MPE-65 + Raynox MSN-202 magnification

[LordV]

Well it was raining so I decided to to see if I could get an image using a Raynox MSN-202 +25 dioptre on the front of my MPE-65. I didn't hold much hope but to my surprise I did get a useable image with the MPE at 5:1. The magnification though was more than I expected at near 9:1 instead of around 6.6:1.
See pics below of ruler mm scale and some focus stacks of a white fly taken at the max mags of the system.

Brian V.



mm ruler scale MPE-65 at 5:1 on 5dmk2


As above plus Raynox MSN-202 +25 diopter

[rjlittlefield]

Brian, it's good to see you back here after a couple of months' absence.

The images look rather better than just "useable", but I'm accustomed to seeing that with your lens combos.

In this case the numbers make good sense. Cranked out to its 5:1 end, the MP-E 65 acts something like a focal length 45 mm lens on 270 mm of total extension. The MSN-202 at +25 dioptre will be 40 mm focal length. As a simple calculation, if the lenses were "thin" and with no separation, then the combo would be roughly 22 mm focal length and 11:1. The separation causes that to drop a bit, giving a convenient hand-waving explanation for the 9:1 that you observe.

What calculation did you do, to get the 6.6:1 prediction?

--Rik

[LordV]

Brian, it's good to see you back here after a couple of months' absence.

The images look rather better than just "useable", but I'm accustomed to seeing that with your lens combos.

In this case the numbers make good sense. Cranked out to its 5:1 end, the MP-E 65 acts something like a focal length 45 mm lens on 270 mm of total extension. The MSN-202 at +25 dioptre will be 40 mm focal length. As a simple calculation, if the lenses were "thin" and with no separation, then the combo would be roughly 22 mm focal length and 11:1. The separation causes that to drop a bit, giving a convenient hand-waving explanation for the 9:1 that you observe.

What calculation did you do, to get the 6.6:1 prediction?

--Rik

Thanks for the info Rik.
Remember I'm just a simpleton so used Existing mag + (65/40)

Brian v.

[TheLostVertex]

Brian, it's good to see you back here after a couple of months' absence.

The images look rather better than just "useable", but I'm accustomed to seeing that with your lens combos.

In this case the numbers make good sense. Cranked out to its 5:1 end, the MP-E 65 acts something like a focal length 45 mm lens on 270 mm of total extension. The MSN-202 at +25 dioptre will be 40 mm focal length. As a simple calculation, if the lenses were "thin" and with no separation, then the combo would be roughly 22 mm focal length and 11:1. The separation causes that to drop a bit, giving a convenient hand-waving explanation for the 9:1 that you observe.

What calculation did you do, to get the 6.6:1 prediction?

--Rik

I think I calculated a different way, but arrive at a similar value as you.

(Lens FL/Diopter FL + 1)(Lens Magnification + 1) -1
(45mm/40mm + 1)(5 + 1) - 1 = 11.75

Was your method different?

[rjlittlefield]

Well, this is certainly a special OCCasion!

I refer of course to the Opaque Calculation Contests that we run from time to time.

Following the rules for Standard Opaque Representation, my calculation could be written this way:

270/(1000/(1000/45+25)) - 1

Taking that apart from the inside out:
1000/45 is the strength in diopters of the MP-E 65,
25 is the strength in diopters of the MSN-202, so
1000/45+25 is the strength in diopters of the combination (assuming the lenses are thin and not separated), so
1000/(1000/45+25) is the equivalent focal length (EFL) of the combination. Then
270/EFL - 1 is the magnification of that equivalent focal length, on 270 mm extension.

Your calculation and mine give the same results no matter what values I plug in for focal lengths and magnification (extension), so I'm quite confident that they're algebraically equivalent.

But I have no idea how to reach your form from first principles.

Can you explain?

--Rik

[TheLostVertex]

But I have no idea how to reach your form from first principles.

Can you explain?

--Rik

Id love to! But unfortunately I can't This is infact just how I read the correct way to calculate a diopter on a lens was, and I do not recall the source off hand.

I should have wrote it a little differently though:
((Lens Fl/Diopter Fl) + 1)(Lens Magnification + 1) -1

Since I know that the first part is the calculation for the magnification of the pair focused at infinite:
(Lens FL/Diopter FL)

So at the very least I know that it is:
(Pair Magnification @ infinity + 1)(Lens magnification + 1) - 1

Beyond this, I think my calculations go completely opaque! Do I win a prize?

It does appear that both have the same information in them(or equivalency there of) and they both produce the same answer. I would guess there is a way to get from one to the other, but I do not think I am able to dedicate that much thought to it at this moment.

[rjlittlefield]

I would guess there is a way to get from one to the other

There is, and it's just a matter of straightforward algebra.

All you have to do is take the deeply nested formula that comes out of first-principles analysis, and start hacking at it to reduce the nesting.

By the time you get down to the minimum nesting of parentheses (one level) and a single division (divisions are terribly hard to do by hand!), what pops out is the formula that you've quoted.

That formula is concise, easy to look at, and easy to evaluate. It's exactly the sort of formula that we'd like to find in a handbook.

There's only one problem: by then the formula is almost completely divorced from its physical origins. If some malicious editor were to swap some +1's and -1's when we weren't looking, it would be pretty hard to spot the corruption. Likewise for figuring out what assumptions the formula is based on.

This bothers me. Maybe I'm the only person in the world that it does bother. I don't know. But I do know that my personal preference is to avoid mistakes and mis-applications, and the best way I know to do that is to avoid formulas that I can't pick apart and make sense of. I much prefer a sequence of simple calculations that all make sense, over a single plug-in-the-numbers calculation that I have to take on faith.

--Rik