Spectrum Scientifics Blog

Very recently, we had a customer in the store who was looking for a high power hand magnifier. Not a problem, as we carry a few models that can get almost as high as hand magnifiers can take a person. But when asked what level of magnification they were looking for, they replied: “30X or more”. This made us do a double-take, as hand lenses just don’t go that high.

To understand this, you have to realize that 30x is well into the inspection microscope range. Heck even Compound Microscopes start at 40X magnification as their low setting.

Hand-held magnifiers, however are not optical systems like microscopes. They are usually a single, doublet or triplet lens acting as a single optical lens, and when using a simple lens as a magnifier there are certain optical rules to be obeyed.

The first is that the focal length (the point from the edge of the lens where light coming through the lens concentrates) determines the magnification of a lens. The shorter the focal length, the higher the power of the lens.

“So no problem” you might think “just make lenses with shorter focal lengths”. Well here is where the laws of optics start to give you grief. You see, as you shorten the focal length of a lens, its curvature must increase. Thus, as you get shorter and shorter focal lengths, the edge of the lens starts to look more and more like a portion of a glass ball. This cause a lot of distortion around the edges of the lens when you look through it. You can hep this situation a bit by using doublets or triplets (two or three lenses cemented together), but even they have limits.

Even more limiting is that as you shorten the focal length you must also decrease the size, or diameter of the lens. This means that as you get higher magnification out of a lens, it starts to become more and more like looking through a pinhole!

This is why most honest magnifier makers are careful about their magnification listings A hand reading lens usually is in the 2-4X range, and specialty magnifiers for people with severe eye problems are in the 5-8x range. At 10X you are dealing with close-up inspection devices for jewelry, botany, or other close up work. The maximum these hand held magnifiers can really get is about 20X. Even then the lens is only about 8mm across and has to be practically on top of the thing being viewed.

But lately, there has been a rather insidious trend of imported lenses coming in with rather ‘generous’ magnification listings on the box or the product itself. If you go to Amazon.com and search for “30X Loupe” you will come up with many results.

And they are all lies.

This isn’t some retailer stretching the truth about the magnification, this is a lie right on the product. Several of these magnifiers say “30X x 21mm”, which means 30X magnification with 21mm diameter lens. THAT IS JUST NOT PHYSICALLY POSSIBLE. A lens 21mm in diameter is unlikely to produce anything close to 30X, and is more likely to be 7X or so.

The frustrating part of this is that some of the magnifiers are actually very good products! At Spectrum we carry a couple of very nice magnifiers, such as this illuminated tabletop magnifier. While this is a great product, the box states incorrectly several times that the lens provides 5x power. Measurements and optics math suggest this is not true and is closer to 3.5X at best. This is still an excellent instrument, but it is sullied by the fact that we must correct the manufacturer’s over-optimistic specs.

This reminds us of the box packaging on cheap telescopes that tries to sell the telescope via very unlikely super-high magnifications (660X!!! 800X!!! on a 50mm telescope) that impress people while more honest manufacturers suffer for telling the truth. Unfortunately, as long as there are sellers out there that don’t know enough or don’t care these fake magnifications will continue to be used to impress folks.

Fortunately for us our customer went away happy and as one of them stated “I think we learned something today”. They had been told about getting these ‘higher power magnifiers’, but didn’t know the truth about them. That’s what we strive for.

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Filed under: Magnifiers, Optics