Stopping down objectives – useful or not? (images added)

[ChrisR]

Very interesting - done well

[Charles Krebs]

Yes John, nicely done. Thanks for posting this. Obviously this can be a very useful technique. The difference is quite dramatic.

[ChrisR]

What's your limiting aperture - A disc of black paper on the back end of the objective?
I suppose if it proved to be important, a disc could be glued to a tube of black paper, and slid into the rear end?

[morfa]

Chris,

That is a very good idea and definitely one I'd like to try! The downside is of course that you need to remove the objective in order to stop down. Apart from being a little inconvenient it is bound to shift the alignment slightly.

I used an enlarger lens with all glass elements removed for stopping down here. Distance between rear element and the diaphragm blades is approximately 18-20mm (in other words: not insignificant).

[rjlittlefield]

If the change in entrance pupil position is troublesome, one possibility is to replace your "wide open" shots with whatever minimal stopping down is necessary to make your added aperture be limiting. Then your "nearly wide open" and significantly stopped down shots would have the same perspective, at least in theory.

--Rik

[seta666]

I saw your aperture "attachment" to the microscope lens on your flicker; first I thought It was a great idea. Then I thought "does not ned the aperture ring to be between the front and rear glass elements?"
May be that is why it is not working as yo expected
Regards

[ChrisLilley]

I would have expected the 0.30 to have a smaller effective aperture than the 0.25. Is the 0.30 a better lens than the 0.25 (Nikon BD Plans)?

Remember that for NA, bigger number is a wider aperture. Just the opposite of f-number.

Well, its the same as when an aperture is specified as an f-number, not the opposite. Bigger number is a wider aperture.

The culprit is the way we contract down the expression to make it shorter to write or easier to say. We rarely say that a lens is "f over four". Instead we say "f four" and sometimes even write f4 instead of f/4.

For a given focal length, say 200mm, f/4 is a bigger number (200mm/4=50mm) than f/8 (200mm/8=25mm).

Apologies for the tardy and pedantic interruption

[rjlittlefield]

Hhmmm....

Since we're into pedantry, I'll point out that the only numbers appearing in "f/4" and "f/8" are 4 and 8, and it is the bigger number that goes along with the smaller aperture.

This distinction is not just pedantry.

If you imagine that f/8 actually means focal length * ( 1/8 ), then indeed the right relationship pops out: 1/8 is a smaller number than 1/4, and an f/8 lens will have worse diffraction-limited resolution than an f/4 lens regardless of their respective focal lengths.

But suppose you take f/8 at face value and do the calculation that Chris suggests, for two lenses with different focal lengths. For a 50 mm f/4 lens, that gives 50/4 = 12.5, while for a 200 mm f/8 lens, it is 200/8 = 25. Well, 25 is bigger than 12.5, but it is the 50 mm f/4 that has a "bigger" aperture. That's because what matters is the 8, or the 1/8 if you prefer, but certainly not the 200/8.

Exercise caution around all things numerical...

--Rik

[ChrisLilley]

Hhmmm....

Since we're into pedantry, I'll point out that the only numbers appearing in "f/4" and "f/8" are 4 and 8, and it is the bigger number that goes along with the smaller aperture.

Hence the confusion.

This distinction is not just pedantry.

If you imagine that f/8 actually means focal length * ( 1/8 ),

Is there any other way?

then indeed the right relationship pops out: 1/8 is a smaller number than 1/4, and an f/8 lens will have worse diffraction-limited resolution than an f/4 lens regardless of their respective focal lengths.

Agreed.

But suppose you take f/8 at face value and do the calculation that Chris suggests, for two lenses with different focal lengths. For a 50 mm f/4 lens, that gives 50/4 = 12.5, while for a 200 mm f/8 lens, it is 200/8 = 25. Well, 25 is bigger than 12.5, but it is the 50 mm f/4 that has a "bigger" aperture.

Agreed. And this is why the aperture is expressed as a fraction f/n rather than a calculated result, even when f is known. Its also why I was careful to say "for a given focal length" before calculating it out. But maybe I should have been more explicit.

I do find that it helps, for those who may not have made the connection, to show that the size of the aperture can indeed be calculated. (Especially when people say things like "what I really want is a 300mm f/1.4 lens" and you ask how big that would be and they have no idea).

That's because what matters is the 8, or the 1/8 if you prefer, but certainly not the 200/8.

Exercise caution around all things numerical...

--Rik

I agree, and the 1/8 is the important part.

[rjlittlefield]

If you imagine that f/8 actually means focal length * ( 1/8 )

Is there any other way?

Yes, there is: focal length / 8

The resulting numbers are the same, but if you believe the concepts are the same then I suspect you have never taught struggling math students. In one case you are dividing by a number greater than 1, in the other case you are multiplying by a fraction less than 1.

Moreover, that fraction is not one that appears in standard explanations of F-number.

Instead, what appears in standard explanations is some statement like "the familiar F-number of the lens, N, which is defined as the ratio of the focal length to the aperture diameter". That one comes from Optics in Photography, by Kingslake. The definition in Wikipedia is the same: N = f/D.

Notice that what these formulas compute for a 200 mm lens with aperture diameter 25 mm is the number 8, not the number 1/8 = 0.125 . The number "1/8" would not be computed by any reference I can remember.

I am curious. Can you point to any reference that would compute the number 1/8 = 0.125 for a 200 mm lens with aperture diameter 25 mm?

--Rik

[ChrisLilley]

Notice that what these formulas compute for a 200 mm lens with aperture diameter 25 mm is the number 8, not the number 1/8 = 0.125 . The number "1/8" would not be computed by any reference I can remember.

I am curious. Can you point to any reference that would compute the number 1/8 = 0.125 for a 200 mm lens with aperture diameter 25 mm?

I can't cite a reference, no. It's just how I have understood aperture, as a diameter expressed as a fraction of focal length. That, plus being annoyed when people write things like "f8", meant that I have always seen the "F number" with a leading "/" as an integral part of it.

[rjlittlefield]

That, plus being annoyed when people write things like "f8", meant that I have always seen the "F number" with a leading "/" as an integral part of it.

Yeah, me too.

Though amusingly, at least the latest printing of Kingslake's book uses "f1.4 f2.8 f5.6" as cover art. The great man must be turning over in his grave. Or maybe not, since the book was published by the International Society for Optical Engineering, 11 years before his death.

I used to be pretty rigid about such things. But from my current standpoint, f/8 versus f8 is just a piece of notation, and I've not seen any cases where that difference prompted any confusion.

What bugs me far more is when people reach into the literature, grab a formula, and misapply it. That happens plenty often enough to keep me occupied with corrections and clarifications.

--Rik

[nielsgeode]

Is it possible at all to stop down when using an infinity-corrected microscope objective on a tube lens?

[rjlittlefield]

Yes, very simple. Place an iris just behind the objective, between it and the tube lens.

--Rik