/evaluation-of-major-brands-of-led-headlights-color-distortion-and-blue-light-hazard/