# Me no understand...'slain this to me.



## 480sparky (Sep 14, 2013)

The other day, during a brief moment when my brain is actually making contact with the rest of my body, I had an idea.

What if?...............


What if.......... I took a close-up filter, and installed it on a lens?  _Backwards_.

Yes, put in on with the 'wrong' side facing the subject.

Would the lens then have the ability to focus _beyond_ infinity?  If so, could I scale the idea up and use a telescope thus outfitted and see beyond the edge of the known universe?  Could I see back further in time than any other astronomer ever has?  Should I contact NASA and sell them the idea?  Have I finally entered the realm of the Great Thinkers?  Am I pondering things beyond the scope of mere mortal comprehension?



WHOA!  Let's not get ahead of myself, I thought.  First things first.... I gotta try it.  Since my body decided to make contact with my brain a few minutes ago, I decided to check out my "Divide by zero" experiment.  I tossed some oddball stuff on the table, threw on the 105 Micro, and took this:









I then spun on a +4 lens the 'right' way onto the lens, and moved it closer (not changing the focus setting), and got this:















Now for the moment of truth: put the +4 onto a 52/52 reversing ring and install it 'backwards onto the 105. I didn't change anything else... not even the focus point.




  (Drumroll, please!)  Net result:








What the!?!?!?  There's _no difference!_ The close-up lens works the same both ways!  Instead of making up some far-fetched conspiracy theory about the Illuminati or NSA or Elvis being involved, I thought I post my results here and ask for a bit o' edyoukayshun:  Why _does _the CU filter work the same both ways?


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## Gavjenks (Sep 14, 2013)

Because a close up lens is usually a simple convex lens in a ring.  It is symmetrical, so it works both ways. (sometimes they may be a bit more complex, like achromats, but even still, the net effect is a basic convergent lens equivalent)

Try taking a pair of reading glasses and holding them in front of any random photography lens, and you will be able to focus on things closer than before.

Flipping things around only has different effects for multiple element, more complex designs that tend not to be symmetrical.


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## 480sparky (Sep 14, 2013)

Gavjenks said:


> Because a close up lens is usually a simple convex lens in a ring............




Nope.  Try again.


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## Gavjenks (Sep 14, 2013)

I have one in my hand right now that is clearly a simple convex lens in a ring. At most it might be a cemented achromat convergent double, but the coloration and curvature look the same on both sides, so i don't think so.  Even if it is a cemented achromat, it would have equal magnifying properties reversed (although different spherical aberration qualities etc.)

I also just now double checked and verified that a lens sitting on my work shelf for homemade lenses, which is 100% absolutely a simple convex lens, held up in front of my 70-300mm Canon lens, reduces the minimum focus distance from 5 feet down to about 4 inches, with reasonable image quality considering I was just holding it in the air.


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## 480sparky (Sep 14, 2013)

Gavjenks said:


> I have one in my hand right now that is clearly a simple convex lens in a ring. At most it might be a cemented achromat convergent double, but the coloration and curvature look the same on both sides, so i don't think so.  Even if it is a cemented achromat, it would have equal magnifying properties reversed (although different spherical aberration qualities etc.)
> 
> I also just now double checked and verified that a lens sitting on my work shelf for homemade lenses, which is 100% absolutely a simple convex lens, held up in front of my 70-300mm Canon lens, reduces the minimum focus distance from 5 feet down to about 4 inches, with reasonable image quality considering I was just holding it in the air.




Well, obviously you're not looking at the lenses I have.


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## Gavjenks (Sep 14, 2013)

Also focusing beyond infinity is already possible with plenty of mediocrely made commercial lenses without adding anything at all.  Just slide the focusing ring to "infinity" on a lens poorly calibrated in the right direction.  All it means is that your lens isn't quite powerful enough to focus parallel incoming rays down to a single point by the time it gets to your sensor. If you were to move the sensor back slightly on such a lens (e.g. with an extension tube of correct length), you would once again be able to focus at infinity



> Well, obviously you're not looking at the lenses I have.


Okay, describe what appears to be the element layout, and it will help in figuring out why reversing has no effect.  It might just be two convergent lenses, which will also act almost exactly the same forward or backward.


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## manicmike (Sep 14, 2013)

Magic?


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## Joeywhat (Sep 14, 2013)

Yes it is possible, but you need a Mr Fusion, or a good source of plutonium.


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## pixmedic (Sep 14, 2013)

it would have worked if you had used the camera with the flux capacitor.


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## 480sparky (Sep 14, 2013)




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## astroNikon (Sep 14, 2013)

480sparky said:


> .... some far-fetched conspiracy theory about the Illuminati or NSA or Elvis being involved,




Actually it was Elvis.  I saw him at the park, I tried taking a picture of him but he was hiding behind a tree with a bag full of lenses and flux capacitors.  He then jumped in his DeLorean and disappeared.


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## pixmedic (Sep 14, 2013)

soooooo...
is anyone actually gonna 'splain this?
'cause now im curious.


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## Designer (Sep 14, 2013)

pixmedic said:


> soooooo...
> is anyone actually gonna 'splain this?
> 'cause now im curious.



Helen will ride in and explain it all.  In terms that even I can understand.


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## manaheim (Sep 14, 2013)

Yeah this is SO a Helen question.

Someone break out the Helen signal.


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## pixmedic (Sep 14, 2013)

Helen will make this all right. 
she is three shades of awesomesauce.


