Image Quality and the Eye's Pupil Size (1 Viewer)

Pinewood said:

Alan,

Perhaps with your pupil dilated, your vision may have suffered from curvature of field, poor depth of field, chromatic aberration, coma and any of a myriad of other problems exhibited by camrera lenses when they are not stopped down.

Clear skies,
Arthur Pinewood

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AlanFrench said:

I had a chance to do a little informal experiment today. I had my annual eye exam, and arrived home with my pupil still dilated to about 8mm.

I like my 7x42 binoculars because they "snap" into focus, and the correct focus is quite obvious. What would happen when my pupil was open wide enough to accept the entire 6mm exit pupil? I found that the no longer snapped into focus and there didn't seem to be a really correct focus. I'd get to a point where they looked almost there, and further motion of the focus knob in that dirrection would just make it worse.

For comparison, I tried my 12x36s. With the smaller 3mm exit pupil serving to stop down my eye's pupil, the correct focus was obvious, although they didn't seem quite as sharp as during normal use. It may well be that I didn't quite have their exit pupil lined up with the center of my pupil where the eye's image quality is best.

Alan

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Leif said:

I've long noticed that as the light fades, so my sight gets worse and I have difficulty getting a sharp image. My optician confirmed it as a general phenomena. The two causes I can see are the reduction in DOF due to the eyes aperture opening (which I suspect is the main cause) and a possible reduction in image quality due to imperfections in the cornea (which will vary between individuals). The other cause mentioned by Arthur - aberrations from a faster lens - undoubtedly plays a role, though goodness knows how significant it is compared with the others.

Leif

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jrhudgins said:

Regretfully, the speculation about the effect of pupil size alone on the ability to focus is left unanswered under the conditions described.

Most eye exams are performed with the use of parasympatholytic drops. These medications do two things to the eye: They prevent the eye's ability to constrict the pupil AND they paralyze the ciliary body. The latter is the muscle and fibers system that is used to change the shape of the lens, or perform accomodation.

If you could find someone who could use a sympathetic agonist eye drop, then the pupil would dilate but the lens could still focus. That would be the only way to check the effects of exit pupil/pupil size matching and image quality (and focus).

As we age, our ability to accomodate decreases, the lens becomes crystalline to one degree or another inducing decreased clarity(even if cataracts cannot be seen on exam), and the pupil is limited in its ability to dilate. Low light conditions exacerbate these limitations in addition to the decreased depth of field, and increased astygmatism noted in earlier replies.

JR

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