A Tale of Three 20's

[Oskar O]

I don't know. I raise this possibility because new member brianc1959, who lists his occupation as lens designer, recently posted a formula for "DOF" that gave half the value of mine

That would most likely be Brian Caldwell...

I understand what he's saying, but I'm a little light on optical theory; optics was never big on my college curriculum and those good lens books are costly... maybe the solution is to buy one, spend a couple of months understanding the stuff and then achieving enlightenment

As for the CoC, I was thinking more on whether manufacturers have some agreed on value they all refer to or not.

Charles, I agree, those two terms are often intermixed. Given the number of variables, it's sometimes hard to make out if people are talking about the same things. Thanks for the link, makes for nice stuff to drool after in addition to being informative... Also interesting how BrianC's formula is almost the same as Mitutoyo's -- my hunch is that the formulas are supposed to be the same, only that BrianC made a small typing error.

David, thanks for the clarification. For some reason I was thinking about smaller format lenses, but it does make sense that they had a Xenar for large format, distant subject and Artar for copy work (both 4 element designs).

[rjlittlefield]

I don't know. I raise this possibility because new member brianc1959, who lists his occupation as lens designer, recently posted a formula for "DOF" that gave half the value of mine

That would most likely be Brian Caldwell...

Yes, it is Brian Caldwell.

As for the CoC, I was thinking more on whether manufacturers have some agreed on value they all refer to or not.

There is some variation between manufacturers.

Also interesting how BrianC's formula is almost the same as Mitutoyo's -- my hunch is that the formulas are supposed to be the same, only that BrianC made a small typing error.

All the formulas are consistent, under the assumption that Mitutoyo's formula #9 on page 32 is to calculate one-sided DOF, that is, maximum deviation from perfect focus. Brian's initial formula computes the same number (within rounding), and he later clarified that it would be only half the total DOF, front and rear combined The diffraction term in microscopyu's DOF formula HERE differs from Mitutoyo's formula only by a factor of 2 in the denominator, and in allowing for immersion with n>1.

--Rik

[dmillard]

Here are a couple of pictures of the basic setup I used to do the test, but with a 240mm Apo-Ronar in place instead of the 210mm Xenar (49mm filter instead of 46mm, but they both screw into the same barrel mount that is secured in the end of the bellows). I used these lenses because I had them on hand, but I'm looking forward to trying one of morfas' excellent finds posted here

[Blame]

...and easily obtained new from Edmund Optics for only $1,830 list.

Don't forget the QV varianty. Probably even more expensive new, but a drag on the market secondhand. Just remove the metal adapter, double the magnification and tube length. Then you have a standard one.

Frankly I suspect all Mitutoyos are a drag on the secondhand market. I don't see many dealers offering them for auction. If they did then maybe they would go for £100 a pair, like I paid.

My advice is to haggle hard.

[ChrisLilley]

Here are a couple of pictures of the basic setup I used to do the test,

I looked for a thread with a general description of your setup, but didn't find one. Did I miss it, or have you not described it yet? It looks interesting and i have a couple of questions about what I am seeing, but they would be better in a separate thread.

[dmillard]

I looked for a thread with a general description of your setup, but didn't find one. Did I miss it, or have you not described it yet? It looks interesting and i have a couple of questions about what I am seeing, but they would be better in a separate thread.

Hello Chris,

You may be right about describing it in another thread, but I'll be very brief (there's already been one digression about DOF, so . . . ).

This is roughly the same setup I described here , (and see also here and here ) but using a Nikon PB6 bellows and PN11 extension tube in lieu of the Olympus telescopic extension tube. The lighting is provided by a Novoflex Macrolight with Vivitar 283 flash, output controlled by a VP1 variable resistor. The specimen position is adjusted with a Newport XYZ stage and Mitutoyo digital micrometer. Going up from the Mitutoyo objective: a Mitutoyo-C-mount adapter, C to T mount disc,T to 39mm adapter, Nikon EL-F adapter, then the bellows standard. Above the front standard is a 52-52mm ring, 52-49mm ring, lens, and barrel mount. The maple cutting board blocks are resting on Sorbothane to help damp vibration.

I've revised my horizontal setup considerably since the image linked above, but it's still in a state of flux.

Please let me know if you have any questions.

Regards,
David

[Oskar O]

David, thanks for this brief clarification of your setup; it looks very nice and gives me a lot to think about on how I should improve my setup(s) for going into the world of high magnification.

I'm still a bit surprised about how many adapters you have between the objective and the lens, but from your description I get the idea that you used adapters that were readily available. Is there anything more to it, e.g. does the arrangement permit you to add filters between lens and objective if you wanted to?

Thanks,
Oskar

[dmillard]

David, thanks for this brief clarification of your setup; it looks very nice and gives me a lot to think about on how I should improve my setup(s) for going into the world of high magnification.

I'm still a bit surprised about how many adapters you have between the objective and the lens, but from your description I get the idea that you used adapters that were readily available. Is there anything more to it, e.g. does the arrangement permit you to add filters between lens and objective if you wanted to?

Thanks,
Oskar

Hello Oskar,

I'm glad you found the description helpful. I suppose I could use screw-in filters at this time if I wished, but I'm trying to make a geared polarizing filter (similar to Nikon's here ) that will fit into the intermediate tube. I'll post the results when (if?) I'm happy with the outcome.

Regards,
David

[ChrisR]

a geared polarizing filter

Just a thought - some Cokin style filter holders have filter threads on the front, to which the objective could be attached. See what I mean?

[dmillard]

a geared polarizing filter

Just a thought - some Cokin style filter holders have filter threads on the front, to which the objective could be attached. See what I mean?

Excellent idea Chris! And Nikon AF-1 gelatin filter holders (like this one) also rotate.

David