Magnifying glass

[AfterDark]

I use the one Trident makes that glues to the inside of the mask. A round plastic disk about quarter (US coin) size. Just enough to read my gauges but small enough not to get in the way of regular vision use. Been using it for 4 years now.

 

[Damselfish]

As far as gauge readers that glue inside your mask, the curved side is in air, so that's a different thing than a magnifying glass in water. Generally not very practical for looking at little critters which I'm pretty sure is what the OP is trying to do, the pygmy seahorse will not be happy.

If you're not using one of the commercial underwater ones with an airspace, or making your own, all else being equal the trick is to get the material with the highest index of refraction you can. Different types of glass and plastic vary, so you can't tell by just looking at it - unless of course you're underwater at the time. I'd heard suggestions for the Harbor Freight ones, but I tried 2 glass ones I found there and they weren't good for much of anything. But who knows if different batches even have the same type of glass. One was a rectangular one, and the other was round, with a larger 3x lens and a smaller 10x lens in the handle. The 10x lens did pretty well underwater, but was too small to be very useful. I've had the best luck with an old slide loupe, which is presumably some kind of high quality glass.

 

[tursiops]

As far as gauge readers that glue inside your mask, the curved side is in air, so that's a different thing than a magnifying glass in water. Generally not very practical for looking at little critters which I'm pretty sure is what the OP is trying to do, the pygmy seahorse will not be happy.

If you're not using one of the commercial underwater ones with an airspace, or making your own, all else being equal the trick is to get the material with the highest index of refraction you can. Different types of glass and plastic vary, so you can't tell by just looking at it - unless of course you're underwater at the time. I'd heard suggestions for the Harbor Freight ones, but I tried 2 glass ones I found there and they weren't good for much of anything. But who knows if different batches even have the same type of glass. One was a rectangular one, and the other was round, with a larger 3x lens and a smaller 10x lens in the handle. The 10x lens did pretty well underwater, but was too small to be very useful. I've had the best luck with an old slide loupe, which is presumably some kind of high quality glass.

Yeah, 10x is a good start! And a glass 10x is likely better than a plastic 10x. The slide loupes tend to be 8x or stronger, and are usually glass.

 

[bowlofpetunias]

Hubby has made a couple used PVC pipe with the lense inside and plastic on either side for the air space. Hardest thing was getting a good seal with silicon sealer. Work really well for seeing the small stuff. Just have to be careful not to get too close and injure/spook the critters!