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Rich V.
scholastic sledgehammer
Reged: 01/02/05
Posts: 988
Loc: Carson Valley, Nevada USA
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Quote:
You may be right, but intuitively this doesn't make sense. Suppose you have a very narrow pencil of light like a laser beam. Are you saying that if you shine this in the eye the beam will spread to cover the entire retina? If so, how do they perform laser surgery on the retina?
Bill T,
With the laser you're talking about a linear "beam" of light; the exit pupil of an eyepiece is a cone of light which has an angle which is the AFOV. A 70° cone from a 2mm exit pupil illuminates the same area of the retina as a 70° cone from a 7mm exit pupil. Only the brightness is different. Any two eyepieces with a 70° AFOV, regardless of exit pupil will give your brain an image whose apparent diameter is identical.
I thought you were saying that a smaller exit pupil illuminated a smaller area of the retina. This is not true. That relationship depends on AFOV only. AFOV is the angular field projected on the retina and is a function of the lens design not exit pupil.
Rich V
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Binoculars:
33-150x100 Saturn III, 16x70FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
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Swedpat
Pooh-Bah
Reged: 02/18/05
Posts: 1033
Loc: Boden, Sweden, Scandinavia
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Quote:
What's at stake here is IF the exit pupil is small it must pass through the central sharpest area of the eye's lens. Our vision is sharpest through the central axis of the lens just like a camera lens operates best through it's central axis. I know of no camera lenses that produce as sharp an image wide open as they do stopped down. Our eye lens operates in much the same fashion.
In daytime a small exit pupil produces a sharp image on the retina rather easily as the eye pupil is constricted and all the light must pass through the central axis. You do get a "black out" effect easily since the constricted pupil is a small target.
At night, things are different; now the eye pupil is dilated and it is easier for the small exit pupil to fall off axis in the eye. Careful IPD adjustment is now more important to get the sharpest views. "Black out" is not such a big issue; the dilated pupil is an easy target to hit but if the light cone doesn't pass through the center of the lens image quality suffers.
Rich V
Rich,
I'm trying to understand the reasoning here...
If a smaller exit pupil gives a sharper image because the light will pass on-axis of eye pupil it sounds like it's in opposite to the statement that larger aperture improves the resolution despite same power; a larger aperture will result in larger exit pupil with same power...
But yes, I have experienced small binoculars as sharper at daytime (a 7x20 seems to be sharper than a 7x50) but my thought about the reason of that is simply that the smaller exit pupil cut down the total light amount and therefore increases the contrast. But I may be wrong.
Patric
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*2,3x40 Constellation View Wide-Bino
*Leupold 6x30 Yosemite
*Leupold Katmai 6x32
*Swarovski SLCNew 7x42B
*Bresser (Lidl) 10x50
*Oberwerk 11x70
*No name (Kunming) 15x70
*Scopos ED APO 66
*Meade 5000 26mm Plössl, Vixen LV 10/5mm
Psalm 19:2
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btschumy
Think Astronomy
Reged: 04/13/04
Posts: 1111
Loc: Austin, TX, USA
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Rich,
Thanks for clarifying this. I wish I could find a good reference that explains this. Most of the optics books just show raytracings and also leave out what happens in the eye. This doesn't give me a good understanding of the overall process.
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Bill Tschumy
Where is M13? Freeware -- Add a new dimension to your observing.
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