# B Trying to buy some simple glass double-convex lenses

1. Nov 15, 2016

### escuta

Dear forum,

I'm currently trying to buy some simple glass double-convex lenses with no optical coatings from China with the help of a colleague over there. I'm trying to match a lens that I already have but which is no longer readily available to me here in Brazil. The lens I have is double-convex, has a diameter of 125mm and a focal length of 318mm.

I'm searching on a website my colleague sent me and have found some lenses with the correct diameter but with what appear to be magnifying strengths, eg. 3X, 5X, 8X, 10X

Is there a way to calculate the focal length from these magnifying ratings?

Thanks!

2. Nov 15, 2016

### pixel

Hard to know exactly what they mean, but if they are saying these are the magnifying strengths for using the lens as a simple magnifier, then the usual equation is:

M = 25 / f where f is in cm.

That's for an image at infinity, which minimizes eye strain; an alternate equation is M = (25/f) + 1 for an image at a standard close focus taken to be 25 cm.

A lens of your focal length is usually not used as a magnifier, and it has an unusual focal length. Can you find out how the website is defining the magnifying strength? Or better still, simply ask them for the focal lengths.

3. Nov 15, 2016

### escuta

Thanks pixel. So using your second equation, a 3X magnification lens (if that what 3X stands for) would have a focal length of 12.5cm? That's too short for my purposes. Unfortunately, with the exception of the numbers, all the text on the site I'm looking at is in Chinese which i don't read. I'm going to China next week however, so when I'm there I'll ask someone to ask the manufacturer if they can provide focal length data. Just to clarify, the lens that I have I bought from Edmund Optics some years ago (they no longer stock it), it has a 318mm focal length rating - and sure enough - when the viewer looks through the lens at close range, objects 318mm from the lens appear in sharp focus. I don't remember it having a magnifying rating. I'm using the lens for an arts project to bring a specific region in a diorama into focus and to produce chromatic aberration. All the best,

4. Nov 15, 2016

### CWatters

5. Nov 16, 2016

### escuta

Perhaps a bit short, but thanks I'll search more on this site.