Ryan <quakeserver149 DeleteThis @yahoo.com> wrote:
>I've been on a project to photograph a few hundred "pages" of a
>16mm microfilm roll.
>
>I've acquired a bellows to use along with a D70 and Tamron f/2.8 35-70mm
>
>Quickly I discovered that I don't get a sharp image across the
>full frame with this method. (It is parallel from lens and flat)
>
>The very center is passable, but very soon the image becomes
>slightly blurry and chromatic abberation becomes glaring. The
>edges of the film are nearly a rainbow.
>
>I've been digging through the archives and as much as they've
>helped, they've raised questions.
>
>Terms like "flat field" are still a bit mysterious.
The flatness of the field of focus is interesting.
If there is a specific point source of light in the lense (think
about when you use f/22, that little small aperture opening),
then light that goes through that point would all be focused at
a particular distance. If that distance is always exactly the
same no matter what part of the image it lands on, then the
field that the image focuses on is curved like a circle
(actually a square patch cut out of a sphere). It won't be in
focus on a flat surface, because the edges of a flat surface are
more distant from the aperture than is the center.
Here is an exaggerated example which is very crude, but about the
best I can do using ASCII text:
, + top of film Use your imagination to
.| draw a curved line between
| the dots. That is the
o + center of film actual field where the
.| image is in focus. The
| straight line between '+'
` + bottom of film points is the flat film.
If that 'o' is the aperture, obviously it is a longer distance
to the top or bottom of the image on the film than it is to the
center.
Lense designs have to be made so that the light headed for the
edges will focus at a longer distance. The designs are not
perfect and in practice vary from having a "focal plane" that is
curved inward smoothly as it goes away from the center, to ones
that sort of wave back and forth and get fairly close at some
distances but are less close at others. None are perfect, they
just get close enough that you can't see any difference.
You have an example of how lense designs are compromises. By
using a bellows to focus yours closer, you've discovered one of
the compromised areas: close focus. And because it is a zoom
lens, there were all sorts of complications trying to make it
work well as the focal length is changed, and they managed to do
that only over a limited range of focusing distances.
Astigmatism and flatness of field are opposing characteristics,
and a design can pick one or the other over the distance range
which a lense can be focused. It can b optimized well enough at
the center of a range (hence is always good at medium
distance). But it is worse at the limits of the focusing
range, so at 1000 yards and at 10 inches it is a case of choose
one or the other to have astigmatism if the design has a flat
field. (Many true macro lenses aren't so good at 1000 yards!)
Most lenses of course are optimized to be sharp when focused at
infinity. The designer then picks the closest focus at which
the lense is still acceptably sharp, and limits the mechanical
linkage for focusing to that distance.
If you put such a lense on a bellows, what you find out is why
the focusing mechanism limited it to 20" instead of 5"! It
suffers all sorts of horrible problems when actually focused at
5 inches.
>I'm now familiar with several terms but don't feel wiser about
>whether I should go about getting a used micro lens, or if I'd
>get tack sharp images by turning my existing lens around with a
>reversing ring.
If it were a fixed focal length lense, I'd suggest going for it.
But many of the problems you are experiencing have to do with it
being a zoom lense. Reversing the lense would probably help,
but not enough. And it is not particularly difficult to find a
lense that would work significantly better.
>With the lens reversed, does this still mean I need to extend
>the bellows to get "closer" or does it change the rules to have
>the lens backwards? Is focusing changed?
Not much.
>Does it matter the internal design of the lens? (yes) What I
>mean is does the "aspherical" design of this lens explain the
>reason for the blurriness at the edges, or is that missing the
>mark completely?
That is a complex subject. Use Google to find tutorials on
lenses, and read several of them. Even a brief discussion is
long, so I'm not going to even try because it isn't actually a
significant part of your problem, though it certainly is an
interesting topic.
>I'm trying to guage whether I should expect better results with
>this lens reversed, or with a macro lens on a bellows. I can
>imagine the answer depends on what I'm shooting, so I'm curious
>which conditions warrant which answer.
Forget about using that particular lense. Think about what
kind you actually do want. The question then becomes a matter
of money and complexity. Are you interested in the least expensive
solution, or the least complex solution?
>If a reversing ring is in my future, do I need to also consider
>the need for a step-up ring?
Depends.
Because 16mm film is fairly small, I'd think that you will want
a solution that is a bit better than average. Nobody needs that
much detail if they are taking pictures of 2" flowers, so what
is a great lense for them might not be acceptable for you at
all.
For that reason, you probably should not bother with any lense
that is not specifically designed for macro work. (Skip all
zoom lenses, and note that "close focusing" is not really the
same as a "macro" design.)
Also, given that taking pictures of a film strip means back
lighting, you do not need to worry much about the distance
between the front of the lense and the object (a common point of
contention in photomacrography). That means shorter focal
length lenses are acceptable for you. Anything from 35mm to
105mm is probably workable, though shorter focal lengths give
greater magnification at any given extension.
You can also probably make use of auto focus capability if you
don't mind the expense. A macro lense in the range from 40 to
60mm used with set of AF extension tubes will give you maximum
convenience (at just about the highest cost of any
solution). Nikon sells a 60mm macro lense, which will cost
something over $300. At least Sigma, Tamron and Tokina also
make AF macro lenses, any of which will probably cost less than
the Nikon lense. I think it is Kenko that has AF extension tubes
for Nikon, at probably just over $100.
If you want to spend less money, there are other solutions which
will provide equal technical results, but at significantly less
convenience for the photographer. There are *many* 50mm, 55mm,
90mm and 105mm macro lenses made by virtually everyone who ever
made a good lense. Almost all of them will provide excellent
images. They are all manual focus, and will work with your
bellows.
And beyond that, virtually any Pentax Screw mount, or T1 mount
macro lense can be adapted to your bellows! There must be several
dozens of different macro lenses that you could find on eBay that
can work. Most of them will cost less than $150.
If you want the lowest cost solution that still provides the
best technical results, buy a good 50mm enlarging lense and the
necessary adapters to fit it to your bellows. Not all enlarging
lenses are made equal, so this takes a little more care or
you'll once again end up with fuzzy edges. But it happens that
older El Nikkor 50mm f/4 or f/2.8 enlarging lenses are forsale
at bargain prices on eBay every day. Another good one is a
Rodagon 50mm enlarging lense.
If you want to get really fancy, get the adapters necessary to
reverse either a 50mm f/4 El Nikkor lense (it has a 34.5mm
filter ring) or a 50mm f/2.8 El Nikkor (40.5mm filter ring). It
will require stepup rings (probably more than one will be
required to get to 52mm) and then a 52mm reversing ring for a
Nikon mount. (Another way is to have a Nikon camera mount to
pentax screw mount lense adapter on the bellows, and convert the
lense filter ring to 49mm (rather than 52mm) and then use a
Pentax screw mount reverse adapter. That has the advantage that
a Leica to Pentax adapter on the other end will allow the lense
to be mounted either direction.)
You already have the bellows, and El Nikkor enlarging lenses
go for $20-30 with regularity. The various adapters are not
easy to find but cost $5-15 each.
If you don't need the absolute best images technically, a BR2A
lense reversing adaptor and a Nikkor 50mm f/1.8 lense is the
simplest solution. Put that on your bellows. It won't be quite
as sharp as the others above, but I'm not sure the difference
would be significant either.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd DeleteThis @apaflo.com
>> Stay informed about: Attempts at micro/macro shots 
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