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GlennLeDrew
Postmaster

Reged: 06/18/08

Why a too-small iris results in reduced aperture

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.

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Chuck Hards
Post Laureate

Reged: 05/03/10

Loc: The Great Basin
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6044403 - 08/24/13 11:30 PM

Well, you were busy today!

Nice illustration, thanks.

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Stacy
Star Partyer

Reged: 09/15/02

Loc: Seattle, WA
Re: Why a too-small iris results in reduced aperture [Re: Chuck Hards]
#6044530 - 08/25/13 01:24 AM

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

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KennyJ
The British Flash

Reged: 04/27/03

Loc: Lancashire UK
Re: Why a too-small iris results in reduced aperture [Re: Stacy]
#6044621 - 08/25/13 02:57 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

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Stacy
Star Partyer

Reged: 09/15/02

Loc: Seattle, WA
Re: Why a too-small iris results in reduced aperture [Re: KennyJ]
#6044633 - 08/25/13 03:13 AM

Quote:

--- 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.

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bremms
Carpal Tunnel

Reged: 08/31/12

Loc: SC
Re: Why a too-small iris results in reduced aperture [Re: Stacy]
#6044960 - 08/25/13 10:53 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.

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MessiToM
scholastic sledgehammer

Reged: 12/21/09

Loc: Huntingdon PA
Re: Why a too-small iris results in reduced aperture [Re: bremms]
#6045266 - 08/25/13 02:09 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.stargazing.net/naa/scopemath.htm

Edited by MessiToM (08/25/13 02:09 PM)

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Chuck Hards
Post Laureate

Reged: 05/03/10

Loc: The Great Basin
Re: Why a too-small iris results in reduced aperture [Re: MessiToM]
#6045278 - 08/25/13 02:15 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.

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GeneT
Ely Kid

Reged: 11/07/08

Loc: South Texas
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6045575 - 08/25/13 05:34 PM

Good job. Nice explanation.

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sopticals
professor emeritus

Reged: 03/28/10

Loc: New Zealand
Re: Why a too-small iris results in reduced aperture [Re: Chuck Hards]
#6046430 - 08/26/13 05:34 AM

Quote:

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.)

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highertheflyer
professor emeritus

Reged: 07/08/05

Loc: Aledo, Texas
Re: Why a too-small iris results in reduced aperture [Re: sopticals]
#6046685 - 08/26/13 10:23 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

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: highertheflyer]
#6046740 - 08/26/13 10:47 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.

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6047059 - 08/26/13 02:05 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,

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highertheflyer
professor emeritus

Reged: 07/08/05

Loc: Aledo, Texas
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6047193 - 08/26/13 03:18 PM

Quote:

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

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MitchAlsup
Pooh-Bah

Reged: 08/31/09

Re: Why a too-small iris results in reduced aperture [Re: highertheflyer]
#6047312 - 08/26/13 04:43 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.

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SACK
member

Reged: 08/11/11

Loc: TX
Re: Why a too-small iris results in reduced aperture [Re: MitchAlsup]
#6047756 - 08/26/13 09:30 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.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: SACK]
#6047899 - 08/26/13 10:57 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.

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Crayfordjon
Vendor - Zerochromat

Reged: 06/17/09

Loc: UK
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048116 - 08/27/13 02:02 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.

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: Why a too-small iris results in reduced aperture [Re: MKV]
#6048129 - 08/27/13 02:15 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,

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: MKV]
#6048165 - 08/27/13 03:18 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.

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