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Effective F aperture for objective NA

 
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zzffnn



Joined: 22 May 2014
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Location: Texas USA

PostPosted: Thu Mar 02, 2017 2:31 pm    Post subject: Effective F aperture for objective NA Reply with quote

Thank you for checking this thread.

I have some confusion about calculating effective F aperture number for objective NA and magnification.

I know F effective = F x (1+m)
Where m = magnification.

And F = 1/ (2 x NA)
Where NA is numeric aperture of microscope objective

Is that m on-sensor magnification or visual magnification (total microscopy magnification, meaning eyepiece power times objective power)?

As an actual example,

if I use an 1x NA 0.05 160TL objective, at 160TL with 10x eyepiece, together with afocal micro four thirds sensor and 30mm m3/4 lens. What effective F aperture am I using the objective at? F/22 or F/110? My gut feeling is that F/22 may be the right answer.

I did experiment and on-senor magnification is about 1.2x. Microscopy total magnification is 10x (1x objective with 10x eyepiece).

Also when you reverse a camera lens, its F number does not change, though effective F number change with sensor size/extension/magnification, correct?

Thank you again!
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ChrisR
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PostPosted: Thu Mar 02, 2017 5:13 pm    Post subject: Reply with quote

This follows some PMs, so I'll chime in early to see if I have it right!

Magnifications for these purposes are just On Sensor.
Quote:

if I use an 1x NA 0.05 160TL objective, at 160TL with 10x eyepiece, together with afocal micro four thirds sensor and 30mm m3/4 lens. What effective F aperture am I using the objective at?

Just 1/(2x 0.05) = eff/10
What you DO with it, is up to you. If you use an afocal setup you're magnifying then shrinking. For diffraction considerations, you aren't changing anything by the enlargment.

If you can cover your sensor without overly enlarging the image circle, that may be your best bet, on your 18x 13.5mm sensor.
I've not seen a "normal" 1x objective with decent coverage.

Quote:
Also when you reverse a camera lens, its F number does not change, though effective F number change with sensor size/extension/magnification, correct?
Its effective F number changes with the PMF, but with the lens I think you have in mind, SC 50mm f/2.8, the PMF will be close to 1.
Only the extension - and hence magnification, changes the effective aperture.

Even if I'm 100% right, Rik would express it differently.
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zzffnn



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PostPosted: Thu Mar 02, 2017 5:36 pm    Post subject: Reply with quote

Thank you, ChrisR.

PMF = pupillary magnification factor?
That is new and complex to a macro beginner like me again Embarassed

Yes, I have the Schneider Componon S 50mm F/2.8 in mind.

It seems that, as you pointed out, the SCS50's effective aperture value will be better than my macro objectives (1x NA 0.05 and 2x NA 0.09).

At 3x on sensor magnification, it probably loses to my 3x NA 0.10 or 3x NA 0.08? Though I am not positive about my calculations.

I am not sure if F effective = F x (1+m) or F effective = F x m (magnification), in this situations. Please kindly correct me.
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ChrisR
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PostPosted: Thu Mar 02, 2017 6:03 pm    Post subject: Reply with quote

If you use a camera type lens, you need the "1+ " in the calculations.
If you use an objective, that's already "built in".
So eg for a 4x NA 0.2, which is eff/10
a camera lens would have to be marked f/2 because

"f/2" * (M+1)
= "f/2" * 5
= eff/10

Sorry about the sloppy terminology, I expect you get it...

And a 4x NA 0.1 is like an f/4 lens, giving eff/20.

I wouldn't llike to predict your outcomes. Worth a few cheap tubes or a bellows to find out. I suspect the enlarger lens will be nicer to use.
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zzffnn



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PostPosted: Thu Mar 02, 2017 7:17 pm    Post subject: Reply with quote

Thank you very much, ChrisR.

I should have put this thread in beginner macro forum. Expectation is too high for me here Shocked

Yes, the enlarger lens seems easier to use and more versatile at around 2x on-sensor.
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rjlittlefield
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PostPosted: Thu Mar 02, 2017 8:48 pm    Post subject: Reply with quote

zzffnn wrote:

if I use an 1x NA 0.05 160TL objective, at 160TL with 10x eyepiece, together with afocal micro four thirds sensor and 30mm m3/4 lens. What effective F aperture am I using the objective at?
...
I did experiment and on-sensor magnification is about 1.2x.

ChrisR wrote:

Just 1/(2x 0.05) = eff/10
...
Even if I'm 100% right, Rik would express it differently.

Probably true! Laughing Laughing

But in this case there's a technical difference also.

The calculation should be 1.2/(2x 0.05) = effective f/12.

That's because it's always the on-sensor magnification that has to be used in the formula that effective f# = mag/(2*NA) .

If you have relay optics that change the magnification, then the effective f# changes also.

It's just like the effect that a teleconverter has on effective f#, where making the image bigger also increases the effective f-number by the same multiplier.

In both cases, there is no effect in the resolution on subject, except for whatever aberrations might be added.

--Rik
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zzffnn



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PostPosted: Thu Mar 02, 2017 9:41 pm    Post subject: Reply with quote

Thank you very much, Rik. You have been very helpful!
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