Olympus-OM
[Top] [All Lists]

Re: [OM] Generic lenses for Oly OM equipment

Subject: Re: [OM] Generic lenses for Oly OM equipment
From: "Doug Nowlin" <wa5ohb@xxxxxxxxxxxxx>
Date: Tue, 24 Feb 1998 10:21:09 -0600
-----Original Message-----
From: Richard Schätzl <Richard.Schaetzl@xxxxxxxxxxxxxxxxxxxx>


>Kennedy wrote:
>
>> The diffraction limited cut-off frequency of a lens is given by
>>                  Wo = 1/(L * f/#)
>>       where Wo is the cut-off frequency in cy/mm at the focal plane
>>       and   L is the wavelength in mm
>>
>> Thus, for an f/32, the cut-off frequency of the diffraction limited
>> optic is 62.5cy/mm at the focal plane, assuming a mean wavelegth for
>> visible light of 600nm.
>>
>> This is independent of focus distance, magnification or whether there is
>> an 'r' in the month.  :-)
>
>Independent of Magnification so far only for the "real or effective
>aperture", because the effective aperture (in contrary to the choosen
>aperture on the aperture ring) will get smaller with higher
>magnification.
>
>For example: choosen aperture = 5,6 effective aperture at 1:1 = 11
>
>Richard
>
I have a different formula for diffraction, but I had enough of complicated
formulas many years ago. Neither of our formulas give any practical data for
photography. Diffraction depends on the hole size and to some degree
diaphragm shape and Richard's statement is correct. A simple focal length/4
formula that is often given is just as effective for photography.  In this
case 90/4 yields f/22 as the minimum stop to prevent image loss from
diffraction. As I said originally, at some point the loss of sharpness from
diffraction is more significant that the gained DOF. The purpose of this
simple rule is a guideline that will be fairly accurate for most situations.
The proof is in the picture, but many pros tend to be even more conservative
and use f/16 for macro lenses of this focal length.

Doug


############################################################
| This message was delivered via the Olympus Mailing List
| To receive the Digest version mailto:listserv@xxxxxxxxxxxxxxx
|    with "subscribe olympus-digest" in the message body.
| To unsubscribe from the current list mailto:listserv@xxxxxxxxxxxxxxx
|    with "unsubscribe olympus" in the body.
| 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