Sunday 27 September, 2015 15:05
FOCUS ADJUSTMENT (Mechanical correction of back/front
to most film and digital changeable lens cameras (SLR and DSLR as
of the reasons I used the Sigma SD9 more than the later SD14 was because
my perception "told" me the images taken with the SD9 were
sharper. It generally went no further than that. However, a recent
closer look at one specific image showed the focus point was slightly
behind where I thought it should have been. I do, of course as taught
on workshops, always focus on the Nearest point and then the Furthest
point and set the lens focus midway on a scale (or midway relative
to the movement) which of course means it is not set midway in distance
but rather midway on the lens mount movement = one third into the
Near/Far distances. All this of course to ensure adequate Depth of
Field for the subject.
I decided to check back through a number of images, I found that the
actual focus did not quite cover the required sharpness in respect
of the Nearest point of the subject. So I set about checking whether
the lens was focusing on the actual points that I perceived as being
sharp in the camera viewfinder. As I was using the same lenses on
the Sigma SD9 and SD14, it clearly was not the lenses that were to
blame but rather that, perhaps at that stage, the SD14 had some problem
with its in-camera focusing mechanism.
first image shows one shot that made me question the focusing. The
image should have been sharp, using the Near/Far focusing method and
an appropriate f-stop (aperture) to suit, being f11 in that shot.
As should be reasonably clear, the Nearest is not quite sharp enough
- the point of focus (meaning where it first became sharp) was slightly
behind the nearest point, meaning the DOF had not quite covered from
Near to Far and as can be seen, the distance is sharp, yet in real
terms, could have been slightly softer in definition, as the main
subject would have benefited being sharp throughout its depth. Here's
that image. Just see how sharp the distance is (well, for that matter,
from the seated "figure" to the far distance) yet the immediate
foreground is just off full sharpness. To keep the image a sensible
size for this TIL, it is shown smaller than perhaps ideal to show
what I mean. You can see a larger size by CLICKING
to check how accurate the in-camera focusing is - I always use manual
focusing - BUT there is information and an illustration at bottom
to show how to adjust both for Manual and Auto focusing, should the
latter also be inaccurate. As will be seen, in my SD14, there was
a problem in that when the in-viewfinder image looked sharp, in reality,
the lens was slightly focusing further away = Back Focusing. A simple
adjustment (most SLR and DSLR cameras, for film and digital, have
similar adjustments) soon corrected the problem, which probably means
I will use the SD14 with more confidence. Here are the shots, the
first showing the actual in-camera focus behind the in-viewfinder
focus point of the "20". Second shows after one adjustment
and last is the final. In each case I took shots to see what was happening.
I used the maximum aperture of f2.8 and the lens (24-70EX) at maximum
70mm. To test it is ESSENTIAL
to copy the method shown using a tape measure set at a slight angle
to the shooting position!
FINAL one below shows actual focus point on 20 in viewfinder now agrees
with actual focus point in the camera. I only did the Manual Focus
because I never use Auto-Focusing.
picture below, which I borrowed from somewhere (sorry, forget where
as long ago) gives an idea of how most reflex cameras can be adjusted
- think it was related to a Nikon D70 film SLR - shows the location
of in-camera adjustments for both the Manual-Focus and Auto-Focus.
Much later DSLRs (well, some) have internal digital adjustments for
correcting for such focusing errors. Taping up the mirror does not
actually apply to many cameras as most have a specific way to raise
the mirror for such events like this and, as another example, sensor
cleaning - so do read your camera Manual! If you should have
mislaid yours, try the manufacturer's online website as most manuals
can be downloaded. Anyone unsure can mail me but of course, I can
only make one reply to non-members of the Zone 8 Photo Society as
members subscribe to obtain such help on many photographic and computing
topics related to same. The "wrench" is actually a 1.5 HEX
(Allan Key) which does not just fit in the end but actually can go
through the adjustment head. To correct for the amount shown in my
examples, I had to make approx two small clockwise turns (meaning
about equal to 10-15% of a full circle) to attain the final adjustments
per the images above. Clockwise to correct
for Back Focus and Anti-Clockwise for correcting Front-Focus.
That applies to BOTH adjusters should you need to adjust both of course.
They are independent, so a camera could have correct focusing for
one or both and thus need no adjustment or different focus adjustments
necessary for both Auto and Manual of course. Please note the adjusters
are CAMS - which means one full turn simply returns to the starting
position. As a rough guide, turn CLOCKWISE if back focusing needs
adjusting and if your HEX key has a long side of, say, around 2½"-3"
move so end only moves around ½" to ¾" maximum
for each trial adjustment before checking as shown above.
a local view shot on 15th June 2010 to prove the adjustment worked
a larger size click on link below
HERE to visit a small gallery of additional pictures taken with
the corrected SD14