| With extension tubes as with virtually
any other lens setup, there’s a plane of sharpest focus,
and in addition, there’s some sort of depth of field.
Without getting any deeper into optical theoretics and confusing
the hell out of both of us, let’s just say that the
plane of focus is sharpest and that planes falling within
the depth of field are kinda sharp enough. Looking at the
field as a line which is on the axis of the lens, the image
will appear "kinda sharp enough" at some measurable
distance in front and in back of the sharpest point of focus,
and this observation will be consistent for a given magnification
at a given f-stop. At higher magnifications, there will be
less that will be acceptably sharp in front and in back og
the point of focus. At larger apertures (lower f-stop numbers),
acceptable sharpness will drop off faster on either side of
the sharpest point.
At high magnification in macro photography, the depth of
field becomes very short. Even when selecting f22, the smallest
aperture available on a Nikonos lens, you’ll have only
about three-sixteenths of an inch between the nearest and
farthest point of acceptable focus when using a 1:1 extension
tube.
Your observation that there seems to be no difference between
setting the focus at nearest or farthest distance on the lens
is probably a function of the difficulty in accurately making
a test. When using a Nikonos with a 1:1 tube and framer, you’re
trying to measure (estimate(guess)) where, in the roughly
one-eighth-inch-plus wire of the framer, at a distance of
a bent arm’s length with a subject about the size of
a postage stamp. Since there is detail in the subject both
in front and in back of the exact part of the thickness of
the wire you think you placed at the exact point on the subject
where you think you placed it, you can’t be sure if
what is sharpest in the photograph is what you expected to
be sharpest when you snapped the shutter and probably moved
the camera ever so slightly, maybe a one-sixteenth of an inch
(nearly half the entire depth of field). Let’s face
it, it’s a tough test.
If you were taking the same photograph with an SLR camera
with through-the-lens focusing, and you were using an extension
tube behind a normal lens which had similar optical characteristics
to the Nikonos lens with extension, you would see the plane
of sharpest focus move in and out as you focused the lens.
The rest of the subject that’s within the depth of field
moves in and out with it. When you look at your subject in
the framer, you see it at roughly your reading distance, and
the subject area fills only a few percent of the viewing area
of your retina. When you view the same composition through
a reflex camera with a large finder, the same image fills
almost your entire viewing area and can thus be much more
precisely evaluated. The framer on your Nikonos is too gross
compared to the small subject within it to be precisely focused
in a controlled manner.
Once the length of the extension is determined, only on point
on the axis of the lens will be in focus, and that point,
along with its accompanying depth of field, is fixed. The
manufacturers arbitrarily position the framer as a reference
to the photographer. It apparently has been the intent of
makers of extension tubes, in order not to thoroughly confuse
the diving public, to agree on a reference for determining
the plane of focus. This intervention has been established
by conventions since the invention of extensions. Most manufacturers
have agreed that the back of the framer should locate the
plane of focus when the lens is set at it’s nearest
focusing distance. If you set the framer, frame down, on a
flat surface, that surface will be at the correct focus distance.
As the magnification is decreased from life-size 1:1, the
depth of field increases proportionately, until at 1:3, f22
depth of field is just about three-quarters of an inch. The
placement of the framer is more precise for selectivity and
much less critical when it comes to missing the subject altogether.
Framers are mounted onto tubes in various ways by different
manufacturers, sometimes allowing for adjustment, either intentional
or inadvertent. You may wish to find the exact plane of focus
for your particular (possibly "readjusted") set
of tubes with a more controlled test. You can do this in the
kitchen sink, bathtub, or any other porcelain or non-porcelain
container which will allow you to submerge the lens and framer
and still get enough light on the test subject to get a usable
exposure. This test will give you a more critical result at
a larger aperture.
Place a credit card or other flat object or material with
surface detail on the bottom of the container with enough
water in it to cover the front of the lens when the camera
rests on the framer. Place a penny on the card and a stack
of two pennies at another place on the card. For 1:1 or 2:1
(twice life-size—about half the size of the 1:1), rest
the framer on the card and take a photo with the lens set
at closest focus and wide open. You can hand-hold your flash,
keeping in mind that very little flash will be required in
mostly air at the large aperture. You can do another shot
at f8, then another at f11, f16, and f22 to see how the depth
of field changes. Depth of field at the back of the plane
of focus can’t be observed unless you shim up the framer
by resting it on more credit cards, stacks of quarters—anything
flat and stable. You can repeat these tests with your 1:2
and 1:3 tubes. See what happens when you change your focus
to infinity while you’re at this: you’ll have
plenty of frames. There’ll be more room for more penny
stacks and they’ll have to be higher. I have not done
these particular tests myself as the bank won’t loan
me the money.
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