The problem here I believe can be two things:
1. Refractive error of the photographer. However the use of microprism/split
prism to focus eliminates this in most cases.
2. When an image is focussed, we are actually appreciating the focus on the
focussing screen that is designed to be at the same distance as the film.
Therefore if your focussing screen is slightly out (frequent screen changes
can warp the frame for holding the screen in place) or more often the film
is not flush with the pressure plate then the small differences you see can
be explained. The problem obviously occurs when the lens is wide open and
the depth of field is unable to cover for the minute film to focus screen
distance differences. The problem is "real" enough that the Contax RTS III
when released showed it has has a suction device to smooth out these film
plane variations as a source of focussing error. The films age could also
play a part perhaps tending to bow out in the centre etc.
Just some suggestions.
Oben
----- Original Message -----
From: "Skip Williams" <skipwilliams@xxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Wednesday, May 09, 2001 11:48 AM
Subject: Re: [OM] Gary's lens test...
> You know, I had the same problem with a 50/1.2 shot wide open one day. I
> took some BEAUTIFUL portraits of my family with window light, but EVERY
one
> was about 2-3" off focus, which effectively ruined them all. I know my
> eyes are failing a bit, but every shot? Geez.
>
> I'd love to hear other stories and/or solutions.
>
> Can an SLR get out of focus calibration? (I know my rangefinders can do
this.)
>
> Skip
>
> At 11:33 AM 5/9/01 +1000, you wrote:
> > > Erwin Voogt <erwin.voogt@xxxxxx> wrote:
> >
> > > lenses should be at F8 or sometimes even at F5.6. I suspect correct
> > > focussing somehow plays a role within the SQF rating.
> >
> >A few days ago I was comparing a few lenses (85mm f2, 100mm f2.8, 135mm
> >f2.8, 50mm f1.4) by taking some portraits of my daughter. She was
sitting
> >in a
> >chair quite still and I was able to focus quite accurately (so I thought)
> >on her
> >eyelashes, lighting was with T32 bounced from an umbrella. Film was
Kodak
> >Portra 160NC, body OM-2SP with 1-4 screen, aperture f2.8 for each lens.
> >
> >When I got the film back about 1/4 to 1/3 showed incorrect focus, with
the
> >area of
> >sharpest focus not the eyelashes.
> >
> >So how do you achieve "correct" focus ?
> >
> >...Wayne
> >
> >< 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 >
>
>
>
> < 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 >
>
>
< 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 >
|