# What's the diff: Entrance Pupil v. Aperture



## christopher walrath (Apr 26, 2008)

I've seen different terminology on this site.  I was always learned that a lens's aperture diameter was found by dividing the lens' focal length by its aperture diameter.  Now I am seeing divide the focal length by the entrance pupil, not the aperture diameter.  Now are there two different parts to a lens by these names or are we tomato/toamto - ing this one?


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## Garbz (Apr 26, 2008)

The terms are used incorrectly interchangeably in this regard. The Aperture of a lens I believe is it's diameter but this is talking about a single element.

The aperture of a camera lens is always the one controlled by the aperture blades, and then we talk about the diameter of the entrance pupil. If we say the aperture of the entrance pupil that means the same.


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## christopher walrath (Apr 26, 2008)

Twice as confused.  Helen, WHERE ARE YOU?!?!?!?!?


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## Alex_B (Apr 26, 2008)

the actual blades of the aperture are usually not located directly at the entrance pupil of the lens. but the maximum aperture opening is usually limited by the entrance pupil of the lens.
 guess we need a graphical illustration now ....


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## christopher walrath (Apr 26, 2008)

I just wanna know which physical part's diameter gets divided into the focal length to give me an f-stop number.


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## Hertz van Rental (Apr 26, 2008)

The entrance pupil is the actual opening in the lens that light comes in through. 
If you open the aperture fully and look at the front of the lens you will see behind the front element(s) a circular hole.
This is sometimes called the entrance pupil and it's diameter is fixed.
Mounted behind it are the blades of the variable aperture.
When the aperture is fully open then the aperture and the entrance pupil are the same diameter.
When you stop down the entrance pupil remains the same but the aperture blades move in to reduce the size of the opening, reducing the amount of light coming in.
The f-number is calculated by dividing the diameter of the aperture - that is to say, the diameter of the circle produced by the aperture blades - into the focal length of the lens.
In effect the entrance pupil just determines the maximum aperture possible for a lens.
Normally you just refer to the hole as the aperture and don't differentiate - unless you are being pedantic.


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## Helen B (Apr 26, 2008)

The entrance pupil is the _image_ of the iris (the aperture blades) created by the lens elements in front of the iris, it is not a physical object. It is the hole you appear to see when you look into the front of the lens (ie the appearance of the hole, not the hole itself). If you stop the lens down, the entrance pupil closes in exact linear proportion. The entrance pupil is not fixed in diameter.

I'm calling the aperture blades themselves the iris.

The entrance pupil is the iris if there are no elements in front of the iris.

The entrance pupil may have neither the same diameter as the iris nor the same location along the lens' optical axis. If there are elements in front of the iris it is unlikely that the iris and the entrance pupil will have the same diameter. The entrance pupil can be in front of, in, or behind the plane of the iris.

The entrance pupil is also the centre of perspective of the lens: ie it is the place the lens 'sees' the world from. It is, therefore, the correct centre of rotation for a stitched panorama. The 'nodal point' is not. It is easy to find the location of the entrance pupil: just look into the front of the lens, and estimate the apparent position of the image of the iris.

The f-number of a lens is the focal length divided by the diameter of the entrance pupil. It is not the focal length divided by the diameter of the physical aperture (the thing that I am calling the iris).

There is also an exit pupil. That is where the image is projected from.

Here is a drawing of a Zeiss Distagon 35 mm f/3.5 for the Contax 645. The iris is the faint pair of lines to the left of the exit pupil. You can see that the diameter of the iris is very different from that of the entrance pupil.

V1 is the front vertex - ie the front of the front element. V2 is the rear vertex. N1 is the first nodal point, or front node. N2 is the second nodal point or rear node.

I should have drawn the lines inside the lens as dotted lines, because they do not represent the path of the incoming or outgoing ray, only where the ray appears to go and where it appears to emerge from.







Enough for now. More if you want it.

Best,
Helen

PS It's just called an f-number, not an f-stop number. The abbreviation for f-number is usually f/#. Therefore you can say that 'the f-number is 16' or 'the aperture is f/16' but 'the f-number is f/16' is more than necessary. Of course they are not the only ways of expressing the aperture, and you can say what you like.

Oh, and the lowest value that is theoretically possible for the kind of lenses we use is f/0.5.

Postscript.
If, having read the correct definition of _entrance pupil,_ you still think that the distinction between the physical aperture diameter and entrance pupil diameter is merely pedantic, here is a note I wrote about variable aperture zooms - how the varying diameter of the entrance pupil affects the aperture rather than the iris diameter, which may be fixed:

_"Even for those lenses that have a variable maximum aperture the diameter of the entrance pupil very rarely, if ever, stays the same as you zoom. Just have a look at the &#8216;hole&#8217; in your lens as you zoom. You can also calculate the diameter of the entrance pupil at the extremes of the zoom range. The kit lens is an 18-70 f/3.5 to f/5.6. That means that at 18 mm the entrance pupil is 5.1 mm diameter and at 70 mm the entrance pupil is 12.5 mm diameter. You should be able to see this when you look into the lens. The change is largely caused by the _[relative]_ movement of the lens elements in front of the iris &#8211; the diameter of the iris itself does not need to change."_

From this thread: link.


