Prescription of Eyeglasses and Custom TTL Loupes

ErgoPractice News – October 2021
By Jin Chang PhD

Properly fitted loupes can play two important roles improving vision and improving working posture. We reviewed the effect of the viewing angle (declination angle) of loupes on the neck posture and identified that improperly fitted loupes are the main cause of chronic neck pain. With this October issue of ErgoPractice eNews, we will review the effects of prescription eyeglasses on the quality of images formed through loupes.

Loupes are a passive vision aid device that magnifies images formed by eye lenses onto the retina of the eyes. The image size of objects seen by eye lenses becomes larger as the distance decreases.¹, ² That means getting closer to an object will help us see more details of that object. If your working distance is longer than your peers, you will need higher magnification power loupes to see the same details as your peers.

Eyes as an Optical Imaging System:

Figure 1 shows the functions of the eyes. The focal length of normal/perfect eyes can be instantaneously adjusted to focus on objects located at any distance, but the minimum focal distance of eyes decreases as age increases.  Figure 2 shows the accommodation power of eyes (in diopter power) as a function of age. If eye lenses are not perfect spheres, images formed by these eye lenses are not sharp and thus these non-spherical eye lenses need prescription eyeglasses to help eyes form sharp images. Old eyes cannot focus on objects located at short distances than young eyes and thus need reading glasses (called diopter lenses).

The average population may start to notice that they feel difficulty in reading fine prints from the age of about 40. Figure 3 shows an example of a bifocal prescription. If the prescription of eyeglasses is not implemented with custom loupes properly, images formed through those loupes may be of poor quality.

Figure 1: Functions of Eyes as an Imaging Optical System

Figure 2: Average Accommodation of Eyes vs Age

Figure 3: Example of Prescription Chart

Implementation of Prescription of Eyeglasses With Loupes:

Front-Lens-Mounted (FLM) loupes are mounted in front of eyeglasses (bifocal areas of eyeglasses) and thus optics of FLM loupes do not require prescription lenses. But Through-The-Lens (TTL) loupes require prescription lenses as a part of the eye lenses of TTL loupe optics. Loupes should use a bifocal prescription, but carrier lenses have several prescription options such as single distance, single read/single monitor, and bifocal prescription for reading or watching monitors.

There is another option that replaces prescription lenses built into loupe optics with drop-in prescription glasses placed behind loupes (Figure 4). If users want to use contact lenses, drop-in prescription glasses can be removed. Note: there are several issues with drop-in prescription glasses when the prescription is strong.

Figure 4: SurgiTel Con2our Loupe with Drop-in Rx 

Potential Issues with Drop-in Prescription:

If prescription lenses are added as a part of eye lenses with loupe optics, then the optical axis of prescription lenses can be precisely aligned. This means that the quality of images formed through loupe optics can be very sharp, but with drop-in prescription glasses, the alignment will be compromised. Therefore, for a strong prescription, this drop-in option is not recommended. Another issue is the added weight of drop-in prescription glasses. Also, drop-in prescription glasses can touch eyelashes or the face creating more fogging problems.


Consideration of Cost for Updating Prescription When Purchasing Loupes:

The major maintenance cost of TTL loupes will be updating prescriptions because only manufacturers can do such. Updating prescriptions may take several weeks and is expensive. When should prescriptions be updated? The first case is the significant change of accommodation of aging eyes which increases the working distance. The second case is the significant change of cylindrical power (astigmatism) which makes images blurry. Second cases should be handled by manufacturers, but for the first case, there is a simple solution with SurgiTel loupes.

The working distance of SurgiTel Loupes can be restored using a new pair of working distance caps without losing time for updating prescriptions by manufacturers. SurgiTel interchangeable working distance caps are designed based on a patented concept. Considering the maintenance cost, the cost of SurgiTel loupes is the most economical. Updating prescriptions with all other brand TTL loupes is required to be done by manufacturers, requiring expensive updating costs and lost time.

SurgiTel Loupes as Multi-Vision Loupes with Interchangeable Working Distance Caps:

Since eyes can see better with a shorter distance, SurgiTel loupes can be used as a multi-vision loupe. For example, if the working distance is reduced by 10%, images projected onto the retina of eyes will be increased by 10% of line, about 20% of the area, and about 30% of volume.²


Why SurgiTel is Your Partner for Your Vision-Aid Products:

The founding mission of SurgiTel has been providing clinicians with truly ergonomic loupes and good headlights. During the last thirty years, SurgiTel introduced many world-first vision aid products based on patented concepts, including customizable FLM loupes, ergonomic frames, truly ergonomic TTL loupes, true-color LED beam-forming optics which allows SurgiTel to offer safer LED headlights with the best color accuracy, LED lights with no-touch switching, multi-vision loupes with interchangeable working distance caps, interchangeable ergonomic nose pad systems, and loupe-mountable digital cameras (SurgiCam® HD). SurgiTel offers unique benefits which may not be available with other brands as follows:

  1. SurgiTel offers the most magnification options (2.5x to 10x)
  2. SurgiTel offers all magnification loupes with either FLM style or TTL style
  3. SurgiTel offers the most LED light options (warm, neutral, and cool) with various styles (wired, wireless, and no-touch switching)
  4. SurgiTel offers truly ergonomic loupes to any clinicians with various facial features which have several benefits over deflection loupes (Designs’ Infinity Vue and LumaDent’s ErgoPrism loupes).³
  5. SurgiTel offers a free evaluation service – posture analysis with your current loupes and potential blue light hazard with your current LED lights

SurgiTel’s free evaluation service can help evaluate your working posture and identify the potential blue light hazard of various brands of LED lights. Click here to schedule a free evaluation.



  1. Anderson, DL, The Science of Vision: Introductory, Link: The Science of Vision: An Introduction (, Content Copy Right 2003 The Mind Project. All Right Reserved.
  2.  Chang, BJ, Choosing Loupes or Dental School: 3 Most Common Myths in the Dental Industry Demystified, Link: Choosing loupes for dental school |3 most common myths | SurgiTel, ErgoPractice News, July 2018
  3. Chang, BJ, Do Deflection Loupes with Steep Deflected Beam Meet Ergonomic Requirements? (SurgiTel Straight Ergonomic Loupes vs Deflection Loupes), Link: Do Deflection Loupes with Steep Deflected Beam Meet Ergonomic Requirements? (SurgiTel Straight Ergonomic Loupes vs Deflection Loupes) – SurgiTel, ErgoPractice News, September 2021