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I am looking to do some experiments with my cellphone and a lens, holding the lens in contact with my cellphone lens. I'm looking for a lens that has the shortest back focal length length possible, so a 0mm BFL being the ideal. This is to image/have in focus the surface of the lens itself, like a Stanhope lens.

If I were to pick a lens from surplusshed.com, what type of lens would get me the closest to this possibility?

I know that a ball lens with a refractive index of 2 equals a BFL of 0, but I've found that the higher refractive ball lenses are quite pricey, and surplusshed does not carry them.

It was suggested that I combine a rod lens with a half ball lens, but I'm looking strictly for a single lens to experiment with, as I don't want to order custom lenses or cement my own, as I am a beginner.

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  • \$\begingroup\$ Had to read up what a Stanhope lens is - I would have assumed it to be a very sharp and rather negative, though pleasing, lens :) \$\endgroup\$
    – rackandboneman
    Jan 9, 2020 at 9:00
  • \$\begingroup\$ Go to Amazon and search for "dome magnifier", some of these appear to be Stanhope lenses, although they are probably larger than what you want. \$\endgroup\$
    – Mattman944
    Jan 9, 2020 at 13:53
  • \$\begingroup\$ A dome magnifier is a pretty good idea actually....do they have them smaller as you say, like 2"? Or is there something similar on surplusshed that functions like a dome magnifier? Thanks! \$\endgroup\$
    – Curiousmarble
    Jan 9, 2020 at 17:06
  • \$\begingroup\$ @Mattman944 Actually what im curious about is, if I have say a 4x dome magnifier in front of my camera lens, will 4x magnification be enough to remedy the minimum focus distance of my phone? (10cm) How do I calculate what magnification of the dome I need? \$\endgroup\$
    – Curiousmarble
    Jan 10, 2020 at 2:02
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    \$\begingroup\$ Why did you vandalize the question? \$\endgroup\$
    – Ross Millikan
    Dec 21, 2020 at 2:11

2 Answers 2

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Because your plan is to mount a supplemental close-up lens, in direct contact with the surface of your camera phone, you believe this added lens must have a very short back focus. This is incorrect thinking. In fact, you can procure most any hand-held magnifier and place it in direct contact with the phone. The air-space between the phone and lens will not have that much influence as to the outcome. A magnifying lens placed before the camera lens, acts almost exactly as reading glasses perform. Best would be a jeweler’s loupe. I suggest you procure a Hasting’s 10X loop. The Hasting’s is a three element color corrected magnifier favored by jewelers, stamp, and coin collectors. This would be your best bet.

The 10x or 10 power translates to the distance eye-to-object or in this case, camera-to-object. A 10X magnifier shortens this distance by a factor of 10. The normal comfortable close viewing distance is about 10 inches. Mount a 10x and this distance shortens to 1 inch; hence the object is seen as 10x larger as compared to the unaided eye (camera).

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  • \$\begingroup\$ Oh! What is the minimum magnification I need then in order to focus on the back surface of the lens? Since the phones minimum focus distance is 10cm, I imagine that plays into it...? You say 10x reduces to 1 inch...Yes, I have a 8x loupe and it seemed to work just fine....but I am wondering about a solid single lens on its own if possible? Would a triplet or doublet suffice/be best? I was looking to get something from surplusshed. Thanks for your reply. \$\endgroup\$
    – Curiousmarble
    Jan 11, 2020 at 2:50
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Most magnifying lenses will reduce the focus distance therefore increasing the magnification. You really need a lens designer, which you might find on physics SE, but since they won't know what is inside the phone, they may not be able to help. Your best bet is trial and error.

iPhone8 Only:

enter image description here

iPhone8 with expensive 8X Loupe

enter image description here

iPhone8 with cheap magnifier. The 45X (smaller lens) in https://www.amazon.com/gp/product/B075WRJPFN Something like this is probably close to what you want, but it is not a Stanhope, it is not in contact with the object. It is recessed into this magnifier so this is the closest that I can get the object.

enter image description here

I don't know what your overall goal is, but consider buying a macro lens attachment for your phone. Sorry, I don't have one or I would test it for you.

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  • \$\begingroup\$ Thanks for the pictures! I had also tried out a 8x loupe I have. Alan Marcus answered above that I actually dont need a lens with a BFL of 0, so thats news to me.....in which case, im still wondering....is a triplet or doublet the best bet from surplusshed? And will it focus on the back surface of the triplet/doublet sharply? You're right that I need a lens designer...I know very little on the subject. \$\endgroup\$
    – Curiousmarble
    Jan 11, 2020 at 2:53
  • \$\begingroup\$ So technically I want just a short focal length/high magnification lens then correct? Like one of: surplusshed.com/… \$\endgroup\$
    – Curiousmarble
    Jan 18, 2020 at 16:30

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