Chuck, I did not mean to suggest that the magnification of the aerial image
increases (or even decrease) the DoF - as you rightly imply, the
magnification has nothing to do with DoF. I was simply describing what
function the macro lens is fulfilling in this application. And you also are
absolutely correct to say that the more you magnify an image the less the
depth of field.
Maybe I should have better expressed myself by saying "enormous depth of
field compared to what is normally expected of a 20/3.5 macro lens".
Granted, that may be very close to a "divide by zero error", but I hope you
know what I mean this time. I will try harder next time :-)
--
Piers
-----Original Message-----
From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
Of Chuck Norcutt
Sent: 11 September 2005 00:42
To: olympus@xxxxxxxxxx
Subject: [OM] Re: OM macro lenses?
Piers Hemy wrote:
> Chuck's question is easier to answer:
>
> Remember two principles:
> The shorter the focal length on a given format, the greater the depth
> of field; The smaller the format on a given focal length, the greater
> the depth of field.
>
> In this case we have a very short focal length on a very small format.
> The image is producedinitially as an aerial image on the focal plane
> of the CCTV lens, which the macro lens then "emlarges" to fill the 35mm
format.
-------------------------------------------------
I must be dense since I have to admit that it didn't help me at all. I
accept that we have a short focal length which is good for depth of field
but I don't see that magnifying the aerial image to fill the 35mm format has
anything to do with it. Another rule about depth of field is that the more
you magnify an image (read this as = longer effective focal length) the less
the depth of field. In other words, the bigger we make it the fuzzier it
looks to our eyes. I don't see any difference between doing that with one
lens or many.
What don't I understand?
Chuck Norcutt
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