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## christopher walrath (Apr 27, 2008)

I'm sorry, I must be reading this wrong but I still see everybody saying the 'iris' is what you base your aperture/f/whatever number from by dividing it into the focal length of the lens, not the entrance pupil that remains fixed.  How can you determine an adjustable ratio by dividing one fixed value (focal length) into another (entrance pupil), the answer would be the same even though the actual aperture diameter gets smaller and thus the f/number should increase.


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## Helen B (Apr 27, 2008)

christopher walrath said:


> I'm sorry, I must be reading this wrong but I still see everybody saying the 'iris' is what you base your aperture/f/whatever number from by dividing it into the focal length of the lens, not the entrance pupil that remains fixed.  How can you determine an adjustable ratio by dividing one fixed value (focal length) into another (entrance pupil), the answer would be the same even though the actual aperture diameter gets smaller and thus the f/number should increase.



I didn't say that the entrance pupil was fixed (_" 		 		The entrance pupil is the image of the iris... The entrance pupil is not fixed in diameter."_), Herz did (_"This is sometimes called the entrance pupil and it's diameter is fixed."_). The definition of entrance pupil given by Herz is incorrect so, to avoid confusion, I suggest that you disregard it.

Best,
Helen


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## Helen B (Apr 28, 2008)

christopher walrath said:


> ... I still see everybody saying the 'iris' is what you base your aperture/f/whatever number from by dividing it into the focal length of the lens, not the entrance pupil...



You should never trust anything you read on the internet (not even the stuff I write), but if you trust me to type out some quotes from standard optical references, here they are, below.

Best,
Helen

 Note: For the kinds of lenses commonly used for photography, the &#8216;aperture stop&#8217; is the modern technical term for the iris, or diaphragm, or physical aperture blades. &#8216;Iris&#8217; is a little old-fashioned.

*Conrady, Applied Optics and Optical Design* (I&#8217;ve quoted a lot, because it is one of the best explanations of the names)

_The human eye is a complete optical instrument, closely resembling a photographic camera in its properties. It possesses an adjustable diaphragm in the centre of a disk of complicated structure which lies just in front of its lens. This disk, which also determines the colour of the eye by the predominating tint of its outside surface, is called the iris, and Abbe therefore introduced this name &#8216;Iris&#8217; for the actual, material, diaphragm which in any given lens system determines the diameter of the axial pencil which can pass through the system. When we look at a person&#8217;s eye, we cannot see the actual iris directly because the cornea and the liquid which fills the space between cornea and lens are interposed; what we see is a virtual image of the actual iris, slightly magnified by the lens-effect of the intervening media, and the central hole of the iris as it appears in this virtual image represents what we always call the &#8216;pupil&#8217; of the eye.

  ...and on account of the complete analogy with the eye Abbe introduced the name &#8216;Pupil&#8217; for these images of the actual diaphragm, calling the pupil seen from the object space the &#8216;Entrance pupil&#8217;..._

   **********************************

*ISO 517:2008 Photography - Apertures and related properties pertaining to photographic lenses - Designations and measurements

* _*f-number*
*stop number*
  the reciprocal value of the relative aperture

*relative aperture of a photographic lens*
  twice the numerical aperture where the numerical aperture is the sine of the semi-angle subtended by the exit pupil at the focal plane
  NOTE For photographic applications, the relative aperture is equivalent (within 1/3 stop) to the ratio of the diameter of the entrance pupil to the focal length.

*entrance pupil*
  image of the aperture stop as viewed from a point in the object space on its optical axis (the image of the aperture stop formed by the front elements of the lens)

*aperture stop*
  physical stop that limits the cross-section of the light beam that can pass through the lens to reach the centre of the on-axis image_

**********************************


*Hecht, Optics

* _The entrance pupil of a system is the image of the aperture stop as seen from an axial point on the object through those elements preceding the stop. If there are no lenses between the object and the aperture stop, the latter itself serves as the entrance pupil.

  The entrance pupil ... diameter D.
  The ratio D/f is known as the relative aperture, and its inverse is the focal ratio or f-number, often written f/#, that is

  f/# &#8801; f/D_

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*Born and Wolf, Principles of Optics

* _The stop which determines the cross-section of the image-forming pencil is called the aperture stop. To determine its position, the Gaussian image of each stop must be found in the part of the system which precedes it; the image which subtends the smallest angle at Po is called the entrance pupil. The physical stop which gives rise to the entrance pupil is the aperture stop. (If it lies in front of the first surface it is identical with the entrance pupil.)

