Olympus-OM
[Top] [All Lists]

Re: [OM] Aperture Modification

Subject: Re: [OM] Aperture Modification
From: "Mark Hammons" <astaire@xxxxxxx>
Date: Fri, 20 Jul 2001 08:54:08 -0500
----- Original Message -----
From: Paul Wallich <pw@xxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Cc: <olympus@xxxxxxxxxxxxxxx>
Sent: Friday, July 20, 2001 8:34 AM
Subject: Re: [OM] Aperture Modification


> At 12:34 AM -0500 7/20/01, Mark Hammons wrote:
>
>
> >>
> >> Diffraction limiting from small apertures seems to kick in sooner and with
> >> greater prominence with the shorter focal lengths than with the longer 
> >> ones.
> >>
> >> -- John
> >>
> >
> >Actually, I believe it is strictly a function of the F-ratio.
> >
> >Assume you have a 50mm Lens at F50.  You have a 1mm aperture
> >hole and the L/A angle is whatever milliradians and the light has to
> >travel 50mm to the film so you have the spot size as (2*L/A*50)mm =
> >(100*L/A)mm.
> >
> >If you now go to a 100mm lens at F50 your aperture is now 2mm.
> >Call this A'.   Well A' = 2A (the aperture size of the 50mm lens at F50).
> >And the total spot size is (2*L/A'*100)mm = (200*L/A')mm =
> >(200*L/2A)mm = (100*L/A)mm, which is the same spot size on the
> >film as the 50mm F50 lens.
> >
> >So the smaller angle of diffraction by going to a longer focal LENGTH
> >lens of the same focal RATIO is exactly offset by the longer DISTANCE
> >the light has to travel to the film plane.  Thus, the spot size is a function
> >of the focal ratio.
>
> I had been wondering about this for a while, because while it seems
> sensible it also doesn't always seem to be the case. My guess is that
> you get some kind complicated effect as a result of the fact that the
> physical distance between film and diaphragm isn't the same as the focal
> length. In particular, most short lenses are retrofocus designs (to clear
> the mirror) and most long lenses are telephoto (to reduce total length
> and weight).

Yes, this is true, however the diameter of the apeture at the blades would
have to be such that it would be scaled to what it would be if it were
at the rear ( or would it be front?) nodal point of the lens.  So even if its
not exactly 1mm for a 50mm F50 then it would be such a size as to still
produce F50 at whatever "optical" distance the aperture is from the focal plane.

> Obviously all of the glass involved makes the problem more complicated
> than just the physical position of the aperture, but it seems to me from
> a first cut at the problem that the back elements wouldn't/couldn't/shouldn't
> (if they're acting properly on light coming through at larger apertures)
> completely compensate for the difference between physical aperture placement
> and focal length.

Remember that with lenses the typical maximum sharpness is a balance
between spherical abberation and diffraction effects.  Spherical abberation is
worse at wide apertures and becomes better as you reduce aperture size.
Diffraction effects are just the opposite -- they are least pronounced at large
apertures and get worse as you decrease aperture size.   If  you are stopped
down enough to where diffraction effects are dominant I don't think the
spherical abberation would come MUCH into play.

> paul  wishing for that old copy of mathematica




< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz