So, I am buying something from a pawn shop...

[Ficer67]

I am looking for a lamp with a 5x (or better) magnifying glass built in to it. Kinda like this one here. Of all of the things that I have to be aware of when I buy used, it occurs to me that I might not be able to determine the power of the magnifying glass.

So there-in kinda lies my question. How do you tell what the power of a magnifying glass is, in a pawn shop.

[Uber_the_Goober]

@Ficer67

I am looking for a lamp with a 5x (or better) magnifying glass built in to it. Kinda like this one here. Of all of the things that I have to be aware of when I buy used, it occurs to me that I might not be able to determine the power of the magnifying glass.

So there-in kinda lies my question. How do you tell what the power of a magnifying glass is, in a pawn shop.

Numbah 1 - linky broken.

Numbah 2 - Measure the height of text without magnification. Then, magnify it. Measure the height of the text while it is magnified. Do math.

If you have text that is 5 mm high unmagnified, which becomes 25 mm high when magnified, my guess is that it’s a 5x magnifying glass.

Unless I’m wrong. In which case, refer back to Numbah 1, and re-post so someone smarter than me can help you.

[ZenBeam]

Here’s the fixed link.

[astro]

Here

Magnification

Some math involved.

Optical magnification is the ratio between the apparent size of an object (or its size in an image) and its true size, and thus it is a dimensionless number.
Linear or transverse magnification — For real images, such as images projected on a screen, size means a linear dimension (measured, for example, in millimeters or inches).
Angular magnification — For optical instruments with an eyepiece, the linear dimension of the image seen in the eyepiece (virtual image in infinite distance) cannot be given, thus size means the angle subtended by the object at the focal point (angular size). Strictly speaking, one should take the tangent of that angle (in practice, this makes a difference only if the angle is larger than a few degrees).

[ftg]

For magnifying glasses, the formula for focal length (f) to magnitude is 25cm / f + 1. A 2x magnifying glass has a focal length of 25 cm. You’d want a focal length around 6 cm. Aim it at a far off light source, focus an image of the source on a piece of paper and measure.

[Ficer67]

Did I bugger the link up? And, I thought I was being careful. Thanks for fixing it zenbeam. Thanks for all the good information people. As it turns out, the store did not have what I want…

[Terry_Kennedy]

@Ficer67

Of all of the things that I have to be aware of when I buy used, it occurs to me that I might not be able to determine the power of the magnifying glass.

Another thing to watch out for when buying used is oddball bulbs (technically, lamps, but saying “lamp lamps” is more confusing than “lamp bulbs” in this context).

If the bulb(s) burn out, you may have difficulty finding suitable replacements. The magnifier I use requires an oddball compact fluorescent which isn’t readily available. Some older ones use incandescent lamps and it is important to use replacements of a similar wattage - ones with a higher wattage can melt the plastic parts of the lamp.

To answer your original question, many of the better magnifiers will have a marking, either a sticker or molded into the case, stating the diopter value of the lens(es). At standardized viewing distances. 3.5d is about 1.88x, 5d about 2.25x.

[Rhythmdvl]

@echo6160

If you have text that is 5 mm high unmagnified, which becomes 25 mm high when magnified, my guess is that it’s a 5x magnifying glass.

But won’t the magnifying glass make the ruler appear larger as well? How will that help?

What you should do is take the lamp and a ruler on a spaceship and approach the speed of light…