Trying to buy some simple glass double-convex lenses

In summary, the lens that the user has is a double-convex lens with a 318mm focal length. The website the user's colleague sent them has ratings for magnification lenses, and the user is trying to find a lens with the same focal length but with a different magnification. The user is unable to find a lens with the correct magnification and instead looks for a lens with a different focal length.
  • #1
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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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  • #2
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
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,
 
  • #5
Perhaps a bit short, but thanks I'll search more on this site.
 

FAQ: Trying to buy some simple glass double-convex lenses

What is the purpose of a double-convex lens?

A double-convex lens is a type of lens that has two outwardly curved surfaces, making it thicker in the middle and thinner at the edges. It is used to converge or focus light rays and is commonly found in cameras, telescopes, and magnifying glasses.

What are the benefits of using glass double-convex lenses?

Glass double-convex lenses are preferred over other types of lenses because they have a higher refractive index, which means they can bend light more effectively. They also have a lower dispersion, which reduces chromatic aberration and produces clearer images.

How do I determine the right size and focal length for my double-convex lenses?

The size and focal length of a double-convex lens are determined by its diameter and curvature. The larger the diameter, the shorter the focal length will be. You can also use a lens calculator or consult with a specialist to determine the appropriate size and focal length for your specific needs.

Are there any special considerations when purchasing glass double-convex lenses?

Yes, there are a few key factors to consider when buying glass double-convex lenses. These include the type of glass used, the lens coating, and the lens tolerance. It is important to ensure that the glass has a high refractive index and low dispersion, and that the lens is coated to reduce reflection and improve image quality. It is also important to check the lens tolerance to ensure it meets your required specifications.

Can I use glass double-convex lenses for other applications besides optics?

Yes, glass double-convex lenses can also be used for non-optical applications such as in laser technology, medical devices, and scientific instruments. However, the specifications and requirements for these applications may differ, so it is important to consult with a specialist to ensure that the lenses you purchase are suitable for your specific needs.

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