----- Original Message -----
From: "Alan Chan" <[EMAIL PROTECTED]>
> >The short conclusion to a long story was that I realigned the screens on
> >all
> >my Pentaxes (3 screws under the screen and not a task to be approached
> >casually) and from that time forward all of my lenses have been as sharp
as
> >their designers intended, even the 35-70 (and even the much hated
A35/2.8)
>
> Just wondering what method you employed to adjust the screens for 100%
> accuracy. Set the lens to infinity did not work for me. :(
>
> regards,
> Alan Chan
>
It's about 7-8 years since I realigned my focussing screens so I hope I
remember the process correctly. It has also been suggested to me that the
mirror rest position is also a source of focussing error. You can determine
this by checking the framing of the viewfinder image for accurate centreing
compared to the film gate. A reference point in the dead centre of the
viewfinder should also be dead centre of the film gate. If it's not then
the reflex mirror is not at exactly 45 degrees vertically and 90 degrees
horizontally to the optical axis and the image is not just inaccurately
focussed but is also skewed off centre, and this method is not the fix you
need.
I've read that viewfinders are calibrated to allow for the film to bow away
from the pressure plate very slightly. This would be especially true for
cameras that wind the exposed film emulsion in on the take-up spool.
Pentaxes (among others) wind exposed film emulsion out on the take-up
spool, the purpose of which is to flex the film against the pressure plate
(and not an evil plot of camera designers to make newly rewound film
difficult to wind onto SS reels). Anyway, any residual error will be
discovered at the test film stage, so if it is a problem at all it is
fixable .
You'll need (apart from the camera to be calibrated):
1 sturdy tripod;
1 locking cable release;
1 ground glass appropriate size to rest between inner film rails. I
have the screen from a scrapped SP1000 :) which is at present stored in
a forgotten place :(
1 loupe or jewellers' magnifier for viewing at the focal plane;
1 jewellers' screwdriver with straight blade head approx. 1mm to 1.5mm;
1 flashlight to see the screws without tilting the camera body towards the
light;
Your widest lens;
Your fastest lens;
(A series lenses are easiest because of their flush barrel designs, F or
FA lenses may be troublesome due to their focus scales being beneath
windows, and because their fast gearing will make fine-tuning
difficult.)
A well lit scene with a wide range of distances including infinity;
1 sharp pencil;
Nail varnish;
1 pair steady hands;
1 film to test your results; and most importantly:
1 notebook to keep detailed records of EVERY adjustment you make.
The screws to be adjusted are:
one at the centre front edge of the screen at the top front edge of the
foam; and one on each side towards the back, behind access slots in the
outer frame of the screen.
Be extremely careful as you will be working very close to the reflex
mirror's surface.
Record every turn of each screw that you make. I worked in very small
increments eg. 1/4 turn clockwise, 3/4 turn counter-clockwise etc., and
turned all the screws the same amount until the very final stage of
fine-tuning. The notebook will enable you to return to the starting point
if things get all pear-shaped.
Wide lenses are particularly good for calibrating the focus screen because
they have very short depth of focus, ie the tolerance for focussing error at
the focal plane. But a fast standard lens is also useful for a similar
reason - the cone produced by the image forming light is very wide so that a
very, very small change in lens extension will make the circle of confusion
unacceptably large. OTOH a long lens has a narrow cone of image forming
light, and will tolerate a quite large change in lens extension without
exceeding the parameters of acceptable circles of confusion. For this
reason your should not use a telephoto lens for this exercise.
Set the camera on the tripod, lens on camera, focus on something (anything
at all but not too close) and draw a mark in pencil across the boundary of
the focussing ring and the fixed part of the lens barrel. Do this several
times to eliminate any wildly wrong focussings and settle upon the mark that
is hit most often.
Now remove the camera back, lock the shutter open and place the ground
glass on the focal plane. Blu-tack helps here but make sure you don't raise
the ground glass off the rails.
Repeat the process of focussing and marking the lens (same point of focus of
course) and compare your first mark (viewfinder result) with the new mark
(focal plane result). If the viewfinder result indicates a closer distance
than the focal plane result this means that the screen is set too low and
the lens needs more extension than it should for the viewfinder image to be
sharp. So turn the screws to raise the screen (remember - small increments)
and repeat the whole process. Conversely, lower the screen if the
viewfinder result is longer than the focal plane result (ie. less extension
of the lens than there should be to get a sharp viewfinder image).
If (and only if) you find that focus alters at different points on the
screen you can make differential adjustments by altering the screws
individually. You are on your own here as I couldn't begin to explain how a
rectangular screen moves when adjustment is made at the points of a
triangle! However, you need to beware of lenses that show curvature of
field, this is where a true macro lens (not a macro-zoom) is useful because
of its flat field.
When you get good matching results then re-run the process with a different
lens, and then re-run the process at different distances. Then shoot a test
film and try especially to include subjects with variable distances, eg. a
receding paling fence with the point of focus marked. That way you can see
which direction any error is leading. And be prepared to repeat the whole
process, I found that I achieved about 90% correction at my first attempt
and got the final 10% the next time around.
When you're satisfied that your screen is accurately aligned then paint a
little nail varnish onto the threads of the adjusting screws, to prevent
them from creeping away from their corrected positions.
Get it right and your ME Super, or any of the family of bodies that share
that basic chassis, will get results as sharp as your lens is capable,
limited only by the steadiness of your hands or the solidity of your tripod.
Regards,
Anthony Farr
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