[pure-silver] Re: Is your camera really in focus ?
- From: Dennis P <dlp4777@xxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Fri, 30 Mar 2012 08:05:45 -0700
On Mar 30, 2012, at 5:03 AM, Peter Badcock wrote:
> Hi folks,
>
> I have just spent the past couple of months off and on re-calibrating the
> focus on my Mamiya C330f TLR. I finally succeeded but needed precision
> instruments/tools (vernier callipers +dial gauge) to do this to what I
> consider an acceptable precision.
>
> Due to the small tolerances required (usually better than 0.1mm), I can't
> help but think there are many cameras out there (both analog and digital)
> which are also not optimally calibrated either (for focus) due to being
> bumped, poor design or poor original manufacturing.
>
> My lens is an 80mm f2.8, but if you have say an f1.4 or lower aperture, your
> depth of focus will be really tiny (like <0.1mm if you use 120 film or
> smaller) and your system could easily get knocked out of alignment.
>
> What are your experiences with fixing focus issues and why aren't more people
> noticing these kind of problems.
>
> thanks
> Peter
I think probably most people aren't experiencing any problems. If you are
talking about your lens focus being less than 0.1mm off the film plane then
that is probably beyond people's ability to focus accurately regardless whether
the lens is that out of calibration. Also there is a small amount of film
curvature or lack of flatness especially in 120 that is probably greater than
the tolerance you are talking about.
I know from experience with lots of tests with my Rolleiflexes that even taking
off the WLF and focusing with a loupe on the focus screen it is very easy to
miss focus by an inch or more from a distance of 10 feet.
People often have the impression that those Rollei lenses aren't very sharp at
2.8 but I think a lot of it is the difficulty of focusing correctly enough for
wide open. Once you stop the lens down a stop or two the
small degree of miss focus is a moot point because the image looks sharp.
I have noticed a couple of times the focus screen is off a bit with my Pentax
67 and I have corrected it by adjusting the tightness of the screws in the
corners of the screen. I think the screen being off by half a mm is probably
enough to make a person think the Pentax lenses just aren't that sharp, which
is probably the conclusion most people come to with any system that has slight
out of adjustment problems.
Anyway that is my opinion.
Dennis
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