On 21 Dec 2002 at 11:01, [EMAIL PROTECTED] wrote:
> My apologies if this appears twice, but I sent it yeaterday and
> messages from 12/21 are already appearing in the digest....so I will
> give it another shot as it has not yet appeared
>
> I was reading a book which had a section on macro work, and it
> mentioned that because of light loss on macro lenses without internal
> focusing that one should not stop down smaller than f 11. The
> reasoning was that with such light loss when the lens changes length
> that the true f stop was not given and that f 11, when you are at a
> 1:1 image size, is the equivalent of approximately f 22 in 'true'
> aperture and any less than f 11 may therefore start to degrade image
> sharpness because of diffraction.
>
> I have one of the older Canon 100mm f 2.8 macros, the one without USM,
> that changes length when focusing. I had always been stopping down to
> f 22 when I wanted to attain maximal DOF and the camera was tripod
> mounted so longer exposures were not of concern in the absence of
> wind. But now I find myself wondering if instead I am "shooting
> myself in the foot" by actually using an exceedingly small effective
> aperture and causing degradation of sharpness and image quality by
> diffraction. With a lens like this is it, in fact, the case that f 11
> at 1:1 size is equivalent to f 22 in terms of diffraction? If so,
> should most macro work using this lens be done at f 11 and just
> consider that DOF is maximized at this level and any smaller yields
> little DOF gain but other problems like diffraction?
Focal length changes with these macro's, so yes, physical properties
in relation to aperture change too.
But: had this been a classic/symmetric macro, you would have lost
roughly 2 stops at 1:1, instead of 1. So classic lenses would be
worse in this respect.
But2: I am not sure whether this actually affects diffraction....the
only way I can imagine this being the case, is when you look at what
happens at the film plane: the 24x36 becomes an ever smaller piece of
the available cone of light, the closer you focus (this is why most
micro lenses easily cover 6x6 and larger)....assuming diffraction on
the entire cone of light stays the same, it gets relatively worse on
a small patch of it....
(assuming everything else stays the same, which it never does....;))
--
Bye,
Willem-Jan Markerink
The desire to understand
is sometimes far less intelligent than
the inability to understand
<[EMAIL PROTECTED]>
[note: 'a-one' & 'en-el'!]
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