Aperture vs entrance pupil

[bclaff]

What is the difference between an aperture and an "entrance pupil?" In a 600mm f/4 lens, is the aperture diameter truly 150mm, as given by the f/D=N formula?

I will be repeating some of the information already provided in an attempt to provide a comprehensive answer.
(This is also my first post so hopefully I won't mess up. )


The term aperture can be ambiguous whereas entrance pupil is clearly defined.


The entrance pupil is a virtual image of the aperture stop (diaphragm / iris) as seen from the front of the lens.
There is a corresponding exit pupil as viewed from the rear of the lens.


The physical diaphragm/iris is typically (much) smaller than the entrance pupil.


The diameter of the entrance pupil is f/f# where f is the focal length and f# is the f-number.
(f# is sometimes called aperture which is the source of some confusion.)


The stated focal length for any lens is at infinity and is often rounded as is the f#.


When you focus closer than infinity the focal length almost always drops.
It is very rare for it to go up and if so only a small amount.


Here's a ray tracing of a Nikon AF-S Nikkor 600mm f/4E FL ED VR from the PhotonsToPhotos Optical Bench:




Note that this 600mm f/4 lens is actually 587.8mm f/4.08
Note that the entrance pupil size is 144.07mm (P_D) whereas the aperture stop diameter is only 34.521mm (A_D)
The exit pupil diameter is only 35.99mm (P'_D) so pupil magnification is 0.25 (35.99mm / 144.07mm)
Pupil magnification is often overlooked but is particularly relevant in close-up photography.


If this subject interests you then you may want to visit the Optical Bench or the PhotonsToPhotos Optics Primer.

[bclaff]

Most sources say the reverse, e.g., the tutorial on this site, which says that focal length roughly doubles at minimum focusing distance with a macro lens:



I don't have a Nikon, but I believe this shows up in larger effective f-stops at minimum distance on Nikon cameras.

I haven't tested this myself.

I haven't looked at the tutorial but that is wrong and sounds like a confusion with effective f# (aka Numeric Aperture (NA))

effective f# is f# * (1 + m / p) where m is magnification and p is pupil magnification

If you assume pupil magnification is 1 (which it rarely is)
And you set m to 1 then you get

effective f# = f# * (1 + 1/1) = f# * 2

And yes, Nikon, unlike most other brands, reports the effective f# rather than f#
This is good but does confuse some people who are just getting into close-up photography.

[bclaff]

.... First, the effect of adding a given length of extension tube to my 100mm macro at minimum working distance is much greater than it ought to be if the actual FL were 100mm. It's more consistent with 70mm or so. I actually took measurements for this but no longer have them. ...

Since the additional magnification you get with an extension tube is
x / f where x is the extension and f is the focal length
your observation is expected.
And, in fact, measuring magnification with one (or more) extension tube(s) is a great way to measure focal length.

.... Second, if the tutorial here is right, one should see a big increase in diffraction ...

I haven't looked at the tutorial but ...
diffraction on the image plane is a function of the exit pupil and therefore is affected by pupil magnification.
Pupil magnification at high magnification is often well under 1, like 0.25
This actually means increased Depth Of Field (DOF0 over what you might expect and less diffraction.

.... If #1 is correct, then the AOV should increase as you approach minimum working distance. ...
If anyone knows the answers to this and has some technical explanation, I'd love to see it.

Angle of View (AOV) goes down with increased magnification.
You can read the gory details in Optics Primer - Angle of View and Magnification Formulas

For example the Nikon AF-S Micro-Nikkor 105mm f/2.8G VR:
23.63 degrees at infinity
13.16 degrees at 0.5x (and focal length 96mm)
7.18 degrees at 1.0x (and focal length 76mm with pupil magnification 0.30)

[xpatUSA]

Welcome, Bill!

Stay safe,

[Manfred M]

Thanks Bill - I had a quick look at this material and your website. Brings back memories from a very long time ago (pushing 50 years) and university physics. Some of this material brings back some very foggy memories and other goes well beyond the scope of what we covered in the course. I'm going to have to spend some more time going through this material. I've bookmarked your site.

I have other vague memories of doing some very simple ray tracing programming in Fortran IV using punch cards on an IBM 360 mainframe computer.

When I look at the topics you have covered, I'm a bit surprised you have not discussed either the impact of anti-reflective coatings or the impact of flange to film / sensor distance on lens performance (or a few more interesting (to me) topics), like vignetting.

[DanK]

Bill,

thanks

Dan

[bclaff]

...
When I look at the topics you have covered, I'm a bit surprised you have not discussed either the impact of anti-reflective coatings or the impact of flange to film / sensor distance on lens performance (or a few more interesting (to me) topics), like vignetting.

Thanks. Sorry for the delay. Thought I was getting email notifications but none arrived.

Coatings aren't a subject I know well nor do they lend themselves to demonstrations with my ray tracer.

I've considered showing how design is influenced by Flange to Focal Distance (FFD).
So that's on my "to do" list. Stay tuned.

I do make some references to light fall-off which is due to hard vignetting of the exit pupil but don't cover that as a topic on it's own.

Thanks for the feedback

[William W]

Welcome

. . . (This is also my first post so hopefully I won't mess up. )

You certainly did NOT mess up.

I have bookmarked and referenced your website for future: Thank you. Excellent.

WW

[Manfred M]

Thanks Bill - I missed your follow up post. I too have bookmarked your website for reference. A well done and comprehensive piece of work!

I will pop in there from time to time to see what you have added / updated.