Ophthalmic Optics and Dispensing Flashcards
6 cards from real CPO practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Ophthalmic Optics and Dispensing flashcards as text
A patient with the prescription OD: -8.00 -1.50 x 180 and OS: -7.50 -1.25 x 175 is fit with new glasses. The refraction was performed at a vertex distance of 12mm, but the new frame sits at 8mm from the cornea. What is the most likely consequence of this uncompensated change?
Answer: The patient will report blurry distance vision due to under-correction.
Moving high minus lenses closer to the eye decreases their effective power. Since the glasses are now 4mm closer than the refraction distance, the -8.00D and -7.50D lenses will behave as if they are weaker, leading to an under-correction of the patient's myopia and resulting in blurry distance vision.
When dispensing to an aphakic patient who requires a +14.00 D spectacle lens, which of the following frame and lens characteristics is MOST critical to manage the significant visual distortions and weight?
Answer: A small, round frame with aspheric, high-index lenses and a short vertex distance.
Aphakic spectacles have very high plus power, causing significant image magnification, a 'ring scotoma' (peripheral blind spot), and lens weight. A small, round frame minimizes lens thickness and weight. Aspheric lens surfaces reduce peripheral distortions and the ring scotoma. High-index materials make the lens thinner and lighter. A short vertex distance helps minimize magnification.
A patient with significant anisometropia (OD: +1.00 DS, OS: +4.00 DS, Add: +2.50) is being fit with a standard lined bifocal. To prevent vertical imbalance at the reading level, a slab-off is ordered. On which lens and in what orientation is a conventional slab-off ground?
Answer: Base Up (BU) prism on the right lens (OD).
A conventional slab-off adds Base Up (BU) prism to the lower portion of the lens with the most minus or least plus power in the vertical meridian. In this case, the OD lens (+1.00 D) is the least plus. The BU prism counteracts the excess Base Down (BD) prism induced by the stronger OS lens when the patient looks down to read, thus correcting the vertical imbalance.
A patient returns with their new progressive addition lenses, complaining that they have to lift their head excessively to read comfortably. The frame fit, prescription, and measurements (PD, fitting height) have all been verified as correct. Which of the following is the most likely underlying cause?
Answer: The corridor length of the PAL design is too long for the chosen frame.
If the fitting height is correct but the patient must tilt their head up to find the full reading power, it indicates the reading zone is positioned too low in the lens. A progressive lens with a corridor that is too long for the vertical depth of the frame will place the widest, most stable part of the reading zone lower than the patient's eye can comfortably reach.
Which of the following markings on a pair of prescription safety glasses indicates they have passed the high-velocity impact test?
Answer: Z87+
The ANSI Z87.1 standard dictates markings on safety eyewear. The 'Z87+' mark signifies that the eyewear has passed both the high-mass and high-velocity impact tests. 'Z87' alone indicates basic impact resistance. 'Z87-2' denotes a prescription frame. 'D3' is for splash protection, and 'W' ratings are for welding shades.
A patient presents with a prescription of R: -10.50 / L: -11.25 and complains of chronic non-adaptation to their progressive lenses, specifically reporting a 'swimming' sensation and narrow fields of vision. All measurements have been verified. Which specialized lens design would be most appropriate to try as a next step?
Answer: A customized free-form progressive lens optimized for a high-minus prescription.
High myopes experience more peripheral distortion and magnification effects, which are exacerbated in standard progressive designs. A customized, free-form (digital) progressive lens can be specifically compensated for the patient's high prescription, frame choice, and position-of-wear measurements. This optimization minimizes peripheral aberrations ('swim') and can create wider fields of view compared to conventional designs.