Resolution / depth of field (was Stellar by sunlight)

[Gary Turner <turner@MORNINGSIDE.EDU>]

Aren't resolution and depth of field two different concepts?

Resolution is the minimum angle at which two objects can be resolved as two
objects before they blur into one.  Depth of field is the range of objects
that can be brought into focus at the same time.

As the pin-hole gets smaller, depth of field increases.  Closer objects can
come into 'focus' because the light paths through the hole are more
constrained.

However, as the hole gets smaller, resolution gets worse.  You may be able
to focus on more things, but may no longer able to distinguish the letters
on the sign across the road (they are still 'in focus', but are now a
focused blob)

I'm not sure whether the explanation of the squinting problem is the
pinhole or the aperture effect, so I'm not arguing one way or the other.
Here is the argument I have heard for the aperture effect though.

theta_min = 1.22 (wavelength / diameter) for a circular aperture.  If you
can make your pupils dilate to twice their original size, theta_min becomes
half of what it was - you can read words that are twice as far away as
before.  Pupils automatically dilate according to the entering light level,
so to make them dilate you just restrict the incoming light - and squint.

The counter-argument is that by squinting you have reduced the 'diameter'
in the vertical direction and should have really bad resolution in that
direction. That can, in principal, be tested u suinting and rotating your
head side to side.  But then you must also include the effect of eyelashes
getting in the way.  They are definitely one-directional and will probably
have a contributing effect.

Just need a volunteer to cut out their eyelashes to try this.  Anyone?


On Wed, 27 Mar 2002 11:34:47 -0600, QUIST, OREN <OREN_QUIST@SDSTATE.EDU>
wrote:

> You say "better resolution" which is a good way to put it, but the way I
> explain it is to say that the pinhole effect increases the "depth of field"
> so that everything is in focus.
>
> -----Original Message-----
> From: Larry Cartwright [mailto:exit60@CABLESPEED.COM]
> Sent: Wednesday, March 27, 2002 10:10 AM
> To: PHYS-L@lists.nau.edu
> Subject: Re: Urban Fables: Stellar by Sunlight
>
>
> Gary Turner wrote:
> > Incidentally, I have been told that squinting helps seeing because
> > it restricts the entering light - causing the pupils to dilate and
> > improving the available resolution.  Is there any truth to this?
>
> I don't believe it has anything to do with dilating the pupils.  When
> you squint, your eyelids and lashes cover most of the pupil; so it
> doesn't matter whether the pupil is dilated or not.
>
> I've always thought that squinting lets the lids and lashes form
> essentially "pinholes", which produces an image on the retina much as a
> pinhole camera produces an image on a piece of film no matter how long
> or short the camera box.  The smaller the pinhole, the better the
> resolution of the image.  I'm ready and waiting to be educated by
> someone who can point out where I have gone wrong.
>
> Best wishes,
>
> Larry
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Larry Cartwright   <exit60@cablespeed.com>
> Retired (June 2001) Physics Teacher
> Charlotte MI 48813 USA
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~