I've got a 25, which is sometimes a bit too much.  the set you're
looking at will give you a bit more flexibility.  I may have to get a
shorter one in the future.

It's allowed me to get very close to subjects.  I was taking pictures
today, and I bumped the lens hood into something maybe four inches
away.

My camera meters fine with the tube.

I've used mine with my 50mm and 25-135mm lenses.  I haven't tried it
with any longer focal lengths.

    O.K.  The 50mm f1.8 is a simple lens (e.g. neither a telephoto
or a wide-angle), so its behavior is fairly predictable.

    It is 50mm focal length, so with an additional 50mm of
separation between the film and the lens, it should get down to a 1:1
ratio -- that is, the image is the same size as the object.  At this
extension, the focus adjustment on the lens is pretty useless -- you have
to focus by moving the camera/lens combination towards/away from the
object.

    So -- your 36mm spacer, plus the 12mm one will get you to 48mm
extension, and your lens focus adjustment should cover the extra 2mm, so
you can get the 1:1 ratio.

    However, the 50mm f1.8 is not optimized for work this close, so
you will probably need to stop it way down.  Macro lenses designed for
this are more often around f3.5 maximum aperture, IIRC.

    And to go beyond this, with your normal lens, you will need a
way to reverse the lens so the front is pointing towards the camera and
sensor, as the image is now larger than the object.  In earlier cameras
(without autofocus), such reversing rings were available.  If you can
find one, you will be totally disconnected from your autofocus and your
metering (as there is no provision for routing the CPU contacts to the
other side, even if your extension tubes couple them through.)  So --
you will be in the position of taking trial exposures, examining the
histogram display, and adjusting exposure until you get it right.  I
hope that what you photograph doesn't have functional wings or legs. :-)

    Note that the "28-105mm f3.5-4.5D" lens has a macro mode which
is quite good -- except that at maximum close-up, it tends to shadow
part of the image from illumination from the built-in flash.

    When *I* want to take extreme close-ups with the D70, I use my
ancient Medical Nikkor, which has a built-in ring flash, and a built-in
exposure computer (mechanical linkages), so I use the camera in manual
mode.  You need the proper flash adaptor to handle the higher voltage
connection from the antique flash.  You will see the recommended one in
the manual as a footnote to the list of lenses which can work with the
camera.  This presumes that you can find a Medical Nikkor these days.
Mine was quite old and well used (and well cared for) when I got it at a
swap meet.

    The two problem are:

1)    Past a certain magnification ratio, there is too much light
    for the lowest ISO that the D70 offers -- though a neutral
    density filter might make the difference, stacked with the
    supplied close-up lenses used on that lens.)

2)    The annotation in which the lens prints the magnification ratio,
    or a frame number on the image is lost -- as it prints in the
    corner, past where the D70's sensor stops.  You've still got
    frame number annotation, but not the magnification ratio, unless
    you edit into the "comments" feature.

    It will let you get closer to the subject than the normal
focusing range.  I'm not sure how well I can calculate that with a zoom,
which is playing all kinds of optical tricks, and which (I think) is
holding a back focal length pretty constant.  Telephotos (in contrast to
plain long lenses) have a negative lens as part of the assembly, so the
lens can be closer to the film (sensor) than would otherwise be
possible, and (extreme) wide angle lenses are sort of a backwards
telephoto lens, so in either case, what focal length is marked may bear
little resemblance to how the extension tubes will work with the lens.

    The way to find out is to add the shortest extension tube, mount
the lens, and move towards the object while looking through the
viewfinder until things come into focus.  Repeat the experiment with the
other individual tubes, and with them in combination.

    *If* the metering will work at all through the tubes.  It
certainly won't with my old set, nor with my bellows.  Both of those
lack the contacts to allow the lens to communicate information to the
camera body.  What you have found appears to have that feature, though
with the exclusion of the 'S' series lens, and the lack of the 'D'
information, maybe not.  (Or maybe only some of the CPU contacts are
brought through, not all of them.)  If you can test them in the photo
shop, do so -- bring all the lenses which you consider it important to
test with the extension tubes, (probably something in the 45-55mm range
would be best.)  See what it does, and bring some reasonable test
subject.

    Get a tabletop tripod.  I think that you'll be in a position
where you will need to repeat test shots until the histogram confirms a
good exposure.  And pray for stationary subjects.

    Note that since focusing at the 1:1 ratio involves moving the
camera/lens combination, sometimes having a focusing rack to which the
camera is mounted before you get to the tripod will help.  Good bellows
setups often had a second set of rails to allow you to move the
combination for such close focusing.  But I don't think that there are
any bellows setups which couple through the AF mechanism and the CPU
connections, so you go fully manual for this.

    I hope that this helps,
        DoN.