Re: Depth of Focus?

From: W. Watson (wolf_tracks_at_invalid.inv)
Date: 02/23/05 

Date: Wed, 23 Feb 2005 15:03:44 GMT

redbelly wrote:

> Not quite. Depth of focus refers to what range you can move an imaging
> screen, **with all lenses and the object fixed in position**, and still
> have a sharp image on the screen. Your eye's lens is one of the lenses
> in the system! Since it's impossible to to move your retina (the
> imaging screen) without also moving your eye lens, moving your head
> around does not determine depth of focus.
Ah, ha. Yes. And I would think then it's my optical system, my eye, that has an
enormous depth of field.
>
> You are correct about depth of field. Move the object (or look at
> objects at different positions) while the lenses and image plane are
> fixed.
>
> FYI, depth of field or focus depends on the spacing between sensor
> elements on the imaging screen (eg. pixel spacing if it is a CCD, or in
> the case of conventional film, the distance between grains). When the
> image from a point source is blurred enough to cover at least two
> sensor elements, the image is no longer in focus. If the image is not
> focussed perfectly, but is still only reaches one sensor element, you
> are considered withing the depth of field or focus.
>
> Offhand, I don't know a good source for the equations and derivations.
>
> Mark
>
> W. Watson wrote:
>
>>I'm looking in a microscope, and can see the object in the eyepiece
>
> from a 1/10"
>
>>out to 3" pretty clearly. Of course, it gets smaller as I go out.
>
> Isn't this
>
>>called the depth of focus? Take instead a magnifying glass and put it
>
> above a
>
>>*** of paper. I can move my head forward and backward. This gives
>
> me the depth
>
>>of focus, right? However, the depth of field is what is sharp in
>
> front of the
>
>>lens, closest tree to farthest tree, right? I see a formula for depth
>
> of focus
>
>>that involves the wave length divided by a quantity involving the
>
> sine square.
>
>>Does this formula have a name, or is named for someone? I'd like a
>
> source where
>
>>it is derived.
>>
>>As an aside, I would guess that if the depth of focus decreases, the
>
> image
>
>>appears brighter?
>>--
>> Wayne T. Watson (Watson Adventures, Prop., Nevada City,
>
> CA)
>
>> (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
>> Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700
>
> feet
>
>> Web Page: <home.earthlink.net/~mtnviews>
>
> 

-- 
              Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
                  (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
                   Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet
                         Web Page: <home.earthlink.net/~mtnviews>