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## 480sparky (Sep 14, 2013)

manaheim said:


> Yeah this is SO a Helen question.
> 
> Someone break out the Helen signal.



Is that like the BatSignal?  :scratch:


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## astroNikon (Sep 14, 2013)

pixmedic said:


> Helen will make this all right.
> she is three shades of awesomesauce.



awesome sauce ?


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## manaheim (Sep 14, 2013)

480sparky said:


> manaheim said:
> 
> 
> > Yeah this is SO a Helen question.
> ...



While you were doing that, I was doing this...


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## manaheim (Sep 14, 2013)

And yes that really is Helen's head on that.


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## Murray Bloom (Sep 14, 2013)

What makes it a close-up lens is that it's thicker in the center, making it the equivalent of a magnifying glass.  It's curved overall to help deliver a sharper image to the flat focal plane.  Reversing the close-up lens still has it thickest at the center, which, at most, will change the magnification factor.  I can do the same thing with my reading glasses.


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## pgriz (Sep 14, 2013)

Sparky, I think you have telescope gear.  Check out your Barlow lens.  It extends the focal point further out (increasing the effective focal length).  As for the magnifying lens, it doesn't have to be symmetric to give the effect - only that the center is "thicker" than the perimeter, causing the light wave to bend inwards.  As Murray already pointed out.

Now as for the looking past the edge of the universe, be careful what you wish for.  This was already done, and for her efforts to explain, she was burned at the stake.  Years later, her explanations scribbled onto the walls of her cell were finally decoded, and the fellow who replicated her results was declared insane and was lobotomized "for his own good".  Now his notes were found and the last person trying to demonstrate was declared a terrorist and vaporized with a Hellfire missile.  That track record is beginning to discourage the truth-seekers, at least those with families and mortgages.  So... don't fool around with the what's behind the edge of infinity.  And if that doesn't deter you, because nowadays, you might be run over by a Google car mapping out the aformentioned corners of the universe.  :mrgreen:


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## 480sparky (Sep 14, 2013)

pgriz said:


> Sparky, I think you have telescope gear.  Check out your Barlow lens.  It extends the focal point further out (increasing the effective focal length).  As for the magnifying lens, it doesn't have to be symmetric to give the effect - only that the center is "thicker" than the perimeter, causing the light wave to bend inwards.  As Murray already pointed out.
> 
> Now as for the looking past the edge of the universe, be careful what you wish for.  This was already done, and for her efforts to explain, she was burned at the stake.  Years later, her explanations scribbled onto the walls of her cell were finally decoded, and the fellow who replicated her results was declared insane and was lobotomized "for his own good".  Now his notes were found and the last person trying to demonstrate was declared a terrorist and vaporized with a Hellfire missile.  That track record is beginning to discourage the truth-seekers, at least those with families and mortgages.  So... don't fool around with the what's behind the edge of infinity.  And if that doesn't deter you, because nowadays, you might be run over by a Google car mapping out the aformentioned corners of the universe.  :mrgreen:



A barlow isn't intended to change anything other than to invert the image so the 'scope can be used for terrestrial views right-side-up.  Besides, a barlow is not in the same place in the optical path as a Close-Up lens.

As for my CUL being thicker in the middle, I have to way to measure it... it just _feels_ like it is.


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## Gavjenks (Sep 15, 2013)

So it's just that one positive meniscus lens, 480?

A positive meniscus lens is still positive (convergent) no matter which way it is facing. What makes it a convergent lens is not whether the front surface is convex or concave.  What makes it convergent is that the middle of the lens is thicker than the edges.

No matter which way you flip it, the middle is still thicker than the edges, so it will still converge rays, and thus have the exact same magnifying properties.

It will NOT have the same affects on distortions, however. Flipping it one way (I think with the convex pointing out, but could have mixed that up) will create less spherical aberration usually than flipping it the other way. Or it interacts in certain ways with other types of shapes to minimize aberrations, or something. I used to know this, but have forgotten the specifics on that, but I one of the main reasons people manufacture positive meniscus lenses is to somehow or other use them to control aberrations. As opposed to just always manufacturing biconvex lenses.

Well, also, a second reason is that you can _fit lots of meniscus lenses inside of each other_, and thus cram a lot more glass into a smaller space, which consumers like, because then your lens isn't bulky.

I can't say for sure what their motivation was here for using a meniscus shape (probably the aberrations). But in any case, the close-up effect depends on the optical strength which depends on the middle versus edge thickness, not the meniscus shape, and thus should be virtually identical either way, because the *difference in thickness is symmetrical*, even if the literal shape of the lens isn't.





If you take your close up lens and walk outside on a sunny day, you should be able to use it to fry ants pointing in either direction, by itself, from about the same distance above the ground. I.e. the individual element's focal length / optical power does not change by direction.


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## Gavjenks (Sep 15, 2013)

Murray Bloom said:


> What makes it a close-up lens is that it's thicker in the center, making it the equivalent of a magnifying glass.  It's curved overall to help deliver a sharper image to the flat focal plane.  Reversing the close-up lens still has it thickest at the center, which, at most, will change the magnification factor.  I can do the same thing with my reading glasses.


Aw, apparently you sniped me on the thicker in the middle thing.

However, I sniped you on mentioning reading glasses in this thread.  So we can call it even *narrows eyes and backs away slowly*


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## snowbear (Sep 15, 2013)

manicmike said:


> Magic?



No; bacon.  It's always bacon.


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