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Aperture and lens effects on stacking
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Charles Krebs



Joined: 01 Aug 2006
Posts: 5805
Location: Issaquah, WA USA

PostPosted: Mon Sep 11, 2006 7:39 pm    Post subject: Reply with quote

Oh yeah.. also in the FWIW department... I wanted to mention that when looking at the focal lengths for the various Olympus 10X objectives (LB series, 160mm tube length) I noted 6 different 10X objectives, with focal lengths of:

16.92
18.96
17.69
16.9
18.98
15.69
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 20455
Location: Richland, Washington State, USA

PostPosted: Mon Sep 11, 2006 9:43 pm    Post subject: Reply with quote

Charlie,

Thanks for the additional information.

I'm letting this problem sit in my head for a while, hoping that a method will occur to me for easily and directly measuring the cone angle.

I'm thinking of putting a small light source in what would be the sensor plane, while creating a little cloud of smoke on the subject side of the lens.

Then viewed from 90 degrees to the side, the light coming through the lens should make a nice tidy double cone, whose apex sits in the subject focus plane and whose sides would directly indicate the angular aperture of the lens.

Unfortunately I'm a bit short on theatrical fog machines at the moment.

You wouldn't happen to know any simple way to make a uniform smoke cloud in air, would you?

--Rik
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rjlittlefield
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Location: Richland, Washington State, USA

PostPosted: Mon Sep 11, 2006 10:58 pm    Post subject: Reply with quote

rjlittlefield wrote:
You wouldn't happen to know any simple way to make a uniform smoke cloud in air, would you?

Well, some questions are barely asked before the answer occurs.

No smoke cloud is needed -- just a time exposure. A piece of white paper waving in the light cone visualizes it quite nicely.

Here's what appears when a laser pointer is used to illuminate a small spot of matte transparent tape stuck across the back of the bellows.



In order, from the top:
  • 20X NA 0.4
  • 10X NA 0.25
  • Luminar f/2.5
  • Olympus f/2.0
This is not carefully done and I haven't tried to quantify it yet, but again we see the same pattern, that the microscope objectives have larger apertures (fatter cones) than the macro lenses, markings and calculations notwithstanding.

--Rik
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Harold Gough



Joined: 09 Mar 2008
Posts: 5787
Location: Reading, Berkshire, England

PostPosted: Wed Mar 12, 2008 7:57 am    Post subject: Aperture and lens effects on stacking Reply with quote

There is some very useful information in this string.

I have 20mm (Olympus auto and Canon manual) and 38mm (Olympus auto) bellows lenses.

I also have a Wild (aka Leitz) M20 microscope with a selection of bright field and phase contrast lenses from 10x to 100x (oil immersion). I know that the phase contrast ones are optimised for green light, so the bright field ones would be the best candidates for photography.

I think my priority lies with fully investigating the potential of my bellows lenses before trying the microscope ones.
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Harold Gough



Joined: 09 Mar 2008
Posts: 5787
Location: Reading, Berkshire, England

PostPosted: Mon Apr 07, 2008 4:53 am    Post subject: The Books of Alfred A Blaker Reply with quote

[quote="twebster"]
I had once read a book on optical design by Alfred Blaker, one of the optical design gurus of modern times.

[quote]

One of the main influences on my macro work has been the books of Alfred A Blaker. The first, which I came across, and borrowed, in the mid 1980s was:

Handbook for Scientific Photography, W.H.Freeman & Co Ltd , 1977, more recently available as Focal Press, 2Rev Ed edition (14 Feb 1989) ISBN-13: 978-0240517902

The other was:

Field Photography: Beginning and Advanced Techniques, W.H.Freeman & Co Ltd (May 1976) ISBN-13: 978-0716705185

These books are full of technical details of equipment and techniques, not the least being formulae exposure factors for magnifications and much besides.

These are books I often refer back to and would be good for any photographer interested in the areas covered by the books to have on their bookshelves.

