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# Why a too-small iris results in reduced aperture

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### #1 GlennLeDrew

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Posted 24 August 2013 - 09:46 PM

In a couple of recent threads I've laboured to describe why a too-small iris (or too-large exit pupil) reduces the aperture of a telescope. Time for a picture, the worth of which the old proverb tells us.

A few introductory notes:

1) The exit pupil is a real image of the entrance pupil (nominally the objective) formed by the eyepiece, and located in space behind the eyepiece at the eye point distance. This is where your iris must be located in order to enjoy a full and evenly illuminated field.

2) The eypeice is an optical coupler which places the reduced objective onto the plane of the iris. But this is a two-way street; the eyepiece also projects anything in the plane of the exit pupil onto the plane of the entrance pupil (objective.)

3) As long as the iris is as large or larger than the exit pupil, the full aperture of the objective is utilized.

4) When the iris becomes smaller than the exit pupil, an aperture stop is introduced into the system. The eyepiece projects the iris aperture onto the objective, which is exactly like placing a physical aperture mask of the same size there. If the iris is half the diameter of the exit pupil, its image on the objective is half the diameter of the objective.

Due to the location of the projected aperture stop in the plane of the objective, all light from all parts of the field must necessarily pass through this one and only reduced aperture; the area of the objective outside this contributes absolutely nothing to image formation.

5) When the too-small iris moves about in the larger exit pupil, its image also moves about on the objective. If the iris meets the edge of the exit pupil, its projected image meets the edge of the objective, on the opposite side. If the iris half overlaps the exit pupil (50% clipped), its image is half inside-half outside the opposite objective edge.

As before, no matter where the reduced aperture is projected onto the objective, the area outside the projected iris does not contribute in any way to image formation, anywhere in the field of view.

One last remark, about the bottom panel in the illustration which deals with the too small iris/too-large exit pupil. The shaded region bounded by the red rays lying between objective and exit pupil is the full envelope of light fielded by the eyepiece as it forms an image of one point on the edge of the iris onto its corresponding point onto the objective. And conversely, a point on the objective projected by the eyepiece onto its corresponding point on the exit pupil/iris.

### #2 Chuck Hards

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Posted 24 August 2013 - 10:30 PM

Well, you were busy today!

Nice illustration, thanks.

### #3 Stacy

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Posted 25 August 2013 - 12:24 AM

So my new 7x50's are really like 7x30 because of course my iris will never be 7.1mm?

### #4 KennyJ

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Posted 25 August 2013 - 01:57 AM

--- And the greatest benefit of all this is that the reduced effective aperture proportionally increases effective focal ratio, reducing astigmatism and other aberrations and producing much "sharper", "tighter" images than if the system were operating at full aperture.

Kenny

### #5 Stacy

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Posted 25 August 2013 - 02:13 AM

--- And the greatest benefit of all this is that the reduced effective aperture proportionally increases effective focal ratio, reducing astigmatism and other aberrations and producing much "sharper", "tighter" images than if the system were operating at full aperture.

Kenny

That sounds like a good thing! So all is not lost. That's great because it's not unlike a lot of the other choices and trade-offs we all make with different optical systems. So many different systems and configurations each with it's own best application.

### #6 bremms

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Posted 25 August 2013 - 09:53 AM

You can see your exit pupil in a defocused star image. move your eye around a little when you have a 6-5mm exit pupil in the scope. you will see the iris cutting of edges of the light cone. I do have a 7mm exit pupil but the outer mm or has a lot more abberations than the rest. Using a 5-6mm exit pupil with 0.5 diopter astigmatism correction works great for me.

### #7 MessiToM

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Posted 25 August 2013 - 01:09 PM

Hmm average exit pupil for a 29 yr old (me)is 6.9mm. My 32mm 2" ep gives me a exit pupil of 7.17mm

Guess Iam missing out on some. BUT maybe this helps with coma near the edge of fov?

I used this calculator http://www.stargazin...a/scopemath.htm

### #8 Chuck Hards

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Posted 25 August 2013 - 01:15 PM

The beauty of this is that it is easily testable. Install a temporary central obstruction, leaving only a thin annulus around the circumference of the objective. Swap out eyepieces until you end up with one that produces an exit pupil that forms an image.

