Low Vision Plus Lens Magnifiers

Flashcards covering the mechanics, magnification formulas, and field of view properties of plus lens magnifiers in low vision clinical practice.

When an object is closer it makes a __________ __________ at the eye, resulting in a larger retinal image.

larger angle

One method of magnification is __________ the viewing distance.

decreasing

Plus lens magnifiers allow for up-close viewing by placing the object at the __________ __________ of the magnifier.

focal point

A more powerful magnifier lens will have a __________ focal length.

short

In standard magnification calculations, the magnifier lens $F$ is compared to a benchmark of __________.

$$+4.00\,D$$

The historical standard viewing distance for magnification is the focal length of a $+4.00\,D$ lens, which is __________.

$$25\,\text{cm}$$

When the object is placed at the focal point of a plus lens, the formula for magnification is __________.

$$M = F/4$$

Magnification remains the same regardless of the __________ distance, as long as the object is at the focal point.

eye-to-lens

A short eye-to-magnifier distance provides a __________ field of view.

greater

A long eye-to-magnifier distance leads to a reduced field of view, which is often described as being __________.

like looking through a keyhole

Vertex powers ($F_v$ and $F_v'$) are measured from the lens surface using a __________.

focimeter

For a thin lens, the equivalent power is equal to both the front and back __________ __________.

vertex powers

For a thick lens, the magnification is determined by the formula __________.

$$M = F_{eq}/4$$

A more accurate way to measure equivalent power involves measuring the size of the object ($h$), the distance ($l$), and the __________ __________.

image size

The formula for equivalent power based on object height ($h$), image height ($h'$), and object distance ($l$) is __________.

$$F_{eq} = h / (h' \times l)$$

For a single lens, equivalent power can be calculated using the formula __________.

$$F_{eq} = F_1 + F_2 - \frac{t}{n}F_1F_2$$

The field-of-view of plus lens magnifiers is defined as the angular subtence at the image of the __________ __________.

ENTRANCE PUPIL

In the simplified formula for linear field of view ($y$), the variable $D$ represents the __________ __________.

magnifier diameter

In the formula $y = D / (a \times F_{eq})$, the variable $a$ represents the distance of the magnifier from the __________ __________.

ENTRANCE PUPIL