Where can one find a Coddington-like lens?

[Nodda Duma]

Yeah I guess I can’t read numbers.

Poked around a bit and found a couple sources of 25mm or larger ball lenses.

There’s this source out of Czechoslavakia (I forget which optical shop that is). Scroll down to the 24-25mm polished:

http://www.glass-sphere.com/en/eshop/kulicky-pro-technicke-a-dekoracni-ucely-10-mm-40-mm.html

Also, search ebay for clear glass ball and you will find plenty of options that will work perfectly well.

 

[jsmoove]

@Maris Do dome magnifiers have a short focal length in general?
If I put a dome magnifier in contact with my mobile phone lens, would the phone be able to focus on the flat side of the dome magnifier?

 

[Maris]

The nominal magnification of a convex lens as a magnifying glass held some distance from the observers eye is merely 25 centimetres divided by the lens focal length in centimetres. The shorter the focal length the higher the magnification; and vice versa. Typical dome magnifier used to help people read print is 3X so focal length is about 8cm. Phone camera? Sorry, don't know, never used one.

 

[jsmoove]

@Maris Thanks, that helps!
The minimum focus distance of my phone is 8cm.
So would that mean I need an 8X-10X dome magnifier in order to focus on the flat surface?

 

[Maris]

Huh? Dome magnifiers don't focus on the flat surface. While a dome magnifier has a focal length and consequently a nominal power these numbers apply to real image calculations. The image seen in a dome magnifier is a virtual image, not a real image, and a virtual image can be photographed like anything else that appears to give off light.

 

[jsmoove]

Sorry, i'll rephrase...
If I wanted to focus on an printed image pasted on the bottom of a dome magnifier, would a certain dome magnifier do the trick since im using a phone and not my eyes?

 

[Maris]

All dome magnifiers are optically alike (when used in the conventional sense) in providing a somewhat enlarged virtual image of whatever they are placed upon. This virtual image can be photographed at any distance from infinity all the way down to the close focussing limit of the photographic device you are using. Of course, the closer you are when you take the photograph the bigger the dome magnifier image appears in your final picture.
If you subvert the purpose of a dome magnifier and use it in the "wrong" way by putting a printed image exactly one focal length behind it then you see nothing readable (total blur) by looking at the top of it. However if you now bend down and place your eye (or camera) nearly in contact with the dome magnifier and look through it you will see an enlarged view of your printed image. But this is optically no different to the old familiar process of just holding a magnifying glass up to your eye and peering at stuff. Dome magnifiers are not designed to be "magnifying glasses" and don't work like them.

 

[jsmoove]

@Maris Thanks, I think that helps. So if I took: https://www.magnifier.com/dome-magnifier-1-8inch-4x-dm1-8.htm?amp=1
Pasted an image on the bottom of this lens,
And put it up to my phone camera lens in direct contact,
The image I capture should be in focus and fill pretty much the entire "frame" depending on the size of the image.
Do I have that right?

 

[Maris]

No. If you want to put a phone camera directly on the dome magnifier and get a picture of the virtual image it needs to have a minimum focussing distance equal to the thickness of the dome magnifier. A dome magnifier is not a "close up" lens. I fear that I am not a good teacher of basic optics and the internet is not a good place to learn basic optics. What am I failing to explain?

 

[jsmoove]

So the dome magnifier would need to be 8cm in thickness then. (my phones minimum focus distance)
The problem im facing is that I don't want the lens I choose to be that thick, I need it under 25mm in thickness, and somewhere around 25mm in diameter. A completely solid lens is what im after (like a dome or a ball, from earlier this thread)
What I am not understanding is, in my situation using a phone, how can one reduce minimum focus distance for a dome lens or plano-convex lens, other than to increase the thickness of the lens?
Or is this the only way?
I appreciate the help either way, I know its basic optics, but I haven't really found an answer for this anywhere.
I had looked at: https://www.surplusshed.com/search_...th_to=1&focal_length_from=5&sort=&sortby=+asc
(which is a smaller diameter than I want) but the focal length is very short, so I imagine my virtual image would then be at 5mm, allowing me to have my image on the bottom of the lens at least somewhat in focus.
I am prepared to be schooled.

 

[Maris]

Dome lenses don't focus. The concept of focus does not apply to dome lenses. You can always make your telephone focus closer than its normal limit by putting a magnifying glass over its little lens. The stronger the magnifying glass you put on the telephone's lens the closer it focusses. Optically it's just like adding a close-up supplementary lens to an ordinary camera lens. The calculations are easy.

 

[jsmoove]

Well thats news to me!
Right....so I need some sort of magnifying glass, that focuses on its own surface. All solid glass. Not a loupe.
If I particularly want a diameter of 25mm, my options are what exactly? I've been going around in circles for a long time now
By "stronger" is this a curvature thing, a refractive index thing....?
I am not sure how to calculate, other than to say, I need a short focal length. Is that all I need to do if I want to focus on the surface?

 

[Maris]

Well thats news to me!

Yes, that's right. Dome lenses do not converge light rays to form a real image.

Right....so I need some sort of magnifying glass, that focuses on its own surface. All solid glass.

Wrong.

Not a loupe.

You need a loupe or some such magnifier to put in front of your telephone camera lens.

If I particularly want a diameter of 25mm, my options are what exactly?

That is a journey of research you must take.

I've been going around in circles for a long time now

I hope I am wrong but I fear that you have insufficient knowledge of basic optics to solve what appears to be a trivially simple problem.