### #9 GeneT

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Posted 25 August 2013 - 04:34 PM

Good job. Nice explanation.

### #10 sopticals

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Posted 26 August 2013 - 04:34 AM

The beauty of this is that it is easily testable. Install a temporary central obstruction, leaving only a thin annulus around the circumference of the objective. Swap out eyepieces until you end up with one that produces an exit pupil that forms an image.

Very good tip. Must try it sometime.

Stephen.(45deg.S.)

### #11 highertheflyer

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Posted 26 August 2013 - 09:23 AM

I have an adjustable iris salvaged from an old camera, with the opening from 50mm fully opened to 3mm fully closed.
Does this iris need to be positioned at the focal point of its primary mirror, or can it be placed in the light cone between the mirror and the eyepiece?
Thx, Jim

### #12 GlennLeDrew

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Posted 26 August 2013 - 09:47 AM

Placing an iris at the focus serves only to change the field size. It would be exactly like having an adjustable field stop in the eyepiece.

The ideal place for an iris is at the entrance pupil, or the objective. But then it has to be as large as the objective. One could place the iris somewhere between the objective and the focus. But then when stopping down the light cone, it becomes the entrance pupil. The closer to the focus this iris lies, the worse the vignetting it introduces.

### #13 MKV

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Posted 26 August 2013 - 01:05 PM

Glenn, digital cameras today come almost exclusively with zoom lenses. It seems the zoom element(s) optically act as an iris, which would make them less than ideal imaging devices for astrophotography except that they help stop down the front aperture and thereby reduce all aberations (as well as theoretical resolution, which is not important in snapshots of the sky). So, in a roundabout way, it is beneficial.

As we have seen in a related thread recently, an afocal image seems to magically "wipe out" chromatic aberration in an otherwise poorly corrected system. Of course, the effective relative aperture is very small and the focal ratio is artificially increated to over f/100!), but ti is no wonder that afocal astrophotography has become popular, since even bad telesciopes produce good images due mainly to pupil issues.

My main interest in optics is in the Bath interferometry and the iris inherent in the digital cameras today was a big problem (especially with the edge of the optic being tested) until I obtained a mirrorless digital camera with a fixed f.l. 35 mm film lens. These old lenses a their fastest setting have only the front aperture as the iris and produce completely unvignetted interferograms.

The zoom lens that came with the camera (22-55 mm) has a 7 mm pupil. My fixed f.l. 50 mm "normal" lens set at f/2 has an exit pupil of about 25 mm.

Regards,

### #14 highertheflyer

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Posted 26 August 2013 - 02:18 PM

Placing an iris at the focus serves only to change the field size. It would be exactly like having an adjustable field stop in the eyepiece.

The ideal place for an iris is at the entrance pupil, or the objective. But then it has to be as large as the objective. One could place the iris somewhere between the objective and the focus. But then when stopping down the light cone, it becomes the entrance pupil. The closer to the focus this iris lies, the worse the vignetting it introduces.

So may I understand that when a large adjustable iris aperture is placed 50mm's forward of the eyepiece field stop and in the line of the light path towards the objective; that constricting with adjustable's smaller than actual eyepiece field stop diameters, there may be a gain in advantage too?
I have an adjustable iris much smaller than the objective diameter in the junk box, and wish to experiment as you might suggest.
Thanks,
Jim

### #15 MitchAlsup

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Posted 26 August 2013 - 03:43 PM

Glenn forgot, also, that the part of the iris that is illuminated (but does not make it to the retina, creates a background glow that diminushes contrast.

### #16 SACK

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Posted 26 August 2013 - 08:30 PM

Hi Mitch,
How does the lit retina create the background glow? How bad is it? I have not noticed before but sounds interesting and good to know for eyepiece selection.

### #17 GlennLeDrew

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Posted 26 August 2013 - 09:57 PM

Jim,
An iris a mere 50mm in front of the focus would introduce *very* pronounced vignetting. The effect would be more like a very blurred field stop edge.

For example, let's consider an f/5 objective. At 50mm from the focus, the light cone for an image point is 10mm wide. In order to stop down an on-axis image point, the iris opening would have to be less than 10mm wide. Illumination immediately off-axis would fall away steeply, and be zero at not much more than 5mm off axis.

