Trying to buy some simple glass double-convex lenses

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SUMMARY

The discussion revolves around sourcing glass double-convex lenses without optical coatings, specifically with a diameter of 125mm and a focal length of 318mm. The user is exploring options from a Chinese supplier and is confused about the relationship between magnification ratings (3X, 5X, etc.) and focal lengths. The equations provided for calculating focal length from magnification are M = 25 / f (for images at infinity) and M = (25/f) + 1 (for standard close focus). The user plans to inquire directly with the manufacturer for precise focal length data during an upcoming trip to China.

PREREQUISITES
  • Understanding of optical lens terminology, specifically double-convex lenses.
  • Basic knowledge of magnification and focal length calculations.
  • Familiarity with optical equations, particularly M = 25 / f.
  • Ability to communicate with suppliers in a foreign language or use translation tools.
NEXT STEPS
  • Research the specifications and applications of double-convex lenses.
  • Learn how to calculate focal lengths from magnification ratings using optical equations.
  • Investigate suppliers of optical components in China, focusing on those with English support.
  • Explore the effects of chromatic aberration in lens design and its applications in art projects.
USEFUL FOR

This discussion is beneficial for optical engineers, artists using lenses in projects, and anyone involved in sourcing optical components for specific applications.

escuta
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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!
 
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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.
 
pixel said:
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.

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

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