  ... a convenient measure of its light-gathering power is the so-called &#8220;F number&#8221; or &#8220;nominal focal ratio&#8221;. It is the ratio of the focal length f of the system to the diameter d of the entrance pupil:

  F = f/d.

_**********************************


*Ray, Applied Photographic Optics

* _The image of the _[aperture]_ stop as given by the preceding elements in the object space form the entrance pupil (En) of diameter e.

  For a simple lens of diameter D

  N = f/D

  But for a compound lens, the entrance pupil diameter (e) and equivalent focal length (EFL), also denoted f, are used, so at infinity focus 

  N = f/e_


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## Sarah23 (Apr 28, 2008)

wow...this is all over my head...lol


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## christopher walrath (Apr 28, 2008)

So if I get this right, the entrance pupil is the diameter of the visible image of the stop as viewed through the front element from in front of the camera/lens. And it is this diameter that, when divided into the lens' focal length, determines the f number. Did I get it?


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## Helen B (Apr 29, 2008)

Yes, I think that you have got it.

There's also an exit pupil, of course. That's the image of the iris when viewed from behind the lens.

Have a look at some of your lenses from the front and the back. Symmetrical lenses will have the same, or roughly the same, size entrance and exit pupil. Telephoto (true telephoto, not just long focus) lenses will have a larger entrance pupil than exit pupil. Retrofocus lenses (wide angle lenses for use on SLRs etc) will have a smaller entrance pupil than exit pupil.

Instead of looking at the pupils straight on, look at them from an angle as well. Notice how the shape changes. Sometimes it looks just like a disk that is held flat in the lens - ie when you look at it from an angle it will appear elliptical. 

With wide angle lenses it may look as if it is following you, and hence stay roughly circular. This is a very clever effect that reduces the natural light falloff at wide angles, known most commonly as  a 'tilting pupil' and the effect is often called the 'Slyussarev Effect' (various spellings are used in optical literature). Slyussarev did some key work on the evenness of illumination of wide angle lenses in the 1940s.


Finally, as I mentioned in a previous post in this thread, the entrance pupil is the centre of perspective of the lens - ie where it sees the world from. This is why it is the correct point of rotation for a stitched panorama. Because you can see the entrance pupil, it is easy to estimate its position.

Best,
Helen


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## Judge Sharpe (Apr 29, 2008)

The confusion comes from the use of an f/# to describe not only the iris (aperture), but the light gathering ability if a lens itself. such as a 135-f/2.8. 
Remember that the number following the f/ is a logarithmic expression and that is the reason that the number goes greater (>) as the light transmission gets smaller (<). The so called f/# is in actuality a function of the area of the open space that allows light through the lens and the focal length of the lens. A faster lens allows more light through at the same opening in a slower lens. It has a lower f/ number value. With every thing else being equal, a faster lens is the better choice. 
Judge Sharpe


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## Helen B (Apr 29, 2008)

Judge Sharpe said:


> The confusion comes from the use of an f/# to describe not only the iris (aperture), but the light gathering ability if a lens itself. such as a 135-f/2.8.
> Remember that the number following the f/ is a logarithmic expression and that is the reason that the number goes greater (>) as the light transmission gets smaller (<). The so called f/# is in actuality a function of the area of the open space that allows light through the lens and the focal length of the lens. A faster lens allows more light through at the same opening in a slower lens. It has a lower f/ number value. With every thing else being equal, a faster lens is the better choice.
> Judge Sharpe



I think that we are on different wavelengths here. Have you actually read this thread?

Best,
Helen


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## christopher walrath (Apr 29, 2008)

Judge Sharpe said:


> The confusion comes from the use of an f/# to describe not only the iris (aperture), but the light gathering ability if a lens itself. such as a 135-f/2.8.
> Remember that the number following the f/ is a logarithmic expression and that is the reason that the number goes greater (>) as the light transmission gets smaller (<). The so called f/# is in actuality a function of the area of the open space that allows light through the lens and the focal length of the lens. A faster lens allows more light through at the same opening in a slower lens. It has a lower f/ number value. With every thing else being equal, a faster lens is the better choice.
> Judge Sharpe


 
Yeah, that just confused it. I got this one. The number behind the f/ is actually a denominator of a fraction that denotes the lens' focal length divided by the entrance pupil (yeah me). The amount of light gets less as the number gets larger because as the number gets larger it takes more entrance pupil diameters to makeup the focal length. And vice versa. A fast lens and a slow lens allow the same amount of light through at f/8 because, though the shorter lens has a smaller aperture diameter, the longer lens makes the light travel twice the distance through its body. A faster lens is a great choice, and also more expensive. Faster film is cheaper than slower film so for those who are financially challenged, a slower lens and faster film will do the job nicely.  FYI, for those wondering, a fast or slow lens refers to its largest aperture setting and it ability or lack thereof to gather light in a low light situation.  Ex. a f/1.4 lens would be considered fast and an f/4 lens would be considered slow.


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