To maintain the same degree of image diminution across the field, the iris must lie at the objective. Otherwise vignetting results, becoming more severe the closer to the focus the iris is located.

Mitch,
I too wonder about the lit portion of the iris introducing a contrast/robbing glow. Is it because of some light is filtering through the iris (which I thought was rather opaque)? If this were the case, then how about normal daytime views as you walk about outdoors? Here the iris is illuminated over its full surface, over an angle approaching 180 degrees, and where the opening is near its smallest; conditions which one would think would be the most conducive to this effect. And of course looking toward the Sun would be the worse case. However, I think the glare here is merely the scatter occurring in the transmissive parts of the eye, not light filtering through the iris.

### #18 Crayfordjon

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Posted 27 August 2013 - 01:02 AM

An iris that is too small will result is a reduction of aperture, but as the iris is at the eye end of the system and not at the OG, the full Aperture is still used, the smaller iris induced apertures will sweep over the OG as the field angle is scanned. This is how a camers lens works. Your optical diagram is drawn strictly along the optical axis and does not take into consideration the field diameter of he image plane. This is a concept I have been labouring in a recent thread without success.

### #19 MKV

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Posted 27 August 2013 - 01:15 AM

Here is a good article on the exit pupil and its effects.

Exit Pupil Effects

If the exist pupil is larger than the sensor, be it your eye or a camera, vignetting will result. It's very likely that both Glenn and John are correct, but are talking considering different configurations. It would help if each made an accurate graphic representation of what they are talking about for direct comparison.

regards,

### #20 GlennLeDrew

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Posted 27 August 2013 - 02:18 AM

Jon,
As I stated, for the reduced iris condition, as the small iris moves laterally across the larger exit pupil, the reduced aperture at the objective correspondingly sweeps across the objective. And so, yes, the full aperture *can* be utilized, but *never at any one time.* If the iris is 1/2 the exit pupil, never more than 1/2 the objective contributes to image formation. As the iris wanders, that 1/2 objective diameter reduced aperture region also wanders about the objective.

I guess I'll have to prepare a diagram like that in the bottom panel, but for the case where the iris is abutting the edge of the exit pupil (as opposed to being in the center.) Then one would see that the reduced aperture at the objective is still just as small, but offset so as to abut the objective edge.

This in no way means that the objective is ever working at full aperture. Just because one has the freedom to let the iris wander, and thereby utilize different parts of the objective at different times, is not the same as utilizing the full objective at one instant. And so at any one time the instrument is always working at reduced aperture.

### #21 GlennLeDrew

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Posted 27 August 2013 - 02:29 AM

Malden,
I've already made a precisely accurate diagram which well illustrates the concept, in the bottom panel. Another version based on the iris having moved to the edge of the exit pupil will be functionally the same, except that the reduced aperture will have moved to the opposite edge of the objective.

The Wiki article you linked to does not get into the detail and the specifics I'm addressing here.

### #22 MKV

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Posted 27 August 2013 - 03:15 AM

Jon,
As I stated, for the reduced iris condition, as the small iris moves laterally across the larger exit pupil, the reduced aperture at the objective correspondingly sweeps across the objective. And so, yes, the full aperture *can* be utilized, but *never at any one time.* If the iris is 1/2 the exit pupil, never more than 1/2 the objective contributes to image formation. As the iris wanders, that 1/2 objective diameter reduced aperture region also wanders about the objective.

I guess I'll have to prepare a diagram like that in the bottom panel, but for the case where the iris is abutting the edge of the exit pupil (as opposed to being in the center.) Then one would see that the reduced aperture at the objective is still just as small, but offset so as to abut the objective edge.

This in no way means that the objective is ever working at full aperture. Just because one has the freedom to let the iris wander, and thereby utilize different parts of the objective at different times, is not the same as utilizing the full objective at one instant. And so at any one time the instrument is always working at reduced aperture.