I had enomous trouble and expense obtaining copies a few years ago but some used copies are now being offered at extremely low prices by e.g. Amazon in various countries.

Harold
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 20455
Location: Richland, Washington State, USA

PostPosted: Tue Aug 19, 2008 7:22 pm    Post subject: Reply with quote

----- Original posting, Aug 19, 2008 -----

In response to recent interest, I've gone back and looked more closely at the cone images shown above (collected some two years ago!).

What I did was to first use Photoshop's "Measure Tool" to get the angles of the edges of the cones. Then I converted the cone angles to NA by using the formula NA = SIN(angle/2). For the macro lenses, I computed the corresponding observed effective f-number using fobs=1/(2*TAN(ASIN(NA))), and I also computed the corresponding expected effective f-number using fexp=f*(1+1/m) based on the lens's nominal f-number and the subject-to-sensor magnification m. All this assumes that I've remembered correctly that the bellows extension put the image plane at 150 mm from the shoulder of the mounting threads, corresponding to a 160 mm microscope tube.

The numbers that I get for the microscope objectives are pretty close to nominal specs: cone angle 41.4 degrees giving NA=0.35 vs 0.40 nominal for the 20X, and 26.5 degrees giving NA=0.23 vs 0.25 nominal for the 10X.

However the numbers for the macro lenses fall quite a bit shorter of expectation. For the Luminar, I measure a cone angle of 17.4 degrees giving NA 0.151 = fobs of f/3.27 vs fexp of f/2.75 from calculation (m=10), and for the Olympus, 18.9 degrees giving NA 0.164 = f/3.00 vs f/2.25 from calculation (m=8 ).

I would really need to re-run the experiment to be sure about the setup, but hopefully this gives a better idea of why I'm still bothered by these results.

It seems clear that the microscope objectives are giving wider apertures than the macro lenses.

That's OK by itself, but what bothers me is that the discrepancy is larger than predicted by straightforward calculation from published specs. Presumably I've messed up the experiment, left something out of the calculation, or the published specs don't mean what I think they do. (All of those are completely possible, and it doesn't have to be just one!)

It's probably relevant that both the Luminar and the Olympus lenses are somewhat asymmetric -- their apertures appear to be different sizes depending on which way you look through them. In other words, the pupillary magnification factors are different from 1. But I don't know yet what the numbers are, and I haven't worked through how to apply them to this situation.

As always, further insight will be most appreciated. It would be nice to get these observations moved into the category of "fully understood".

--Rik

----- Edited to add, Sep 17, 2018 (10 years later!) -----

I'm tired of periodically reading 'would be nice to get these observations moved into the category of "fully understood" ', when in fact I've understood this issue for about 9 years as of this writing. I'll explain now, but I'm choosing to edit the post rather than bump the thread.

Indeed the answer lies in the pupillary magnification factors. For the Luminar 16 mm f/2.5, that number is 0.87 (rear pupil smaller), and for the Olympus 20 mm f/2, it is 0.75 (rear pupil smaller). Both of these lenses are used in their "normal" orientation, not reversed, so the effect of rear-pupil-smaller is to make the subject-side NA smaller than it would be with PMF=1.

For the Luminar, at magnification 10.2 (based on the whole-frame images, compared to the 10X objective), I calculate subject-side NA = 0.160 versus measured 0.151. This means that at this magnification, the lens acts the same as an f/2.8 lens with PMF=1, despite its rating of f/2.5 based on FL and entrance pupil diameter.

For the Olympus, at magnification 9.04, I calculate subject-side NA = 0.173 versus measured 0.164 . This means that at this magnification, the lens acts the same as an f/2.6 lens with PMF=1, despite its rating of f/2.0 based on FL and entrance pupil diameter.

Considering measurement error, these numbers strike me as being close enough to declare success in understanding these lenses.

For more information about PMF, see the FAQ: What is "pupil ratio" and why would I care?.

--Rik
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