4.2.1 Flashcards
Who typically benefits from Low Vision Aids (LVAs)?
Patients with:
* Reduced visual acuity not corrected by standard spectacles
* Significant visual field loss
* Difficulty with near or distance tasks
* Good cognitive and physical ability to use the aid
What is magnification in the context of LVAs?
The increase in image size to compensate for vision loss.
Define linear magnification.
Linear magnification = Image size with aid / Image size without aid.
List the types of magnification.
- Relative size
- Relative distance
- Angular
- Electronic magnification
What is acuity reserve?
The difference between the patient’s acuity and the required task acuity, usually needing to be 2x for fluent reading.
How is Field of View (FOV) related to magnification?
FOV is inversely related to magnification; higher magnification leads to a smaller FOV.
What advantages do stand magnifiers offer?
Wider FOV than high-powered hand magnifiers and fixed focus.
What is a disadvantage of hand magnifiers?
Requires steady hands and has a limited field of view.
What are the advantages of spectacle magnifiers?
Hands-free use and good for continuous tasks.
What is a key disadvantage of telescopes?
Small field of view and requires practice.
What are the benefits of electronic LVAs?
Adjustable magnification and high contrast options.
What lighting is recommended for using LVAs?
Bright, even, and directed lighting improves contrast.
Fill in the blank: For near aids, reading adds of _______ or higher are needed.
+2.50D
What are examples of non-optical aids?
- Large print books
- Audio books
- Bold markers
- Talking watches
- Tactile labels
- Voice-assist devices
What is the relationship between working distance and magnification?
Higher magnification results in a shorter working distance.
What is the nominal magnification formula for simple lenses?
M = (D/4) + 1, where D is the dioptric power of the magnifier.
What factors affect the field of view?
- Lens size
- Distance from the eye
- Optical design
True or False: Higher magnification increases the field of view.
False
What is working distance?
The distance at which the user must hold the object from the aid to achieve a clear image.
What is a key consideration for hand magnifiers?
Variable working distance but requires steady hands.
When is the Nominal Magnification Formula used?
When calculating the magnification of simple plus lenses with the object at the focal point of the lens
This includes handheld and stand magnifiers.
What is the formula for Nominal Magnification?
M = (D/4) + 1
D is the dioptric power of the lens, and 4 represents the reference dioptric power of the standard near working distance.
What does the variable ‘D’ represent in the Nominal Magnification Formula?
Dioptric power of the lens
What is the reference dioptric power for the standard near working distance?
4D
What conditions must be met to use the Nominal Magnification Formula?
- The magnifier is not held close to the eye
- The object is placed at the focal point of the magnifier
- The eye does not accommodate and views the image in a relaxed state
Calculate the nominal magnification for a +10D hand magnifier.
3.5× magnification
M = (10/4) + 1 = 2.5 + 1 = 3.5
List the limitations of Nominal Magnification.
- Does not account for the eye being close to the lens
- Assumes the object is positioned correctly at the focal point
- Less accurate for high-powered lenses
What is Effective Magnification?
Used when a magnifier is held close to the eye, providing a more accurate measure of actual enlargement
What is the formula for Effective Magnification?
M = D/4
D is the dioptric power of the magnifier.
Calculate the effective magnification for a +10D handheld magnifier.
2.5× magnification
M = 10/4 = 2.5
Compare Nominal and Effective Magnification.
- Nominal assumes the object is held at the focal length and viewed from a distance
- Effective assumes the magnifier is as close as possible to the eye
True or False: Nominal magnification is higher than effective magnification when the magnifier is held close to the eye.
False
Why is it important for low vision patients to hold the magnifier close to their eye?
Maximizes usability, improves field of view, and provides a more natural viewing experience