Glenn this is exactly what I see when I use SONY DSC HX-1 camera and a relay scope to capture interferograms. The picture below shows a pupil that cuts off part of an 8-inch f/3 mirror. You can move left and right, as if peeking through a keyhole, and see more of the mirror, but not at the same time. So clearly, I am seeing only a small section of the image, however the image is s till a full-aperture image; its just that you don't see all of ti at once.

regards,

### #23 Crayfordjon

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Posted 27 August 2013 - 03:15 AM

Here is an optical diagram which explains clearly what happens when the iris is smaller than the exit pupil of an eyepiece. The resolving aperture diameter is generated by the diameter of he iris "d", rays entering at an angle from the edge of the OG define the field angle of the focal plane, although the resolving aperture is smaller than the OG aperture"D", the whole of the OG aperture is still used, as is illustrated.

### #24 freestar8n

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Posted 27 August 2013 - 04:05 AM

Hi-

I think this original post is overall correct but I would add a few things that are relevant to the discussion. First, I think that talk about entrance and exit pupils should be in the context of object space and image space. In this view, the pupils are defined differently from the way Glenn presents.

Normally, an optical system has a single physical aperture in it that limits its throughput, and the entrance pupil is the image of that stop as seen from object space. The exit pupil is the image of that stop as seen from image space. Since they are both images of the same thing, the two pupils are conjugate - and any other images of that stop are also pupils. But the entrance and exit pupils are defined in different conceptual spaces, and they are formed from entirely different optical components viewing the same physical stop.

This is important because it's not just the size of a pupil that matters, but its location in the corresponding space. It's ironic that Glenn is providing details on the definition of these pupils, because I have been making the same points in discussions of the so called "flashlight test" to measure aperture - the accuracy of which depends critically on the location of the pupil when some stop near the image plane is acting as the aperture stop.

The location of the pupils is similarly important when doing afocal imaging with a digital camera. It's not just the size of the pupils that matters, but their locations. The exit pupil of a normal afocal telescope will have some size and location in the image space behind the eyepiece. Meanwhile, if you look into a digital camera, you will "see" the entrance pupil of that camera as an image of its iris, formed by the lenses in front of that iris. It will have some apparent size and apparent depth into the camera - indicating its location and size in object space as you look into it.

In order to avoid vignetting, the entrance pupil of the camera needs to be big enough *and* close enough to the exit pupil of the telescope. Typical digital camera lenses have the entrance pupil fairly deep inside the lens, behind the front element. In that case, even if the camera entrance pupil is bigger than the telescope exit pupil - if it is too far back the vignetting can be severe.

If the entrance pupil of the camera is smaller than the telescope exit pupil, then now the iris of the camera is the aperture stop of the overall system, and the entrance pupil has a size and location unrelated to the telescope objective. The entrance pupil is the image of that iris as seen from object space, looking through the objective, the eyepiece, and the front elements of the camera lens ahead of the iris.

When I see afocal images taken through a telescope, the vignetting is a mixture of the small size of the entrance pupil of the camera, combined with its distance from the eyepiece, limiting the result. If you can't get the camera lens physically close enough to the eyepiece then there will be vignetting. If the camera lens aperture is too small, then the aperture of the system is reduced and the location of the entrance pupil is unknown without a ray trace.

Similar problems show in Foucault test images, where it is difficult to get the entrance pupil of the camera close enough to the knife edge to get a full view of the mirror.

In all these discussions, the physical size of the front lens of the camera isn't what's important, but instead its the stated aperture of the lens - i.e. its entrance pupil diameter - combined with the location of that entrance pupil (which itself is an image) in image space behind the eyepiece, or knife edge.

Frank

### #25 freestar8n

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Posted 27 August 2013 - 04:25 AM

Although your diagram shows that the full aperture is "used" - different parts of the aperture are used for different points in the field. This means the objective is acting more like a field stop than an entrance pupil. This is in fact the case for a Galilean telescope, where the human iris is the aperture stop of the system. The field becomes vignetted and the objective acts as the field stop.

In terms of both the entrance pupil diameter and diffraction performance, the full objective aperture is not being used. The aperture is only helping to reduce vignetting across the field.

Since the entrance pupil is much smaller than the objective, the f/ratio of the system is much greater than would appear using the objective aperture as the entrance pupil diameter. The objective has no connection, in size or location, to the entrance pupil of the overall system - and it's the entrance pupil diameter that determines throughput, diffraction performance, and the effective speed of the system. Since the system is slower, it improves the aberration performance by being stopped down.

Frank

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