By "stronger" is this a curvature thing, a refractive index thing....?

Stronger means having a higher power measured in dioptres.

I am not sure how to calculate, other than to say, I need a short focal length. Is that all I need to do if I want to focus on the surface?

You need to learn how to do basic optics calculations. Lenses are not cheap; guesswork gets expensive and disappointing.

 

[jsmoove]

I was saying that I would essentially like to have it all solid glass and for it to not be a loupe (since I want all solid glass), not that that was the only answer to the problem.
I definitely have insufficient knowledge in optics.
I was given these diopter numbers: https://imgur.com/a/2iYqUlP
So from what I can tell, I need a diopter of over 50X in order to focus on the surface.
So this lens: https://www.surplusshed.com/search_...th_to=1&focal_length_from=5&sort=&sortby=+asc
Has a magnification power of 50X.
But its diameter is a bit too small.
So I guess im on the right track.

*Im reading: https://www.mymagnifier.com/en/faq/...ation-of-a-magnifying-lens/E-TAY-faq-003.html
The backwards formula to get the diopter would then be -1 (x4)
So then that surplus shed lens would be a diopter of 196.

It says "It is not possible to get a high power lens in a large diameter"
So, I wonder what the max diameter is for shortest focal length...."large" could mean anything.

Maybe a ball lens really is the best option after all

 

[jsmoove]

@Nodda Duma
Just re-reading your message from a while back:
"You either need to be looking at a ball lens, or if you don't like that, then select and bond a short focus Fresnel lens masked down to about f/5.6 or even f/11 and bonded to an acrylic block with the same length as the focal length of the Fresnel (accounting for the index of refraction of the acrylic block)."

Two possible dumb questions about the fresnel part, could I use something like https://www.materialsampleshop.com/products/fresnel-lens-film
and attach it to a block?
Is it theoretically possible to micromachine a fresnel lens at the top of a acrylic block? Instead of bonding a fresnel with a block?

 

[Nodda Duma]

Theoretically yes, but it would require a precision diamond-turning machine

 

[jsmoove]

@Nodda Duma Neat. Im not sure itd work in my case though, since I'd want to hold the fresnel in direct contact with the phone. Even if the fresnel had tiny microstructures, im not sure itd work. So far I haven't been able to find any other modern single lens solution for a stanhope other than a ball lens. :<

 

[jsmoove]

Heres where I am. The application: https://imgur.com/a/DZA3fyo
I read that a stanhope has a focal length equal to its length (assuming they mean edge thickness)

Currently my options for a single lens with a large-ish diameter seem to be:
-A ball lens with a refractive index of 2. Problem: large diameters are very expensive
-A half ball lens (same problem)
-An elongated plano convex. Problem: Can't seem to find one in modern times
-A dome magnifier. Problem: There don't seem to be any with the focal length the same as its thickness
-Fresnel. Problem: Can't be in contact with the phone, I think
Other options:
-A half sphere glued onto a glass blank. Problem: Was hoping to find a single lens
-Gelatin grin lens? (tiny grin lenses seem to be very expensive)

 

[jsmoove]

@Nodda Duma Is there a type of lens that is a lens built into a glass slide? Like a microscope slide or similar

 

[Donald Qualls]

Would it work to mount a simple plano-convex or biconvex lens (or even a triplet loupe) in a tube with length set so the lens focuses at the end of the tube? Seems a lot easier than making your own lens. I presume you're trying to use your phone as a microscope...

 

[jsmoove]

@Donald Qualls Yeah thatd work no doubt, but im aiming for a single piece of glass if possible, or a pre-cemented piece of glass. All glass, no plastic.
Yeah kind of, more of an "eyepiece" of sorts, where I hold up the lens to my phone in contact and then image what's on the surface of the lens itself.
An example with a doublet loupe I have and my phone: https://drive.google.com/open?id=1G9qdoZsOg0JFdeEeW6ypt_Dsnk3eIjKm
It reduces my phones minimum focus distance, but not quite enough.

 

[Donald Qualls]

For a Stanhope or similar, the convex end would go to your eye and the slide would be cemented to the flat end. You'd need a tube of some kind to give your phone's camera the correct relief from the lens surface and center it. Might as well put a good quality lens inside a tube with the correct clearances on both ends. Easier to get, easier to use.

 

[jsmoove]

@Donald Qualls I appreciate the help. Is there a modern option that doesn't include a tube, or is something solid out of the question ?
Someone had recommended to me a dome magnifier, but I'm not sure where to find one with the focal length equal to its thickness

 

[Donald Qualls]

Most of the dome magnifiers I've seen had a focal length a little less than their thickness -- when placed directly on a page, they'd make the page look larger, but wouldn't change the focus to infinity (as would be the case if the text were at the focal plane of the front surface).

But the bigger question is what are you trying to view or photograph with your phone -- and how will it get light if the lens is right against it? You might have what I've heard called an X-Y problem -- you're asking about solution X when there's another, easier/better solution for problem Y.

 

[jsmoove]

Hmm yeah that seems to be the case so I've found as well.
Oh, I was going to hold the phone and the lens towards a light source, specifically the sun.
Ah, im trying to view a piece of 35mm film, though it does not have to be this size nor film for that matter...eventually I'd like to etch my image on the glass itself, since I'm aiming for an all-glass solution.
The image itself is data, like the photostore chip: http://www.computer-history.info/Page4.dir/pages/Photostore.dir/
(see bottom images)